Michael A. Reshchikov

Virginia Commonwealth University | VCU · Department of Physics

Experimental studies of Be-doped GaN by photoluminescence (PL) confirmed theoretical predictions that the BeGa acceptor in GaN has dual nature, namely a coexistence of a deep state with a localized hole and a shallow state with a delocalized hole. Electron transitions via the deep polaronic states of this defect cause the broad yellow luminescence...

GaN samples are implanted with Be and F and annealed in different conditions to activate the Be Ga acceptors. Photoluminescence spectra are studied to recognize the defects. The UVL Be band with a maximum at 3.38 eV and the YL Be band with a maximum at 2.15 eV are observed and associated with Be. The sequential implantation of Be and F ions into Ga...

We present a systematic photoluminescence study of Be‐doped GaN grown by metalorganic chemical vapor deposition. All Be‐doped samples show the ultraviolet luminescence (UVLBe) band with a maximum at 3.38 eV and the yellow luminescence (YLBe) band with a maximum at ∽2.15 eV in GaN:Be having various concentrations of Be. The UVLBe band is attributed...

In this chapter, a critical analysis of point defects in GaN and their manifestation in such experiments as photoluminescence (PL), deep-level transient spectroscopy (DLTS), positron annihilation spectroscopy (PAS) is presented. Only a few PL bands are attributed to specific defects. The dominant defect-related PL band in GaN grown by metalorganic...

The yellow luminescence (YL) band with a maximum at 2.2 eV is the dominant defect‐related luminescence in unintentionally doped GaN. The discovery of the mechanism responsible for this luminescence band and related defects in GaN took many years. Eventually, a consensus has been reached that the CN acceptor is the source of the YL band (the YL1 ban...

The current status and future challenges in the field of point defects in GaN are presented. The properties of point defects, such as thermodynamic transition levels, optical energies, and charge carrier capture coefficients, are obtained from the analysis of photoluminescence, while other experimental methods are used to confirm the identification...

Beryllium has been considered a potential alternative to magnesium as a p-type dopant in GaN, but attempts to produce conductive p-GaN:Be have not been successful. Photoluminescence studies have repeatedly shown Be to have an acceptor level shallower than that of Mg, but deep Be defects and other compensating defects render most GaN:Be materials n-...

Photoluminescence (PL) bands in GaN associated with point defects involving nitrogen or gallium vacancy (VN or VGa) are reviewed. The VN–containing defects, including the isolated VN and its complexes with acceptors, are often observed in PL from semi‐insulating GaN and are responsible for the green (GL2) and red (the RL2 family) bands. The complex...

The CaGa acceptor in GaN was studied using photoluminescence (PL) experiments and first-principles calculations. The experimentally found –/0 transition level of the CaGa at 0.50 ± 0.02 eV above the valence band reasonably agrees with the value calculated using the Heyd-Scuseria-Ernzerhof functional tuned to fulfill the generalized Koopmans conditi...

Photoluminescence studies reveal three CN-related luminescence bands in GaN doped with carbon: the YL1 band at 2.17 eV caused by electron transitions via the −/0 level of the CN, the BLC band at 2.85 eV due to transitions via the 0/+ level of the CN and the BL2 band at 3.0 eV attributed to the CNHi complex. The BLC band studied here has the zero-ph...

GaN samples were implanted with Be and annealed in different conditions in order to activate the shallow Be Ga acceptor. Low-temperature photoluminescence spectra were studied to find Be Ga -related defects in the implanted samples. A yellow band with a maximum at about 2.2 eV (the YL Be band) was observed in nearly all samples protected with an Al...

Ammonothermal GaN samples with the concentration of free electrons of 10¹⁸ and 10¹⁹ cm⁻³ were annealed in a wide range of temperatures (Tann = 300–1400 °C) under atmospheric N2 pressure and under ultra-high N2 pressure conditions to avoid the GaN decomposition. Photoluminescence (PL) studies reveal the YL2 band with a maximum at 2.3 eV before annea...

