Ultraviolet Luminescence Band Research Articles

Bandgap narrowing and hole self-trapping reduction in Ga2O3 by Bi2O3 alloying

Ga2O3 is an emerging material with attractive electrical properties for improving the performance of high-voltage power electronics, and it is widely accepted that engineering its band edge energies will expand its applications, especially for bipolar devices. In this work, monoclinic phase (BixGa1−x)2O3 ternary alloys are fabricated on c-plane sapphire via magnetron co-sputtering of Ga2O3 and Bi2O3. The bandgap of Ga2O3 is found to decrease from 4.97 to 4.78 eV by alloying with a Bi x-fraction of 0.04, consistent with the theoretical prediction that alloying with Bi2O3 causes the up-shift of the valence band. Concurrent temperature-resolved cathodoluminescence (CL) measurements of the Ga2O3 and (Bi0.04Ga0.96)2O3 are employed to investigate their intrinsic ultraviolet (UV) luminescence and defect-related visible bands with increasing temperature from 80 K. The CL results reveal a more rapid thermal quenching of the self-trapped hole (STH) emission with an activation energy of only 32 meV, suggesting that alloying with Bi2O3 lowers the self-trapping energy for holes. Temperature-dependent electrical measurements reveal the conductivity of the (Bi0.04Ga0.96)2O3 film is one order of magnitude lower than that of the undoped Ga2O3 counterpart; however, both possess an electrical activation energy of ∼ 26 meV, likely associated with an impurity-related shallow donor. The low electrical conductivity of (Bi0.04Ga0.96)2O3 can be attributed to the compensation of shallow donors by acceptors activated by the Bi2O3-induced upward shift of the valence band edge. The frequency-dependent conduction and ionic polarization mechanisms up to 107 Hz are found to be identical for Ga2O3 and (Bi0.04Ga0.96)2O3, indicating that the conduction in this ternary alloy does not involve the activation of Bi acceptors.

Photoluminescence from Be‐Doped GaN Grown by Metal‐Organic Chemical Vapor Deposition

A systematic photoluminescence study of Be‐doped GaN grown by metal‐organic chemical vapor deposition is presented. 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 to the shallow state of the BeGa acceptor with a delocalized hole. The YLBe band is caused by a Be‐related defect, possibly the polaronic state of the BeGa acceptor with the charge transition level at 0.3 eV above the valence band. This broad band exhibits unusual properties. In particular, it always shows two steps in its thermal quenching. The second step at T ≈ 200 K is attributed to the emission of holes from the 0.3 eV level to the conduction band. The origin of the first step remains unexplained.

Thermal annealing of GaN implanted with Be

GaN samples were implanted with Be and annealed in different conditions in order to activate the shallow BeGa acceptor. Low-temperature photoluminescence spectra were studied to find BeGa-related defects in the implanted samples. A yellow band with a maximum at about 2.2 eV (the YLBe band) was observed in nearly all samples protected with an AlN cap during the annealing and in samples annealed under ultrahigh N2 pressure. A green band with a maximum at 2.35 eV (the GL2 band), attributed to the nitrogen vacancy, was the dominant defect-related luminescence band in GaN samples annealed without a protective AlN layer. The ultraviolet luminescence (UVLBe) band with a maximum at 3.38 eV attributed to the shallow BeGa acceptor with the ionization energy of 0.113 eV appeared in implanted samples only after annealing at high temperatures and ultrahigh N2 pressure. This is the first observation of the UVLBe band in Be-implanted GaN, indicating successful activation of the BeGa acceptor.

Defect-related photoluminescence from ammono GaN

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 BL1 band with a maximum at 2.9 eV, and the yellow luminescence band (labeled YL2) with a maximum at 2.3 eV. The YL2 band belongs to an unknown defect and is different from the CN-related YL1 band. Its maximum blueshifts by 0.06 eV with increasing excitation intensity and redshifts by more than 0.1 eV with a time delay after a laser pulse. The YL2 band is preliminarily attributed to a defect complex containing the gallium vacancy.

