Free Exciton Binding Energy Research Articles

Effect of Calcination Temperature on Crystalline phase and Grain size of ZnO Particles

In today's technology, ZnO as a semiconductor, has emerged as a leading candidate in green environmental management systems, owing to its strong oxidation ability, good photocatalytic properties, and large free-exciton binding energy. ZnO is a costeffective, stable, and high photocatalytic material with good electrical properties and light transmittance, making it useful for various applications such as solar cells, photocatalysts, and electrical equipment. ZnO, being a photocatalyst, can be used for environmental clean-up efforts such as air purification, water purification, and deodorization. In this study, the zinc oxide (ZnO) was synthesised with a forced hydrolysis method and characterised by different characterization techniques. X-ray diffraction (XRD) was used to investigate the change in phase and crystal structure of ZnO particles at different calcination temperatures. Moreover, the average crystallite size of the particles was calculated at various calcination temperatures, using Scherrer equation. Also Scanning Electron Microscopy (SEM) used to image the surface morphology and size of the particles and it is found that the calcination temperature had a significant effect on the morphology of the particles. Additionally, differential Thermal Analysis (DTA) and Thermogravimetric Analysis (TGA) were used to observe the material transitions and changes in mass as the temperature increased.

Excitonic and deep-level emission from N- and Al-polar homoepitaxial AlN grown by molecular beam epitaxy

Using low-temperature cathodoluminescence spectroscopy, we study the properties of N- and Al-polar AlN layers grown by molecular beam epitaxy on bulk AlN{0001}. Compared with the bulk AlN substrate, layers of both polarities feature a suppression of deep-level luminescence, a total absence of the prevalent donor with an exciton binding energy of 28 meV, and a much increased intensity of the emission from free excitons. The dominant donor in these layers is characterized by an associated exciton binding energy of 13 meV. The observation of excited exciton states up to the exciton continuum allows us to directly extract the Γ5 free exciton binding energy of 57 meV.

(Invited) Spectroscopic Properties of Black Phosphorus Crystals and Layers

Black Phosphorus stands out in the 2D-materials panorama by its unique semiconducting properties: direct bandgap which can be tuned by the layer number in a wide range of wavelengths from visible (monolayer) to midinfrared (bulk) [1]. This tunability combined the anisotropy of the structure therefore offers promising perspectives in various fields such as electronics and photonics. However, the fast photooxidation in ambient condition, coupled to a high sensitivity to quantum confinement and dielectric environment in ultrathin BP, make very difficult the investigations on its intrinsic optical properties [2]. Further, as screening effects may strongly affect electronic and spectroscopic properties of 2D materials, it is highly desirable to investigate intrinsic properties of free-standing layers as well the ones of the bulk material which remain poorly known.To start with, we have investigated the infrared photoluminescence of BP single crystals at very low temperature [3]. Near-band-edge recombinations are observed at 2 K, including dominant excitonic transitions at 0.276 eV and a weaker one at 0.278 eV. The free-exciton binding energy is calculated with an anisotropic Wannier-Mott model and found equal to 9.1 meV. On the contrary, the PL intensity quenching of the 0.276 eV peak at high temperature is found with a much smaller activation energy, attributed to the localization of free excitons on a shallow impurity. This analysis leads us to attribute respectively the 0.276 eV and 0.278 eV PL lines to bound excitons (I°X) and free excitons (X) in BP. As a result, the value of bulk BP electronic bandgap is refined to 0.287 eV at 2K, to serve as reference for future work on thin BP layers [3].As far as the thinnest layers are concerned, which cannot manipulated in air, we have shown that Angular resolved Electron energy loss spectroscopy implemented in Transmission Electron Microscopy (Ar-EELS-TEM) offers a unique way to investigate dielectric response of free-standing layers related to valence band and plasmon excitations with the advantage to get access to their q dispersion and their symmetry properties [4]. By combining this technique with suitable ab initio calculations, we have studied the dielectric response of free-standing BP layers as a function of the number of layers. We found optical bandgap values of 1.9 eV, 1.4 eV and 1.1 eV for the mono- bi- and trilayer respectively. Moreover, by combining our results with a simple variational model, we correlate the exciton energy with the dielectric screening. We hence demonstrate that the variations of the electronic gap are sizeably larger than the variations of the binding energy. Finally, we probe and analyze the volume and surface plasmons dispersion as a function of momentum for the 1-3 BP layers and bulk and highlight a deviation and linearization of the parabolic dispersion with strong anisotropic fingerprints [5].[1] G. Zhang et al., Nat. Com., 8, 14071, (2017)[2] A. Favron, E. Gaufres et al Nature Mat. 14 (2015) 826.[3] E. Carré et al, 2D Materials (2020) :doi.org/10.1088/2053-1583/abca81[4] F. Fossard et al, Phys. Rev. B 96, 115304 (2017)[5] E. Gaufres et al, Nanoletters 19, 8303 (2019); DOI: 10.1021/acs.nanolett.9b03928