Detailed analysis of the defect-related photoluminescence (PL) in semi-insulating and n-type Be-doped GaN samples grown by molecular beam epitaxy is presented. The dominant PL bands in these samples are the ultraviolet (UVLBe), yellow (YLBe), red (RLBe), and green (GL2) bands. The UVLBe band is caused by electron transitions via the 0/− level of th...

The dissociation of the CNHi complex in GaN is studied in detail by using photoluminescence measurements and first principles calculations. The BL2 band with a maximum at 3.0 eV is caused by electron transitions from an excited state located at 0.02 eV below the conduction band to the ground state of the CNHi donor with the 0/+ level 0.15 eV above...

Using theory and experiment, we address a possible p-type doping route of GaN using the Be acceptor. Hybrid functional calculations suggest that straightforward incorporation of Be in GaN will likely lead to compensated high resistivity samples. In this case, beryllium interstitials and nitrogen vacancies are the most likely compensating donors. Ho...

Photoluminescence (PL) spectroscopy is a powerful tool in studying semiconductor properties and identifying point defects. Gallium nitride (GaN) is a remarkable semiconductor material for its use in a new generation of bright white LEDs, blue lasers, and high-power electronics. In this Tutorial, we present details of PL experiments and discuss poss...

Photoluminescence (PL) from GaN substrates fabricated by the ammonothermal growth method was studied in a wide range of temperatures and excitation intensities, both with steady-state and time-resolved PL techniques. Three defect-related PL bands were detected: the ultraviolet luminescence band with the zero-phonon line at 3.27 eV, the Zn-related B...

Currently, only one shallow acceptor (Mg) has been discovered in GaN. Here, using photoluminescence (PL) measurements combined with hybrid density functional theory, we demonstrate that a shallow effective-mass state also exists for the Be_Ga acceptor. A PL band with a maximum at 3.38 eV reveals a shallow Be_Ga acceptor level at 113±5 meV above th...

The intensity of defect‐related photoluminescence (PL) in semiconductors changes with temperature, and it usually decreases exponentially above some critical temperature, a process called the PL quenching. Here, main mechanisms of PL quenching are reviewed. Most examples are given for defects in GaN as the most studied modern semiconductor, which h...

Hybrid density functional theory has become a standard method for calculations of defects in semiconductors. The majority of work in this field is done using hybrid functionals tuned to reproduce the experimental bandgap of the host material. This approach usually yields results in reasonable agreement with the experiment. Alternatively, hybrid fun...

Photoluminescence (PL) was used to estimate the concentration of carbon in GaN grown by hydride vapor phase epitaxy (HVPE). The PL data were compared with profiles of the impurities obtained from secondary ion mass spectrometry (SIMS) measurements. Comparison of PL and SIMS data has revealed that apparently high concentrations of C and O at depths...

Unusually large shifts of photoluminescence (PL) bands with excitation intensity or with temperature are sometimes observed in GaN. In particular, the ultraviolet luminescence band with a maximum at 3.28 eV is attributed to transitions from the conduction band (or shallow donors) to the shallow MgGa acceptor. In semi-insulating GaN samples doped wi...

We report on the enhanced incorporation efficiency of magnesium dopants into facets of hexagonal hillock structures in N-polar GaN, studied by comparative analysis of GaN:Mg films grown by MOCVD on high and low hillock density GaN template layers. Total magnesium concentration in planar regions surrounding a hillock structure is comparable to that...

A blue luminescence band, labeled BL3, has been found in undoped GaN samples grown by hydride vapor phase epitaxy. It has a maximum at 2.8 eV and a phonon-related fine structure at its high-energy side. The zero-phonon line of this band consists of a duplet with two sharp lines at 3.0071 and 3.0147 eV. Three phonon modes, including the LO mode with...