Effect of DNA on the optical properties of ZnO:SiO2:La3+ films

ZnO–SiO2 composite films doped with La3+ ions (0.1%–5.0% mass.) were obtained using the sol–gel method. The absorption and ultraviolet luminescence spectra of the films, as well as the change in spectral characteristics under the influence of DNA, were studied. Doping of ZnO:SiO2 films with La3+ ions leads to an increase in the transmittance in the UV range (λ = 357 nm) from 4.5% (for undoped films) to 32%–34%, and the transmittance is inversely proportional to the amount of dopant. In all doped films, the optical band gap increases in comparison with undoped films (3.35 eV) and amounts to 3.830–3.853 eV. The application of DNA to the surface weakly affects the value of optical band gap. The maximum of the ultraviolet luminescence (UVL) band in ZnO:SiO2:La3+ films is slightly shifted depending on the amount of dopant and is located at λem = 363–367 nm. An increase in UVL intensity was observed upon doping by a factor of 8.4–9.6 compared to ZnO:SiO2 films. A change in the amount of dopant does not lead to sharp changes in the magnitude of the intensity. Thus, the UVL intensity (λex = 260), compared with films with 0.1% La3+ content, varies by 5%–11%. The effect of small amounts of DNA was studied for films doped with La3+. In the case of ZnO:SiO2:La3+, both extinguishing (up to 12%) and ignition (up to 5%) of UVL were observed. Monotone dependencies (which are desirable for the biosensor sensing element) were obtained for samples with La3+ 0.1 and 1.0% content. SEM images show that the images for zinc oxide are white crystallites of different sizes: from 0.5 µm to conglomerates of crystallites −1 µm. In the presence of DNA, the surface structure changes due to the formation of rhomboid structures, whose sizes vary from 3 µm to 35 µm. It is known that lanthanum ions induce the compaction of DNA with the formation of nanoscale structures.

Carbon related hillock formation and its impact on the optoelectronic properties of GaN/AlGaN heterostructures grown on Si(111)

The integration of GaN on Si as large scale substrates still faces many hurdles. Besides the large difference in the lattice constant and the high thermal mismatch existing between GaN and Si, spiral hillock growth phenomena are common problems in the development of thick GaN layers. In this work, hexagonal hillocks were observed on GaN/AlGaN high-electron-mobility transistor heterostructures grown on Si(111) by metal-organic chemical vapor deposition. The presence of these morphological and structural defects is attributed to the presence of localized contamination at the AlN/Si interface. These carbon-based defects cause highly defective regions in the AlN seed layer, which propagate through all the AlGaN buffer layers inducing the formation of V-shaped pits at the AlGaN interfaces. In hillock regions of the wafers, Raman spectroscopy indicates disturbed two-dimensional electron gas characteristics resulting from GaN/AlGaN interface roughness and a decreased amount of free carriers in the potential well. Energy-dispersive x-ray spectroscopy reveals Ga accumulation inside the V-pits and in nanopipes below, which is responsible for defective areas and the increased GaN growth rate resulting in hillock formation. Photoluminescence measurements confirm the presence of Ga-rich material reducing the inherent gallium vacancy concentration. Here, the reduced amount of Ga-vacancies acting as a shallow acceptor suppresses the ultraviolet luminescence band from donor–acceptor pair transition.

Giant shifts of photoluminescence bands in GaN

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 with Mg, enormous redshift (up to 0.6 eV) and broadening of this PL band can be observed with variation of temperature or excitation intensity. A similar behavior of PL was observed in semi-insulating GaN doped with Zn. The blue band with a maximum at 2.9 eV in GaN:Zn is attributed to electron transitions from the conduction band or from shallow donors to the ZnGa acceptor level located at 0.4 eV above the valence band. In conductive n-type GaN, this PL band redshifts by only 7 meV with decreasing excitation intensity by five orders of magnitude, and the shift is explained with the donor–acceptor pair model. However, in semi-insulating GaN:Zn samples, a gradual shift by up to 0.8 eV with decreasing excitation intensity has been observed. Finally, the yellow luminescence band shifts by not more than 7 meV in high-quality, undoped GaN samples, while a redshift by more than 0.1 eV can be observed in GaN heavily doped with Si. The giant shifts are attributed to the presence of electric fields in high-resistivity or heavily doped n-type GaN.

