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Showing papers on "Cathodoluminescence published in 1997"


Journal ArticleDOI
TL;DR: In this article, the relationship between microstructure and luminescence efficiency for heteroepitaxial films of GaN grown on c-axis sapphire substrates by metalorganic chemical-vapor deposition was discussed.
Abstract: We discuss the relationship between microstructure and luminescence efficiency for heteroepitaxial films of GaN grown on c-axis sapphire substrates by metalorganic chemical-vapor deposition. We directly characterize the correlation between threading dislocations as observed by transmission electron microscopy, surface morphology as observed by atomic force microscopy, and wavelength-resolved cathodoluminescence imaging. We show that the inhomogeneity in the luminescence intensity of these films near band edge can be accounted for by a simple model where nonradiative recombination at threading dislocations causes a deficiency of minority carriers and results in dark regions of the epilayer. An upper bound for average diffusion length is estimated to be 250 nm.

512 citations


Journal ArticleDOI
TL;DR: In this article, a spatially resolved cathodoluminescence (CL) spectrum mapping revealed a strong exciton localization in InGaN single-quantum-wells (SQWs).
Abstract: Spatially resolved cathodoluminescence (CL) spectrum mapping revealed a strong exciton localization in InGaN single-quantum-wells (SQWs). Transmission electron micrographs exhibited a well-organized SQW structure having abrupt InGaN/GaN heterointerfaces. However, comparison between atomic force microscopy images for GaN-capped and uncapped SQWs indicated areas of InN-rich material, which are about 20 nm in lateral size. The CL images taken at the higher and lower energy side of the spatially integrated CL peak consisted of emissions from complementary real spaces, and the area was smaller than 60 nm in lateral size.

360 citations


Journal ArticleDOI
TL;DR: In this article, a series of Europium doped yttrium oxide (Eu:Y2O3) phosphor thin films were grown using a pulsed laser deposition (PLD) technique at varying growth conditions.
Abstract: Europium doped yttrium oxide (Eu:Y2O3) phosphor thin films were grown using a pulsed laser deposition (PLD) technique at varying growth conditions. The structural characterization carried out on a series of Eu:Y2O3 films grown on (100) silicon at substrate temperatures in the range of 250–600 °C and oxygen pressure in the range of 10−5 Torr to 200 mTorr indicated that films were preferentially (111) oriented. Measurements of photoluminescence and cathodoluminescence properties of laser deposited Eu:Y2O3 thin films and powder used for laser target showed that the best in situ grown films were ∼10%–22% as bright as Eu:Y2O3 powder. A postdeposition annealing treatment of Eu:Y2O3 films led to further improvements in their brightness (up to ∼70% with respect to Eu:Y2O3 powder), with cluster sizes of <400 nm.

263 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated electrical and optical properties of the highconductivity layers formed in both undoped and B-doped diamond films prepared by chemical vapor deposition, and they found that both hydrogenated undoped (HDE) and Bdoped (B-DDE) diamond films have high-concentration holes of ∼1018 cm−3 at 297 K, while that of the oxidized HDE has a strong temperature dependence with activation energy 0.38 eV.
Abstract: We have investigated electrical and optical properties of the high-conductivity layers formed in both undoped and B-doped diamond films prepared by chemical vapor deposition. It is found that both hydrogenated undoped and B-doped diamond films have high-concentration holes of ∼1018 cm−3 at 297 K. These films exhibit little temperature dependence of the hole concentration between 120 and 400 K, while that of the oxidized B-doped film has a strong temperature dependence with an activation energy 0.38 eV. The Hall mobility of all the hydrogenated films of ∼30 cm2/Vs at 297 K is one to two orders of magnitude smaller than that of the oxidized B-doped film and increases with increasing temperature. The I−V characteristics of Al–Schottky contacts to the hydrogenated undoped film show excellent rectification properties and the temperature dependence of their forward characteristics is well explained by a junction theory inclusive of the tunneling process, i.e., thermionic-field emission theory, indicating that the depletion layer becomes thin due to high-density space charge in the depletion layer. We have also found a broad cathodoluminescence peak at around 540 nm in the hydrogenated films which disappears with subsequent oxidation treatment, indicating the existence of hydrogen-related gap states in the subsurface region of as-deposited homoepitaxial diamond films. High density hydrogen is detected in the subsurface region of the hydrogenated films by secondary ion mass spectroscopy. These experimental results suggest the existence of hydrogen-induced shallow acceptors in the surface region of as-deposited (hydrogenated) diamond films and that the difference between the hydrogenated and the oxidized films observed in both electrical and optical properties originates from hydrogen incorporated in the subsurface region.