The investigation and identification of point defects in GaN is crucial for improving the reliability of light-emitting and high-power electronic devices. The RY3 defect with a characteristic emission band at about 1.8 eV is often observed in photoluminescence (PL) spectra of n-type GaN grown by hydride vapor phase epitaxy, and it exhibits unusual...

We have found a photoluminescence (PL) band with unusual properties in GaN. The blue band, termed as the BLC band, has a maximum at about 2.9 eV and an extremely short lifetime (shorter than 1 ns for a free-electron concentration of about 1018cm−3). The electron- and hole-capture coefficients for this defect-related band are estimated as 10−9 and 1...

Two yellow luminescence bands related to different defects have been revealed in undoped GaN grown by hydride vapor phase epitaxy (HVPE). One of them, labeled YL1, has the zero-phonon line (ZPL) at 2.57 eV and the band maximum at 2.20 eV at low temperature. This luminescence band is the ubiquitous yellow band observed in GaN grown by metalorganic c...

Defect-related photoluminescence (PL) is analyzed in detail for n-type, p-type, and semi-insulating Mg-doped GaN grown by different techniques. The ultraviolet luminescence (UVL) band is the dominant PL band in conductive n-type and p-type GaN:Mg samples grown by hydride vapor phase epitaxy (HVPE) and molecular beam epitaxy. The UVL band in undoped...

The problem of magnesium acceptor in gallium nitride is that experimental photoluminescence measurements clearly reveal a shallow defect state, while most theoretical predictions favor a localized polaronic defect state. To resolve this contradiction, we calculate properties of magnesium acceptor using the Heyd-Scuseria-Ernzerhof (HSE) hybrid funct...

We observed varying thermal quenching behavior of the yellow luminescence band near 2.2 eV in different GaN samples. In spite of the different behavior, the yellow band in all the samples is caused by the same defect—the YL1 center. In conductive n-type GaN, the YL1 band quenches with exponential law, and the Arrhenius plot reveals an ionization en...

Photoluminescence (PL) was used to estimate the concentration of point defects in GaN. The results are compared with data from positron annihilation spectroscopy (PAS), secondary ion mass spectrometry (SIMS), and deep level transient spectroscopy (DLTS). Defect-related PL intensity in undoped GaN grown by hydride vapor phase epitaxy increases linea...

Point defects in high-purity GaN layers grown by hydride vapor phase epitaxy are studied by steady-state and time-resolved photoluminescence (PL). The electron-capture coefficients for defects responsible for the dominant defect-related PL bands in this material are found. The capture coefficients for all the defects, except for the green luminesce...

The surface photovoltage (SPV) technique was used to study the surface and electrical properties of Mg-doped, p-type AlxGa1−xN (0.06 < x < 0.17) layers. SPV measurements reveal significant deviation from previous SPV studies on p-GaN:Mg thin films and from the predictions of a thermionic model for the SPV behavior. In particular, the SPV of the p-A...

The yellow luminescence band was studied in undoped and Si-doped GaN samples by steady-state and time-resolved photoluminescence. At low temperature (18 K), the zero-phonon line (ZPL) for the yellow band is observed at 2.57 eV and attributed to electron transitions from a shallow donor to a deep-level defect. At higher temperatures, the ZPL at 2.59...

The abrupt and tunable quenching of photoluminescence (PL) in bulk ZnO samples is reported, a phenomenon which has been previously observed only in GaN and a few phosphorescent materials. ZnO samples grown by the hydrothermal method contain Li defects responsible for the orange luminescence (OL) band with a maximum at 2.0 eV. The LiZn acceptor is w...

An unusual temperature dependence of the photoluminescence lifetime for the green luminescence (GL) band in GaN is explained. This GL is caused by an internal transition of electrons from an excited state to the ground state of the 0/+ transition level of the isolated C N defect. The excited state appears only after the C N defect captures two phot...