Boosting the doping efficiency of Mg in p-GaN grown on the free-standing GaN substrates

The Mg doping efficiency is found to be drastically enhanced in the p-GaN films grown on the free-standing GaN substrates by metal organic chemical vapor deposition. The free hole concentrations are five and ten times higher in the lightly and heavily Mg-doped homoepitaxial p-GaN, respectively, than those in the p-GaN-on-GaN/sapphires grown and activated at the same conditions although the Mg incorporation concentrations {[Mg]} are much lower. The indication of the p-type conductivity in the photoluminescence spectra at room temperature in p-GaN-on-GaN substrates is found to be the dominant ultraviolet luminescence band located at around 3.26 eV. This behavior is different in the heteroepitaxial p-GaN, for which the fingerprint of the p-type conductivity is the emergence of blue luminescence bands at around 2.9 eV. The markedly enhanced activation efficiency is attributed to the suppression of self-compensation centers in the high-quality homoepitaxial films. The Mg-Ga-O disordered layer, which is typically observed on the surface of p-GaN-on-GaN/sapphires due to the Mg diffusion along edge-type dislocations, is also inhibited on the homoepitaxial p-GaN film, which is beneficial for the stable operation of the vertical-type metal-oxide-semiconductor field effect transistors.

Temperature-dependence of X-ray excited luminescence of β -Ga2O3 single crystals

We investigated the temperature-dependence of X-ray excited luminescence of β-Ga2O3 single crystals. The recombination of free-electrons with self-trapped holes (STHs) responsible for the ultraviolet luminescence (UVL) band was observed. For the as-grown sample with a high electron concentration, the Auger recombination becomes important, which accelerates the decay dynamics and reduces the ultraviolet luminescence band. The ultraviolet luminescence band is controlled by temperature quenching due to the migration of self-trapped holes to VGa3− centers. Annealing in an oxygen environment can increase the VGa3− concentration, which contributes to the blue-green luminescence band when VGa3− captures a self-trapped hole forming a VGa2− center at rising temperature.

Magnesium acceptor in gallium nitride. I. Photoluminescence from Mg-doped GaN

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 and Mg-doped GaN samples is attributed to the shallow $\mathrm{M}{\mathrm{g}}_{\mathrm{Ga}}$ acceptor with the ionization energy of 223 meV. In semi-insulating GaN:Mg samples, very large shifts of the UVL band (up to 0.6 eV) are observed with variation of temperature or excitation intensity. The shifts are attributed to diagonal transitions, likely due to potential fluctuations or near-surface band bending. The blue luminescence $(\mathrm{B}{\mathrm{L}}_{\mathrm{Mg}})$ band is observed only in GaN:Mg samples grown by HVPE or metalorganic chemical vapor deposition when the concentration of Mg exceeds ${10}^{19}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. The $\mathrm{B}{\mathrm{L}}_{\mathrm{Mg}}$ band is attributed to electron transitions from an unknown deep donor to the shallow $\mathrm{M}{\mathrm{g}}_{\mathrm{Ga}}$ acceptor. Basic properties of the observed PL are explained with a phenomenological model.