240 citations


Journal ArticleDOI
TL;DR: In this article, the authors report room-temperature cathodoluminescence and photoluminecence spectra originating from ZnS overcoated CdSe nanocrystals, 33 and 42 A in diameter, embedded in a znS matrix.
Abstract: We report room-temperature cathodoluminescence and photoluminescence spectra originating from ZnS overcoated CdSe nanocrystals, 33 and 42 A in diameter, embedded in a ZnS matrix. The thin-film quantum dot composites were synthesized by electrospray organometallic chemical vapor deposition. Cathodoluminescence and photoluminescence are dominated by the sharp band-edge emission characteristic of the initial nanocrystals. The emission wavelength can be tuned in a broad window (470–650 nm) by varying the size of the dots. The cathodoluminescence intensity depends on the crystallinity of the ZnS matrix and the voltage and current density applied.

138 citations


Journal ArticleDOI
TL;DR: In this article, the effect of hydrogenation on the luminescence property of ZnO crystal was investigated by means of cathodoluminescence (CL) and it was found that hydrogen plasma treatment strongly passivates the green emission and enhances the band edge luminance.
Abstract: The effect of hydrogenation on the luminescence property of ZnO crystal was investigated by means of cathodoluminescence (CL). It was found that hydrogen plasma treatment strongly passivates the green emission and enhances the band edge luminescence. The band edge luminescence of ZnO crystals in the as-grown state does not show any significant temperature dependence. However, that of hydrogenated ZnO crystals shows strong temperature dependence and the intensity increases significantly at low temperatures. At 30 K, the number of luminescent photons exceeds several tenths of generated electron-hole pairs.

134 citations


Journal ArticleDOI
TL;DR: In this article, the synthesis of Eu(III)-and Tb(III)doped yttrium oxide phosphor powders by supercritical CO2-assisted aerosolization and pyrolysis (at 1010°C) from aqueous solutions of Y(NO3)3, Eu (NO3), and Tb (NO 3)3 was presented, and X-ray diffraction data indicate that the powders are crystalline and consist of a single phase.
Abstract: The synthesis of Eu(III)- and Tb(III)-doped yttrium oxide phosphor powders by supercritical CO2-assisted aerosolization and pyrolysis (at 1010 °C) from aqueous solutions of Y(NO3)3, Eu(NO3)3, and Tb(NO3)3 is presented. Dense, submicron aerosols are formed by contacting aqueous solutions of the precursor metal nitrates with supercritical CO2 at 1500 psi, and decompressing the mixture through a restrictor nozzle into a heated tube. X-ray diffraction data indicate that the powders are crystalline, and consist of a single phase. Scanning electron microscope shows that they are spherical in morphology and less than 1 μm in size with average diameters of ∼0.2 μm. The cathodoluminescence intensity of the Y1.91Eu0.09O3 powders, annealed at 1200 °C for 1 h, is more than twice greater than that of the industrial standard (P56, 3–10 μm in diameter) phosphor. The phosphors synthesized by this method are promising candidates for use in high-resolution low-voltage flat panel displays.

93 citations


Journal ArticleDOI
TL;DR: In this paper, a single-crystal hexagonal GaN with its basal plane parallel to the Si(111) plane is shown to be grown epitaxially by atmospheric pressure metalorganic chemical vapor deposition on an aluminum oxide compound layer utilized as an intermediate layer between GaN and a Si( 111).
Abstract: We have demonstrated that GaN can be grown epitaxially by atmospheric pressure metalorganic chemical vapor deposition on an aluminum oxide compound layer utilized as an intermediate layer between GaN and a Si(111). X-ray diffraction measurement indicates that single-crystal hexagonal GaN with its basal plane parallel to the Si(111) plane is grown. Using a scanning electron microscope, the macroscopic evolution of GaN grown on the AlOx/Si(111) substrate is found to be similar to that of GaN grown on a sapphire(0001) substrate. Cathodoluminescence (CL) spectrum shows a unique emission that consists of several peaks with the intensity comparable to that of the near-band-edge emission. Unique characteristics in CL spectrum are discussed in terms of a possible oxygen contamination of GaN grown on the AlOx/Si(111) substrate.