The blue luminescence band with a maximum at 3.0 eV and the zero-phonon line at 3.33 eV (labeled BL2) is observed in high-resistivity GaN. Under prolonged ultraviolet (UV) light exposure, the BL2 band transforms into the yellow luminescence (YL) band with a maximum at 2.2 eV. Our hybrid functional calculations suggest that the BL2 band is related t...

In this chapter, a critical analysis of point defects in GaN and their manifestation in such experiments as photoluminescence (PL), deep-level transient spectroscopy (DLTS), positron annihilation spectroscopy (PAS) is presented. Only a few PL bands are attributed to specific defects. The dominant defect-related PL band in GaN grown by metalorganic...

Gallium nitride, grown by hydride vapor phase epitaxy and capped with a thin AlGaN layer, was studied by photoluminescence (PL) methods. The concentration of free electrons in GaN was found from the time-resolved PL data, and the concentrations of point defects were estimated from the steady-state PL measurements. The intensity of PL from GaN decre...

DOI:https://doi.org/10.1103/PhysRevLett.115.029701

We have investigated point defects in GaN grown by HVPE by using steady-state and time-resolved photoluminescence (PL). Among the most common PL bands in this material are the red luminescence band with a maximum at 1.8 eV and a zero-phonon line (ZPL) at 2.36 eV (attributed to an unknown acceptor having an energy level 1.130 eV above the valence ba...

We have studied photoluminescence (PL) from undoped GaN films grown by HVPE technique on sapphire. Several defect-related PL bands are observed in the low-temperature PL spectrum. The concentrations of the defects responsible for these PL bands are determined from the dependence of PL intensity on excitation intensity. The RL band with a maximum at...

Periodically oriented GaN surfaces have been investigated using scanning probe microscopy techniques to compare Ga- and N-polar regions on a common surface. An epitaxial layer comprising 16 μm-wide stripes of alternating surface polarity was grown by utilizing a patterned AlN layer on an N-polar GaN template. The regions of different polarity are e...

Hydride vapor phase epitaxy (HVPE) is used for the growth of low-defect GaN. We have grown undoped films on sapphire and investigated them using steady-state and time-resolved photoluminescence (PL). One of the dominant PL bands in high-quality GaN grown by HVPE is the green luminescence (GL) band with a maximum at 2.4 eV. This PL band can be easil...

In high-purity GaN grown by hydride vapor phase epitaxy, the commonly observed yellow luminescence (YL) band gives way to a green luminescence (GL) band at high excitation intensity. We propose that the GL band with a maximum at 2.4 eV is caused by transitions of electrons from the conduction band to the 0/+ level of the isolated C N defect. The YL...

A majority of the point defects in GaN that are responsible for broad photoluminescence (PL) bands remain unidentified. One of them is the green luminescence band (GL2) having a maximum at 2.35 eV which was observed previously in undoped GaN grown by molecular-beam epitaxy in Ga-rich conditions. The same PL band was observed in Mg-doped GaN, also g...

In this work, we report the observation of abrupt and tunable thermal quenching of photoluminescence (PL) in high-resistivity, undoped GaN. The ultraviolet luminescence (UVL) band in these samples exhibited abrupt and tunable quenching, which is similar to behavior observed for p -type GaN:Mg samples. Such behavior has never been observed for undop...

Point defects in GaN were studied with time-resolved photoluminescence (PL). The effects of temperature and excitation intensity on defect-related PL have been investigated theoretically and experimentally. A phenomenological model, based on rate equations, explains the dependence of the PL intensity on excitation intensity, as well as the PL lifet...

In agreement with predictions from a model that explained an abrupt thermal quenching of the blue luminescence (BL) band in high-resistivity Zn-doped GaN [Reshchikov et al., Phys. Rev. B 84, 075212 (2011) and Phys. Rev. B 85, 245203 (2012)], we observed the stepwise decrease of photoconductivity in this material with increasing temperature. For th...