Electronic Band Transitions in γ-Ge3N4

Electronic band structure in germanium nitride having spinel structure, γ-Ge3N4, was examined using two spectroscopic techniques, cathodoluminescence and synchrotron-based photoluminescence. The sample purity was confirmed by x-ray diffraction and Raman analyses. The spectroscopic measurements provided first experimental evidence of a large free exciton binding energy De≈0.30 eV and direct interband transitions in this material. The band gap energy Eg = 3.65 ± 0.05 eV measured with a higher precision was in agreement with that previously obtained via XES/XANES method. The screened hybrid functional Heyd–Scuseria–Ernzerhof (HSE06) calculations of the electronic structure supported the experimental results. Based on the experimental data and theoretical calculations, the limiting efficiency of the excitation conversion to light was estimated and compared with that of w-GaN, which is the basic material of commercial light emitting diodes. The high conversion efficiency, very high hardness and rigidity combined with a thermal stability in air up to ~ 700 °C reveal the potential of γ-Ge3N4 for robust and efficient photonic emitters.

Excitons in bulk black phosphorus evidenced by photoluminescence at low temperature

Atomic layers of black phosphorus (BP) present unique opto-electronic properties dominated by a direct tunable bandgap in a wide spectral range from visible to mid-infrared (IR). In this work, we investigate the IR photoluminescence (PL) of BP single crystals at very low temperature. Near-band-edge recombinations are observed at 2 K, including dominant excitonic transitions at 0.276 eV and a weaker one at 0.278 eV. The free-exciton binding energy is calculated with an anisotropic Wannier–Mott model and found equal to 9.1 meV. On the contrary, the PL intensity quenching of the 0.276 eV peak at high temperature is found with a much smaller activation energy, attributed to the localization of free excitons on a shallow impurity. This analysis leads us to attribute respectively the 0.276 eV and 0.278 eV PL lines to bound excitons and free excitons in BP. As a result, the value of bulk BP bandgap is refined to 0.287 eV at 2 K.

Anisotropic optical properties of single Si2Te3 nanoplates

We report a combined experimental and computational study of the optical properties of individual silicon telluride (Si2Te3) nanoplates. The p-type semiconductor Si2Te3 has a unique layered crystal structure with hexagonal closed-packed Te sublattices and Si–Si dimers occupying octahedral intercalation sites. The orientation of the silicon dimers leads to unique optical and electronic properties. Two-dimensional Si2Te3 nanoplates with thicknesses of hundreds of nanometers and lateral sizes of tens of micrometers are synthesized by a chemical vapor deposition technique. At temperatures below 150 K, the Si2Te3 nanoplates exhibit a direct band structure with a band gap energy of 2.394 eV at 7 K and an estimated free exciton binding energy of 150 meV. Polarized reflection measurements at different temperatures show anisotropy in the absorption coefficient due to an anisotropic orientation of the silicon dimers, which is in excellent agreement with theoretical calculations of the dielectric functions. Polarized Raman measurements of single Si2Te3 nanoplates at different temperatures reveal various vibrational modes, which agree with density functional perturbation theory calculations. The unique structural and optical properties of nanostructured Si2Te3 hold great potential applications in optoelectronics and chemical sensing.