86 citations


Journal ArticleDOI
TL;DR: In this article, a very thin In2O3 conductive layer was applied to the phosphor surface to improve the luminance of low voltage cathodoluminescence.

70 citations


Journal ArticleDOI
TL;DR: In this paper, the surface degrades during prolonged electron bombardment and the growth of a non-luminescent ZnO surface layer is one reason leading to the degradation in cathodoluminescence intensity.

68 citations


Journal ArticleDOI
TL;DR: In this paper, single crystalline aluminum nitride films were grown on 4H and 6H SiC substrates using metal-organic chemical-vapor deposition at 1200 °C.
Abstract: Undoped single crystalline aluminum nitride films were grown on 4H and 6H SiC substrates using metal–organic chemical-vapor deposition at 1200 °C. From in situ reflection high-energy electron diffraction, x-ray diffraction rocking curves, and cathodoluminescence spectra, the crystallinity of the films was confirmed. Atomic force microscopy showed that some films were substantially dominated by island growth, rather than step flow growth. Aluminum was evaporated to form metal–insulator–semiconductor (MIS) capacitors for high-frequency capacitance voltage measurements carried out at room temperature. Low leakage made it possible to measure the structures and characterize accumulation, depletion, deep depletion, and, in some cases, inversion. From independent optical thickness measurements, the relative dielectric constant of aluminum nitride was confirmed at 8.4. The flatband voltage of the AlN MIS capacitors on p-type SiC was close to the theoretical value expected. The films were stressed up to 60 V (3 MV...

Journal ArticleDOI
TL;DR: In this article, a theoretical expression for the brightness of powder phosphor is derived in order to investigate the effects of phosphor's properties on low-energy cathodoluminescence.
Abstract: A theoretical expression for the brightness of powder phosphor is derived in order to investigate the effects of phosphor’s properties on low-energy cathodoluminescence. It contains the particle size, surface recombination rate, lifetime of holes, carrier diffusion coefficients, and anode voltage. As such, it can be applied to systematically optimize the design parameters of phosphors for maximum efficiency at low-voltage excitation. Use of the given expression is illustrated and a typical case is analyzed in this study.

Journal ArticleDOI
TL;DR: In this article, a Ga1-xBxN with compositions from 0 to 0.07 were grown by organometallic vapor phase epitaxy on sapphire substrates using trimethylgallium, triethylboron (TEB) and NH3 as precursors.
Abstract: Layers of Ga1-xBxN with compositions from x = 0 to x = 0.07 were grown by organometallic vapor phase epitaxy on sapphire substrates using trimethylgallium, triethylboron (TEB) and NH3 as precursors. Growth was done in the temperature range from 450 to 1000°C. The presence of boron was detected by the shift in the (0002) peak position in x-ray diffraction, by x-ray photoelectron spectroscopy, secondary ion mass spectrometry measurements, and by the changes in the band gap as measured by optical transmission. It was found that for the studied range of compositions the band gap varied from 3.4 eV for x = 0 to 3.63 eVfor x = 0.05. At certain TEB concentrations in the gas phase, the growth rate decreased abruptly, most likely because of a growth poisoning by the onset of growth of a very slow growing B-rich phase. The threshold TEB concentration for this growth poisoning decreased with increasing growth temperature; and at 1000°C, less than 1% of B could be incorporated as a result. GaBN alloys with about 7% substitutional boron were also produced by implantation of 5 × 1016 cm−2 B ions at 60 keV into GaN, as evidenced by the shift of the band edge emission in cathodoluminescence spectra from 3.4 eV for GaN to 3.85 eV for GaBN.