In n-type GaN, an upward band bending of about 1 eV is caused by negative charge at the surface. UV light reduces the band bending by creating a surface photovoltage (SPV), which can be measured by a Kelvin probe. Previously, we reported a fast SPV signal of about 0.6 eV in undoped and moderately doped GaN. In this work, we have studied degenerate...

In this work, we report on the effect of temperature on near-surface band bending and surface photovoltage (SPV) in GaN. Band bending and SPV during illumination and after switching the illumination off are calculated for n -type and p -type GaN by using a thermionic model. The temperature and temporal dependencies are compared with experimental da...

From the analysis of photoluminescence (PL), we evaluated the carrier-capture and vibrational characteristics for a number of defects in GaN and ZnO. The methods of determining the carrier-capture coefficients are reviewed. The properties of the ZnGa acceptor in GaN and the LiGa acceptor in ZnO are studied in more detail. The hole-capture coefficie...

Many point defects in GaN responsible for broad photoluminescence (PL) bands remain unidentified. Their presence in thick GaN layers grown by hydride vapor phase epitaxy (HVPE) detrimentally affects the material quality and may hinder the use of GaN in high-power electronic devices. One of the main PL bands in HVPE-grown GaN is the red luminescence...

The surface photovoltage (SPV) behavior for Mg-doped, p-type GaN was studied using a Kelvin probe at temperatures from 80 to 300 K. Under band-to-band UV illumination at room temperature, the measured SPV signal for p-type GaN becomes negative as electrons are swept to the surface. However at low temperatures, the SPV signal becomes positive under...

Mechanisms of thermal quenching of photoluminescence (PL) related to defects in semiconductors are analyzed. We conclude that the Schön-Klasens (multi-center) mechanism of the thermal quenching of PL is much more common for defects in III-V and II-VI semiconductors as compared to the Seitz-Mott (one-center) mechanism. The temperature dependencies o...

In this paper, we present a comparison of exchange-tuned hybrid density functional calculations with experimental data obtained for the Zn acceptor in GaN. Since this acceptor is one of the few reliably identified defects in GaN, we use Zn-doped GaN as a test case for the widely used HSE06 hybrid functional method of calculations of defect properti...

We have observed a superlinear increase of photoluminescence (PL) intensity in a narrow range of excitation intensities for Zn-doped GaN. The characteristic intensity at which the abrupt increase occurs increases with increasing temperature. This is unlike the usual observations for defects in semiconductors in which the PL intensity increases line...

We have studied the thermal quenching of the ultraviolet luminescence band with a maximum at about 3.25 eV in p-type Mg-doped GaN. The characteristic temperature of the thermal quenching of photoluminescence (PL) gradually shifted to higher temperatures with increasing excitation intensity. This effect is explained by a population inversion of char...

We have investigated the N- and Ga-polar faces of bulk GaN substrates with photoluminescence (PL) and the surface photovoltage (SPV) technique using a Kelvin probe attached to an optical cryostat. Experiments were conducted in vacuum. Some of the surfaces were mechanically polished (MP), while others were epi-ready after a chemical-mechanical polis...

Gallium nitride samples codoped with zinc and silicon were fabricated by metal organic chemical vapor deposition. Optical properties of these samples with lowlevel doped acceptor are investigated by temperature dependent photoluminescence (PL) measurements. PL spectrum shows three peaks labeled as near band edge (NBE), blue luminescence (BL), and y...

Using hybrid functional theory compared with experimental measurements, we demonstrate that yellow luminescence often observed in both carbon-doped and pristine GaN is the result of electronic transitions via CN-ON complex. In contrast to the common isolated defects, CN-ON complex is energetically favorable, and its calculated optical properties as...

Photoluminescence (PL) from high-quality GaN codoped with silicon and zinc was investigated in detail. The internal quantum efficiency (IQE) of PL was determined from the analysis of the dependencies of the PL intensity on the excitation intensity and temperature, and the simulation of these dependencies with a phenomenological model based on rate...