Defect-Controlled Scintillation Process in Undoped Y3Al5O12Crystals

Optical absorption, luminescence spectra, scintillation decay curves, and scintillation light yield were measured in a series of undoped yttrium–aluminum garnet (YAG) crystals of different origins. This paper reveals a correlation between luminescent properties and scintillation efficiency of undoped YAG crystals. All the samples were separated into three groups, Type I, Type II, and Type III, based on their optical properties and scintillation efficiency. The intrinsic ultraviolet luminescence band peaking around 300 nm is found in the samples with high scintillation yield (Type I). Luminescence intensity of this band depends on the crystal purity and on the stoichiometry of the garnets. The UV emission may be related to e–h recombination in vicinity of vacancies. The garnets of Type II contain more impurities which quench the 300-nm emission. The samples of Type III do not exhibit any significant radioluminescence. Light yield of the finest undoped YAG samples in the series, Type I, is similar to the yield of Ce-doped YAG. This Type of crystals can potentially be used in scintillation detectors.

Structural and optical properties of highly (110)-oriented non-polar ZnO evaporated films on Si substrates

Zinc oxide (ZnO) films with thicknesses from (9.1±0.7) to (145±7)nm were deposited on silicon substrates by ion-assisted electron beam evaporation (EBE), and transformed to a highly (110)-oriented non-polar phase through thermal treatment at temperatures of 300, 500, and 700°C. The structural, morphological and optical properties of the ZnO thin films were systematically characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), spectroscopic ellipsometry (SE), and photoluminescence (PL). The XRD and AFM results illustrate that the deposited ZnO films evolved from an X-ray amorphous structure to the polycrystalline hexagonal wurtzite structure with increasing thickness and experienced a crystallographic plane improvement from (102) to the (110) non-polar phase during annealing. The dielectric functions of the whole layer are mainly affected by the dielectric responses near the surface of ZnO nanocrystals and show an upward trend with nanocrystal growth, induced by thickness and annealing temperature. The optical band gaps were also investigated and correlated with the structural properties. In the PL spectra, a strong ultraviolet luminescence (UVL) band and a weak visible green luminescence (GL) band were observed. The intensity ratio of UVL and GL—associated with crystal quality—agrees with the results acquired from other measurements. The two determinants for highly (110)-oriented non-polar ZnO films are the suppression of growth in the (002) direction by ion bombardment and the promotion of the (110) plane induced by diffusion energy during annealing.

Investigation of Isoelectronic Doping in p-GaN Based on the Thermal Quenching of UVL Band

In this paper, isoelectronic doping with indium (In) in p-GaN is investigated on the basis of the thermal quenching of ultraviolet luminescence (UVL) band. A phenomenological rate-equation model is proposed as a nondestructive diagnostic technique to acquire the dopant ionization energy of p-GaN. In terms of this model, we analyze the evolution of the ionization energy of p-GaN samples grown with different trimethyindium flow rate and find that isoelectronic doping with appropriate In incorporation is able to significantly reduce the ionization energy of the acceptor from 245 to 112 meV, which are consistent with the results of the variable temperature Hall-effect measurements. The comparison between the samples’ full width at half maximum of the UVL band indicates that excessive In incorporation will degenerate the conductivity of p-GaN.

Electrical properties and emission mechanisms of Zn-doped β-Ga2O3 films

We study the electrical properties and emission mechanisms of Zn-doped β-Ga2O3 film grown by pulsed laser deposition through Hall effect and cathodoluminescence which consist of ultraviolet luminescence (UV), blue luminescence (BL) and green luminescence (GL) bands. The Hall effect measurements indicate that the carrier concentration increases from 7.16×1011 to 6.35×1012cm−3 with increasing a nominal Zn content from 3 to 7at%. The UV band at 272nm is not attributed to Zn dopants and ascribed as radiative electron transition from conduction band to a self-trapped hole while the BL band is attributable to defect level related to Zn dopant. The BL band has two emission peaks at 415 and 455nm, which are ascribed to the radiative electron transition from oxygen vacancy (VO) to valence band and recombination of a donor–acceptor pair (DAP) between VO donor and Zn on Ga site (ZnGa) acceptor, respectively. The GL band is attributed to the phonon replicas’ emission of the DAP. The acceptor level of ZnGa is estimated to be 0.26eV above the valence band maximum. The transmittance and absorption spectra prove that the Zn-doped β-Ga2O3 film is a dominantly direct bandgap material. The results of Hall and cathodoluminescence measurements imply that the Zn dopant in β-Ga2O3 film will form an acceptor ZnGa to produce p-type conductivity.