Journal ArticleDOI
TL;DR: In this paper, the authors show that electron bombardment in the presence of O2 and H2O caused depletion of sulfur and accumulation of oxygen on the surface of ZnS-based phosphors.
Abstract: Phosphor thin films of ZnS:Mn on Si(100) have been subject to electron bombardment (0.6–4 keV) over a range of pressures from 1×10−6 to 5×10−8 Torr. Various gases including hydrogen, oxygen, and water vapor were introduced into the ambient during bombardment, to assess their effects on the phosphor surface. Auger electron spectroscopy data indicate that electron bombardment in the presence of O2 and H2O caused depletion of sulfur and accumulation of oxygen on the surface. Hydrogen also caused depletion of sulfur. Removal of sulfur was shown to be consistent with electron-beam dissociation of molecular species to atomic hydrogen and/or oxygen, followed by a surface reaction to form high vapor pressure sulfur compounds (e.g., SOx and H2S). In the case of oxygen and water vapor, ZnS was converted to ZnO or ZnSO4. These changes in surface chemistry reduced the intensity of cathodoluminescence from ZnS-based phosphors. The mechanisms leading to loss of CL intensity are discussed.


Journal ArticleDOI
TL;DR: In this paper, the NiO emission bands in the visible and near infrared spectral ranges are attributed to Ni(2+) intraionic transitions and some of the features observed in the CL spectra recorded from vacuum annealed samples are tentatively attributed to electronic transitions involving defect-induced states located inside the charge-transfer gap of this transition metal oxide.
Abstract: Cathodoluminescence (CL) and photoluminescence (PL) of nickel oxide (NiO) have been investigated. The observed emission bands in the visible and near infrared spectral ranges are attributed to Ni^(2+) intraionic transitions. Some of the features observed in the CL spectra recorded from vacuum annealed samples are tentatively attributed to electronic transitions involving defect-induced states located inside the charge-transfer gap of this transition metal oxide.

Journal ArticleDOI
TL;DR: In this paper, a ZnCdSe quantum dot was created on a cleavage-induced ZnSe (110) surface by depositing a ZSe/ZncdSe /ZnSe heterostructure under growth conditions that cannot lead to layer-by-layer growth.
Abstract: We successfully realized ZnCdSe quantum dots on a cleavage-induced ZnSe (110) surface by depositing a ZnSe/ZnCdSe/ZnSe heterostructure under growth conditions that cannot lead to layer-by-layer growth of ZnSe. This growth mode introduces surface roughness to the newly deposited ZnSe layer, and ZnCdSe quantum dots are then formed. Cathodoluminescence and microphotoluminescence measurements demonstrate the formation of quantum dots.

Journal ArticleDOI
TL;DR: In this paper, a model of precursor transformation via a metastable interlevel was proposed for the peak dose dependence of cathodoelectroluminescence peak dose in a wide range of current density (10 −5 to 10 −3 A cm −2 ) and temperature.
Abstract: Cathodoluminescence of amorphous SiO 2 films thermally grown on a Silicon substrate has been observed in a scanning electron microscope using wavelength dispersed registration by a charge coupled device (CCD) camera. Spectra have three bands: at 650 nm (red), 460 nm (blue), and 285 nm (UV) whose intensities change during the initial period of electron beam excitation. Luminescence peak dose dependence has been investigated in a wide range of current density (10 −5 to 10 −3 A cm −2 ) and temperature (90 to 500 K). An interpretation of the dose-temperature dependence is made by a model of precursor transformation via a metastable interlevel. Application of an electric field during continuous electron excitation (cathodoelectroluminescence) causes an enhancement up to five times of the blue band intensity. On the other hand, the red band decreases in the electric field. Based on these phenomena, the UV and the blue luminescence band are attributed to an internal electron impact excitation within a localized center, probably twofold-coordinated silicon in the SiO 2 network, whereas the red band is ascribed to band-to-band-recombination via localized levels attributed to non-bridging oxygen.

Journal ArticleDOI
TL;DR: In this article, a red form of band-A luminescence, related to electron-hole recombination at substitutional phosphorous and a phosphorus-vacancy complex, is revealed, in the cathodo luminosity spectra of homoepitaxial and polycrystalline films.
Abstract: Luminescence studies have been performed on phosphorous-doped diamond films deposited by hot-filament chemical vapor deposition. A broad luminescence band, centered around 1.9 eV is revealed, in the cathodo luminescence spectra of homoepitaxial and polycrystalline films, whereas the blue band-A luminescence, which is characteristic for undoped diamond, is quenched in the presence of phosphorus. The 1.9 eV luminescence band could not be excited by Ar+ laser light (2.54 eV) and did not show a zero-phonon line in the spectrum. Therefore, we suggest that this band is a red form of band-A luminescence, related to electron-hole recombination at substitutional phosphorous and a phosphorus-vacancy complex.