We demonstrate that yellow luminescence often observed in both carbon-doped and pristine GaN is the result of electronic transitions via the C_{N}-O_{N} complex. In contrast to common isolated defects, the C_{N}-O_{N} complex is energetically favorable, and its calculated optical properties, such as absorption and emission energies, a zero phonon l...

Porous SiC (PSiC) substrates were used for the growth of GaN by reactive molecular-beam epitaxy with ammonia as the nitrogen source. Improved quality of GaN films has been demonstrated for growth on PSiC substrates, as compared to that on standard 6H-SiC substrates. Cross-sectional transmission electron microscopy and electron diffraction showed a...

The effects of polarity and surface treatment on the morphological, electrical, and optical behaviors in bulk GaN have been investigated. Kelvin probe, atomic force microscopy (AFM), and photoluminescence (PL) techniques were utilized to examine a set of freestanding, bulk GaN samples, which were grown by halide vapor phase epitaxy. The Ga- and N-p...

The band bending in a Mg-doped, p-type GaN film grown by hydride vapor phase epitaxy was studied at various temperatures. At 295 K, the band bending in dark was calculated to be approximately −1.5 eV. However, when the sample was heated to 600 K for 1 h in dark before performing a measurement at 295 K, the calculated value of band bending in dark b...

We report on the dramatic thermal quenching of the blue luminescence (BL) band in high-resistivity Zn-doped GaN. In conductive n-type GaN samples, the thermal quenching of this band begins at T-0 = 200 K, and the activation energy of this thermal quenching is about 0.35 eV, which agrees with the ionization energy of the Zn-Ga acceptor. However, in...

The optical properties of high-quality GaN co-doped with silicon and zinc are investigated by using temperature-dependent continuous-wave and time-resolved photoluminescence measurements. The blue luminescence band is related to the ZnGa acceptor in GaN:Si,Zn, which exhibits an exceptionally high absolute internal quantum efficiency (IQE). An IQE a...

Photoluminescence is one of the most powerful tools used to study optically-active point defects in semiconductors, especially in wide-bandgap materials. Gallium nitride (GaN) and zinc oxide (ZnO) have attracted considerable attention in the last two decades due to their prospects in optoelectronics applications, including blue and ultraviolet ligh...

N-type GaN exhibits upward, near-surface band bending that can be decreased by generating a surface photovoltage (SPV). Fitting SPV measurements with a thermionic model based on the emission of charge carriers over the nearsurface barrier provides information about the band bending in dark. We have studied the temperature dependent SPV behavior fro...

Over the past three decades wide-bandgap semiconductors such as GaN and ZnO have attracted great interest as materials for short-wavelength light-emitting diodes (LEDs), laser diodes, UV detectors, high-power and high-frequency amplifiers, and other optoelectronic applications. Significant efforts are currently applied to increase the internal quan...

This article reviews the advances in the growth of III-nitride quantum dots achieved in the last few years and their unique properties. The growth techniques and the strcutural and optical properties associated with quantum confinement, strain, and polarization in GaN/AlxGa1-xN and InxGa1-xN/GaN quantum dots are discussed in detail.

We report on the fabrication of GaN co-doped with silicon and zinc by metalorganic vapor phase epitaxy and a detailed study of photoluminescence in this material. We observe an exceptionally high absolute internal quantum efficiency of blue photoluminescence in GaN:Zn,Si. The value of 0.93±0.04 has been obtained from several approaches based on rat...

Tunable and abrupt thermal quenching of photoluminescence by increasing temperature has been observed for the blue band in high-resistivity Zn-doped GaN. The photoluminescence intensity dropped by several orders of magnitude within a few Kelvins, and the temperature at which that drop occurred could be tuned by changing the incident light intensity...