Photoluminescence efficiency of Al-rich AlGaN heterostructures in a wide range of photoexcitation densities over temperatures up to 550 K

Time-resolved photoluminescence and light-induced transient grating techniques were applied for temperature and excitation dependent internal quantum efficiency and carrier diffusion investigation in silicon-doped epilayer and AlxGa1-xN MQWs (x > 0.6). Decrease of photoluminescence efficiency at high excitations correlated with increase of diffusion coefficient as well as nonradiative recombination rate, indicating excitation and temperature dependent localized exciton density reduction. Excitation-dependent lifetime decrease was less pronounced than the corresponding drop of the time-integrated photoluminescence efficiency dominated by thermal dissociation of excitons. At lowest excitations excitons were found captured to vacancy complexes. With increase of excitation exciton ionization appeared, leading to enhanced nonradiative recombination of holes on aluminum vacancies and simultaneous droop of PL efficiency due to saturated bimolecular free carrier plasma recombination. At initial recombination stages and high excitations diffusive recombination on dislocations was also revealed. Modelling provided free exciton binding energies of 104, 140 meV in the MQWs and in the layer; exciton localization energies in 12-35 meV range; localized exciton lifetimes in 2-4 ns range; free exciton and carrier radiative recombination rate coefficients of rex = (0.6 0.2)109(T/300)-1 1/s and Brad = (7 1)10-10(T/300)-3/2 cm3/s, respectively. Vacancy complex capture cross section for excitons was determined to be = (2 1)10-16(300/T)2 cm2. Electron and hole capture cross sections to aluminum vacancy were modeled (1.5 1)10-13 cm2 and (7 1)10-13 cm2, respectively. Dislocation Coulomb radius for free carrier recombination was found to be 12-15 nm.

Fast and Efficient Sun Light Photocatalytic Activity of Au_ZnO Core-Shell Nanoparticles Prepared by a One-Pot Synthesis.

Gold nanostructures absorb visible light and show localized surface plasmon resonance bands in the visible region. Semiconducting ZnO nanostructures are excellent for ultraviolet detection, thanks to their wide band gap, large free exciton binding energy, and high electron mobility. Therefore, the coupling of gold and ZnO nanostructures represents the best-suited way to boost photodetection. With the above perspective, we report on the high photocatalytic activity of some Au_ZnO core–shell nanoparticles (NPs) recently prepared by a one-pot synthesis in which a [zinc citrate]− complex acted as the ZnO precursor, a reducing agent for Au3+, and a capping anion for the obtained Au NPs. The overall nanostructures proved to be Au(111) NPs surrounded by a thin layer of [zinc citrate]− that evolved to Au_ZnO core–shell nanostructures. Worthy of note, with this photocatalyst, sun light efficiently decomposes a standard methylene blue solution according to ISO 10678:2010. We rationalized photodetection, reaction rate, and quantum efficiency.

(Invited) Hexagonal Boron Nitride Epilayers

As a member of the III-nitride wide bandgap semiconductor family, BN has received much less attention in comparison with other nitride semiconductors. The stable phase of BN synthesized at any temperature under ambient pressure is hexagonal. Due to its wide bandgap (> 6 eV) and layered structure, hexagonal BN (h-BN) is an ideal platform for probing fundamental 2D properties of wide bandgap semiconductors. In this talk, a brief overview of the synthesis and electrical and optical properties of wafer-scale h-BN epilayers will be presented [1-7]. It was shown that the unique 2D structure of h-BN induces exceptionally high density of states, large exciton binding energy and high optical absorption and emission intensity. By growing h-BN under high V/III ratios, epilayers exhibiting pure free exciton emission have been obtained [4]. Photocurrent excitation spectroscopy results directly provided a room temperature bandgap value for h-BN in between 6.4 and 6.5 eV and the free exciton binding energy (Ex) of 0.73 eV [5]. Layer-structured B-rich BGaN alloys, heterostructures, and quantum wells have also been successfully grown and their properties will be discussed. Thermal neutron detectors fabricated from 100% B-10 enriched h-BN epilayers of thicknesses exceeding 50 mm have attained the highest detection efficiency to date among solid-state detectors at about 58% [6, 7]. These solid-state neutron detectors have become increasingly desirable for a wide range of applications from fissile materials sensing to well logging, because 3He gas detectors are inherently bulky, require high pressurization and high voltage application, slow response time, and expensive. It is our belief that h-BN will lead to many potential applications from deep UV optoelectronics, radiation detectors, to novel layered-structured photonic and electronic devices. [1] R. Dahal, J. Li, S. Majety, B.N. Pantha, X. K. Cao, J. Y. Lin, and H.X. Jiang, Appl. Phys. Lett. 98, 211110 (2011). [2] S. Majety, J. Li, X. K. Cao, R. Dahal, B. N. Pantha, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 100, 061121 (2012). [3] H. X. Jiang, and J. Y. Lin, Semicon. Sci. Technol. 29, 084003 (2014). [4] X. Z. Du, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 106, 021110 (2015); 108, 052106 (2016). [5] T. C. Doan, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 109, 122101 (2016). [6] A. Maity, T. C. Doan, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 109, 072101 (2016). [7] A. Maity, S. J. Grenadier, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 111, 033507 (2017) and J. Appl. Phys. 123, 044501 (2018).