Tunable thermal quenching of photoluminescence in GaN

AbstractIn 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 undoped GaN and was very rarely reported for other semiconductors. We describe the effect of temperature and excitation intensity on PL in undoped GaN with a phenomenological model involving three defect species. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Correlation between blue luminescence intensity and resistivity in β-Ga2O3 single crystals

Temperature-dependent cathodoluminescence spectra were measured from (001) unintentionally doped, (100) Si-doped, and (010) Mg-doped β-Ga2O3 substrates prepared by either the floating zone growth or edge-defined film-fed growth methods. Although β-Ga2O3 is expected to be an indirect bandgap material, direct Γ-Γ transitions were found to be dominant in the optical transmittance spectra. The substrates exhibited no near-band-edge emission and instead exhibited ultraviolet luminescence, blue luminescence (BL), and green luminescence bands. Since the BL intensity strongly depended on the resistivity in the crystals, there was evidence of a correlation between the BL intensity and formation energy of oxygen vacancies.

Tunable thermal quenching of photoluminescence in Mg-dopedp-type GaN

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 charge carriers at low temperatures, which suddenly converts into a quasiequilibrium population as the temperature increases above the characteristic value. Tunable quenching of PL has been observed only in some of the GaN:Mg samples. The absence of the tunable quenching of PL in another group of GaN:Mg samples is preliminarily attributed to different types of dominant nonradiative defects in the two groups of samples.

Hole traps and thermoluminescence in Li2B4O7:Be

A trapped-hole centre with a high thermal stability is studied in Li2B4O7:Be. EPR spectrum demonstrates a hyperfine splitting due to interaction of the trapped hole with a 9Be nucleus. The hole is trapped at the common vertex (bridging oxygen) of two adjacent BO4 tetrahedra when Be2+ substitutes for B3+ in one of them. The centre is stable up to ≈500 K. The high-temperature thermoluminescence peak at 510–575 K occurs when the released holes recombine with trapped electrons. This peak shows an ultraviolet exciton-like luminescence band and also luminescence bands of casual impurities.

Mg Doping Effect in Nonpolara-Plane GaN

The properties of nonpolar a-plane GaN layers grown with different Mg doping levels were investigated. With increasing the Mg flow rate, the hole concentration initially increased and then decreased, indicating the formation of compensation centers. The dominant photoluminescence (PL) emission at relatively low Mg doping levels is the blue luminescence (BL) band due to the donor-acceptor pair (DAP) transition with MgGaVN (deep donor) and MgGa (acceptor). In addition to the weak BL band, both the ultraviolet luminescence (UVL) and yellow luminescence (YL) bands are observed at higher Mg doping level. The UVL band, especially dominant at 10 K, can be related to bound excitonic emissions involving Mg-induced extended defects, whereas one or more mechanisms may contribute to the YL band.

Mg Doping Effect in Nonpolar a-Plane GaN

The properties of nonpolar a-plane GaN layers grown with different Mg doping levels were investigated. With increasing the Mg flow rate, the hole concentration initially increased and then decreased, indicating the formation of compensation centers. The dominant photoluminescence (PL) emission at relatively low Mg doping levels is the blue luminescence (BL) band due to the donor-acceptor pair (DAP) transition with MgGaVN (deep donor) and MgGa (acceptor). In addition to the weak BL band, both the ultraviolet luminescence (UVL) and yellow luminescence (YL) bands are observed at higher Mg doping level. The UVL band, especially dominant at 10 K, can be related to bound excitonic emissions involving Mg-induced extended defects, whereas one or more mechanisms may contribute to the YL band.