Using a Kelvin probe, the authors have studied changes in surface contact potential during illumination, i.e., surface photovoltage (SPV), for n- and p-type GaN films grown by hydride vapor phase epitaxy. Short ultraviolet (UV) exposures (3 s) generate a positive SPV of about 0.5 eV for n-type and a negative SPV of about -0.6 eV for p-type GaN, whi...

We have studied the electrical and optical properties of undoped and Si-doped n -type GaN layers grown on c -plane sapphire by Kelvin probe and photoluminescence (PL) techniques. The maximum SPV value of ∼0.7 eV indicates that the band bending decreased by this amount. The PL intensity depended on ambient and surface treatments such as cleaning, st...

Comparison of the room-temperature photoluminescence (PL) spectra from a high-quality freestanding GaN template and an average-quality GaN layer on sapphire substrate shows that the width of the exciton band in the former may be twice larger than that in the latter. The surprising fact can be explained by a wide depletion region and large diffusion...

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 spec...

We report on the effects of substrate temperature and surface morphology of p-GaN templates on the properties of ZnO:Ga (GZO) layers grown by plasma-assisted molecular beam epitaxy. Substrate temperature varying from 200 °C to 450°C was found to have only a moderate effect on the electrical properties of GZO films but it greatly affects the surface...

We report on a strong effect of p-GaN surface morphology on the growth mode and surface roughness of ZnO:Ga films grown by plasma-assisted molecular-beam epitaxy on p-GaN/c-sapphire templates. A range of ZnO:Ga surface morphologies varying from rough surfaces with well defined three-dimensional islands, capable to enhance light extraction in light-...

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...

We have investigated polarization of photoluminescence from the VGaTeAs complexes in n-GaAs induced through resonant excitation by polarized light. Experimental data in temperature range from 77 to 240 K were described by classic one-dipole approximation within the model of the VGaTeAs complex subjected to the Jahn-Teller distortion in the ground a...

ZnO 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, pr...

Structural, electrical and optical properties of free-standing 200-μm thick GaN films grown by hydride vapor phase epitaxy (HVPE) have been investigated. After laser lift-off, the GaN substrates were mechanically polished on both Ga and N-sides and dry etched only on the Ga-side to obtain a smooth epi-ready surface. Hot H3PO4 chemical etching on bo...

We report a new defect-related photoluminescence (PL) band peaking at 2.56 eV at 15 K in undoped GaN layers grown by molecular beam epitaxy (MBE) on GaN templates. The maximum of the aquamarine luminescence (AL) band shifts to higher energies by about 100 meV with increasing temperature from 15 to 300 K. In spite of the fact that the shape and the...

Sharp intense peaks are sometimes detected in the low-temperature photoluminescence (PL) spectrum of undoped GaN samples in the photon energy range of 3.0 - 3.46 eV. Some of these peaks can be attributed to excitons bound to dislocations and inversion domains, whereas some others originate from the GaN surface because they can be affected essential...

We studied photoluminescence (PL) and PL excitation (PLE) spectra in a large number of undoped GaN layers grown on sapphire by molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE). The HVPE-grown GaN layers with thickness of ∼200 μm were separated from the sapphire substrate by laser l...

Epitaxial ZnO layers heavily doped with Ga (GZO) were grown at 400 °C under metaland oxygen-rich conditions in terms of metal-to-reactive oxygen ratio by plasma-assisted molecular beam epitaxy (MBE). Several atomic force microscopy (AFM) techniques were used to characterize the surface morphology and electrical properties of these GZO films in ambi...

Ga-doped ZnO layers were grown on sapphire substrates by molecular beam epitaxy (MBE). Low-temperature photoluminescence (PL) and room-temperature Raman spectra were investigated. Defect-related modes at 277 and 510 cm−1 appeared in the Raman spectrum for Ga-doped layers. The PL spectrum is dominated by a donor-bound exciton peak at 3.356 eV. A wea...

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 temperat...