Photoluminescence and electronic transitions in cubic silicon nitride.

A spectroscopic study of cubic silicon nitride (γ-Si3N4) at cryogenic temperatures of 8 K in the near IR - VUV range of spectra with synchrotron radiation excitation provided the first experimental evidence of direct electronic transitions in this material. The observed photoluminescence (PL) bands were assigned to excitons and excited and centers formed after the electron capture by neutral structural defects. The excitons are weakly quenched on neutral and strongly on charged defects. The fundamental band-gap energy of 5.05 ± 0.05 eV and strong free exciton binding energy ~0.65 eV were determined. The latter value suggests a high efficiency of the electric power transformation in light in defect-free crystals. Combined with a very high hardness and exceptional thermal stability in air, our results indicate that γ-Si3N4 has a potential for fabrication of robust and efficient photonic emitters.

Spectroscopic determination of the bandgap crossover composition in MBE-grown AlxGa1−xAs

The aluminum concentration dependence of the energies of the direct and indirect bandgaps arising from the Γ and X conduction bands are measured at 1.7 K in the semiconductor alloy AlxGa1−xAs. The composition at which the bands cross is determined from photoluminescence of samples grown by molecular-beam epitaxy very close to crossover at x ≈ 0.4. The use of resonant laser excitation and the improved sample linewidth allows excitation intensities as low as 10−2 W/cm2, giving a precise determination of the bound exciton transition energies and their Γ and X crossover. Photoluminescence excitation spectroscopy is then used to measure the binding energies of the donor-bound excitons and the Γ free exciton binding energy. After correcting for the Γ- and X-dependence of these quantities, the crossover of the bandgap is determined to be at x = 0.401 and E = 2.086 eV.

Contactless electroreflectance studies of free exciton binding energy in Zn1-xMgxO epilayers

Contactless electroreflectance (CER) has been applied to study optical transitions in Zn1-xMgxO layers with magnesium concentration ≤44%. CER resonances related to free exciton and band-to-band transitions were clearly observed at room temperature. For ZnO the two transitions are separated by the energy of ∼65 meV, which is attributed to the free exciton binding energy in ZnO. Due to magnesium incorporation, the CER resonances broaden and shift to blue. The energy separation between excitonic and band-to-band transitions increases up to ∼100 meV when the magnesium concentration reaches 22%. For larger magnesium concentrations, CER resonances are significantly broadened and the excitonic transition is no longer resolved in the CER spectrum.

Improved value for the silicon free exciton binding energy

The free exciton binding energy is a key parameter in silicon material and device physics. In particular, it provides the necessary link between the energy threshold for valence to conduction band optical absorption and the bandgap determining electronic properties. The long accepted low temperature binding energy value of 14.7 ± 0.4 meV is reassessed taking advantage of developments subsequent to its original determination, leading to the conclusion that this value is definitely an underestimate. Using three largely independent experimental data sets, an improved low temperature value of 15.01 ± 0.06 meV is deduced, in good agreement with the most comprehensive theoretical calculations to date.

Elevated temperature dependence of energy band gap of ZnO thin films grown by e-beam deposition

We report the surface, structural, electronic, and optical properties of the epitaxial ZnO thin films grown on (0001) sapphire substrate at 600 °C by an electron-beam deposition technique. ZnO thin films have been deposited in an oxygen environment and post-deposition annealed to improve the stoichiometry and the crystal quality. In order to investigate the free exciton binding energy and the temperature dependence of the energy bandgap, we carried out variable temperature (78–450 K) transmittance measurements on ZnO thin films. The absorption data below the energy bandgap have been modeled with the Urbach tail and a free exciton, while the data above the gap have been modeled with the charge transfer excitations. The exciton binding energy is measured to be E0 = 64 ± 7 meV, and the energy band gaps of the ZnO film are measured to be Eg ~ 3.51 and 3.48 eV at 78 and 300 K, respectively. The temperature dependence of the energy gap has been fitted with the Varshni model to extract the fitting parameters α = 0.00020 ± 0.00002 eV/K, β = 325 ± 20 K, and Eg (T = 0 K) = 3.516 ± 0.0002 eV.