Journal ArticleDOI
TL;DR: In this paper, the luminescence properties of GaN films grown by metalorganic chemical vapor deposition using depth-resolved and excitation power dependent cathodoluminescence spectroscopy were studied.
Abstract: We have studied the luminescence properties of GaN films grown by metalorganic chemical vapor deposition using depth-resolved and excitation power dependent cathodoluminescence spectroscopy. In the thickness range we studied (∼3 μm), the luminescence properties, measured by the ratio of band-edge emission to the yellow band intensity, improve as the layer thickness increases.

Journal ArticleDOI
TL;DR: In this paper, the authors studied time-resolved cathodoluminescence (CL) and electron beam induced current (EBIC) on AlGaAs, GaAs/GaAs/InGaAs heterojunction phototransistors under operating conditions, i.e., at room temperature and under bias.
Abstract: We have studied time-resolved cathodoluminescence (CL) and electron beam induced current (EBIC) on AlGaAs/GaAs/InGaAs heterojunction phototransistors under operating conditions, i.e., at room temperature and under bias. Devices from four wafers, with a different amount of lattice relaxation, were tested. It is shown that the CL intensity increases more than one order of magnitude as the voltage is increased and the current gain of the device turns on. The voltage dependence of the CL signal is analogous to the current–voltage curve of the transistor. The buildup in CL intensity was found to be much less in devices with low current gain showing that the CL intensity correlates to the electrical gain of the device. Time resolved CL showed two distinct CL decay times, one very short, a few nanoseconds, and one long, of the order of microseconds. This indicates that two fundamental recombination processes are present, which we attribute to a spatially direct recombination between carriers in the base and a sp...

Journal ArticleDOI
TL;DR: In this article, an additional UV line at 364nm/3.4eV can be connected to the occurrence of stacking faults by a model based on the concept of excitons bound to stacking faults that form a quantum well of cubic material in the wurtzite lattice of the layer material.
Abstract: We correlate structure analyzed by transmission electron microscopy with photo- and cathodoluminescence studies of GaN/Al2O3(0001) and GaN/SiC(0001) and show that an additional UV line at 364nm/3.4eV can be connected to the occurrence of stacking faults. We explain the occurrence of this line by a model that is based on the concept of excitons bound to stacking faults that form a quantum well of cubic material in the wurtzite lattice of the layer material. The model is in reasonable agreement with the experimental observations.

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate unambiguously that the defect-related Y luminescence band at 1.476 eV in CdTe originates from the polar Te(g) glide dislocation segments.
Abstract: We demonstrate unambiguously that the well-known defect-related Y luminescence band at 1.476 eV in CdTe originates from the polar Te(g) glide dislocation segments. Crystallographically defined glide dislocation arrangements produced by local plastic deformation on (111)Te surfaces using Vickers microindentation were characterized by temperature-dependent cathodoluminescence (CL) microscopy as well as CL and PL spectroscopy. The identification of the Te(g) dislocation was obtained by determining the surface polarity applying X-ray diffraction and subsequently revealing the volume glide geometry observed by CL imaging after stepwise depth-etching of the (111) sample surface. From the spectral and recombination-kinetic properties of the dislocation-bound Y luminescence the model of radiative decay of dislocation-related excitons is supported. Particularly, we may conclude that they are bound to energy levels in the fundamental gap localized at the 90° Te(g) partial dislocations.

Journal ArticleDOI
TL;DR: In this article, a selective-area metalorganic chemical vapor deposition of GaN stripes in the size range of 50 to 125 urns was performed and the characterization of the morphology, topography, and optical properties of these stripes were presented.
Abstract: We report on the selective-area metalorganic chemical vapor deposition of GaN stripes in the size range of 50 to 125 urn and the characterization of the morphology, topography, and optical properties of these stripes. GaN films (∼1–3 μm) grown on (0001) sapphire are used as the substrates. Excellent surface morphology is achieved under optimized growth conditions which include a higher V/III ratio than broad area growth. It is found that, under certain growth conditions, (0001) terraces of ~5 µm in width develop at the edges of all stripes, independent of stripe size and orientation. The selectively grown GaN yields stronger band-edge emission than the “substrate” GaN which indicates an improvement in optical quality. However, the donor-acceptor pair recombination (or conduction band to acceptor transition) and yellow emission are also enhanced in certain areas of the stripes. The spatial correlation of these emission bands is established by cathodoluminescence wavelength imaging, and the origin of these emissions is speculated.

Patent
05 Sep 1997
TL;DR: In this paper, the integrated intensity ratio of the diamond films, CL1 /CL2, is equal or greater than 1/20, where CL1 is defined as the intensity of the emission band in the wavelength region shorter than 300 nm while CL2 is the intensity from 300 nm to 800 nm.
Abstract: Diamond films and novel method to grow the diamond films can improve the performance of products utilizing diamond films. In the cathodoluminescence taken at room temperature, the integrated intensity ratio of the diamond films, CL1 /CL2, is equal or greater than 1/20, where CL1 is the integrated intensity of the emission band in the wavelength region shorter than 300 nm while CL2 is the integrated intensity of the emission band in the wavelength region from 300 nm to 800 nm. Such high quality diamond films with intensive coalescence on the surface can be obtained by deposition on the substrates or films, made of at least one member selected from the group consisting of platinum, platinum alloys, iridium, iridium alloys, nickel, nickel alloys, silicon, and metal silicides.


Journal ArticleDOI
TL;DR: In this paper, a GaAs quantum dot array coupled to quantum wire networks is demonstrated, where the GaAs buffer layers are grown on GaAs(001) substrates with SiN x square masks in 400 nm periodicity to [100] and [010] directions.

Journal ArticleDOI
TL;DR: In this paper, the authors examined the kinetics of carrier relaxation in GaAs/AlGaAs quantum wells (QWs), quantum wires (QWRs), and quantum boxes (QBs) with time-resolved cathodoluminescence (CL).
Abstract: We have examined the kinetics of carrier relaxation in GaAs/AlGaAs quantum wells (QWs), quantum wires (QWRs), and quantum boxes (QBs) with time-resolved cathodoluminescence (CL). In the cases of QWRs and QBs, the nanostructures were grown via a size-reducing growth approach on pre-patterned GaAs(001) substrates composed of stripes and mesas, respectively. The growth involved deposition of multiple GaAs/AlGaAs layers in order to establish both structural and optical markers which facilitated the identification of important features in transmission electron microscopy (TEM) and CL experiments. In TEM measurements, the lateral dimensions of the top-most GaAs layers in typical stripe and mesa structures comprising the QWRs and QBs delineate GaAs regions expected to exhibit 2D and 3D quantum confinement effects, respectively. Time-delayed CL spectra of all three structures reveal that the initial capture of carriers in the active regions occurs on a time scale less than the temporal resolution of the CL system...

Journal ArticleDOI
TL;DR: In this article, the effect of nitrogen incorporated in chemical vapor deposited diamond films on their electrical properties was studied, showing that the Hall mobility was increased by 4.3 times at room temperature.
Abstract: The effect of a small amount of nitrogen incorporated in chemical vapor deposited diamond films on their electrical properties was studied. Upgrading the purity of CH4 gas from the conventional ultra-high purity to the grade with no detection of nitrogen resulted in improvement of electrical properties of the boron-doped homoepitaxial diamond films; the Hall mobility was increased by 4.3 times at the room temperature. Decrease in nitrogen concentration in the diamond films was confirmed by investigating the 2.16 eV center of cathodoluminescence induced by ion beam irradiation and subsequent annealing.

Journal ArticleDOI
TL;DR: In this paper, the authors measured the surface potential of commercial phosphors by monitoring secondary and Auger electron spectra during electron beam irradiation at energies between 0.5 and 5 keV.
Abstract: Shifts of the surface potential were measured for several commercial phosphors by monitoring secondary and Auger electron spectra during electron beam irradiation at energies between 0.5 and 5 keV. The insulating phosphors charge strongly negative above 2 keV, whereas the conductive ones do not. This charging behavior is exaggerated after prolonged beam exposure. Measurements using two coincident electron beams to control the surface potential demonstrate that aging-enhanced surface charging decreases the phosphor cathodoluminescence. The dependence of the phosphor surface potential on the voltage applied to the metal plate on which they are deposited was also determined. These data suggest the presence of substantial space charge in the phosphors, even at low electron beam energies where no beam-induced shifts of the surface potentials are seen. We suggest that the electric fields due to these near-surface space charge regions are crucial in altering secondary emission and in establishing a steady state ...