Featured issue: zinc oxide

In the first two issues of 2013 we are highlighting fields which are topics of particular current interest in our discipline, as we did in the first two issues of 2012. The February featured papers will be devoted to electronic materials related to Solar Energy Generation and Energy Storage, but in this (January) issue we again highlight the material featured in the February 2012 issue—Zinc Oxide. Reasons for the high interest in this material was outlined in my editorial in the February 2012 issues as follows. While this II–VI semiconductor has been known for many years, the modern appreciation of its potential began in the 1950s, utilising properties such as piezoelectric, luminescent, and UV absorption characteristics, as well as its semiconductor features. The growth of single crystals facilitated the development of devices such as piezoelectric transducers, strain gauges, and pressure sensors, in addition to earlier devices such as varistors. Further work on the growth of large single crystals was stimulated by the potential for lattice matched epitaxial growth of optoelectronic III–V nitrides, but more recently it has become apparent that ZnO might be a good optoelectronic material on its own account, owing to similarities with GaN. Having a similarly wide band-gap, and similar lattice dimensions, the key advantageous feature of ZnO is that it has a free exciton binding energy of 60 meV, more than twice that of GaN. This combination offers the possibility of UV laser action at room temperature, and has given rise to much research to address outstanding problems, including that on doping and thin film growth by several different growth techniques. Further work is also pursuing the potential of ZnO-based dilute magnetic semiconductors in spintronics’. Continued interest in this subject has led to this second Featured issue from papers submitted as regular submissions, and updates the position last year. We hope you will find this issue of interest, and that it will help to guide further work on this important topic.

Strain Effects on Exciton Transition in CdTe Films on GaAs(100) Substrates

We performed photoluminescence (PL) and reflectance characterization of CdTe films on GaAs substrates. Separate emission peaks for heavy- and light-hole free excitons due to induced compressive strain were observed. The strain in a CdTe film is estimated to be ε=-5.28×10-4. By analyzing the PL and reflectance spectra, both heavy- and light-hole free exciton binding energies are estimated to be 9.9 meV. By analyzing the energy separation between the heavy- and light-hole free exciton reflectance dips, the magnitude of the residual compressive strain is determined to be homogeneously distributed within a CdTe film. From the CdTe film thickness dependence of heavy- and light-hole free exciton linewidths, the CdTe film quality is observed to become poor with decreasing CdTe film thickness.

Low-Temperature Photoluminescence of Nanostructured ZnO Crystal Synthesized by Pulsed-Laser Ablation

The optical properties of ZnO nanorods and nanowires grown by our newly developed nanoparticle-assisted pulsed-laser deposition (NAPLD) technique were studied by photoluminescence (PL) spectroscopy. This NAPLD technique, which does not require a catalyst for crystal growth, is expected to synthesize high-quality nanostructured ZnO crystals. The green luminescence (GL) band and ultraviolet luminescence (UVL) observed at low temperature were investigated for these crystals. The UVL band consists of several sharp emission lines due to free excitons, donor-bound excitons, donor-acceptor pair (DAP) transitions, and their longitudinal-optical (LO) phonon replicas. The free-exciton binding energy was estimated to be 59 meV and the band gap of these ZnO crystals was determined to be 3.435 eV at 10 K. The temperature variation for the energy positions of free-exciton emission was fitted by the Manoogian–Woolley equation and the Debye temperature was estimated to be 505 K. The low-temperature PL investigation of our nanostructured ZnO crystals that were easily synthesized by our newly developed NAPLD technique demonstrated their excellent crystallinity.

Effect of annealing temperature on the optical and electrical properties of aluminum doped ZnO films

abstract Aluminum doped ZnO thin films were successfully deposited on the silicon substrates by spin coatingmethod. The effects of an annealing temperature on electrical and optical properties were investigatedfor 1.5 at.% of aluminum. Refractive index profile has been obtained for the film annealed at 350 C usingellipsometry and it has shown minimum refractive index of 1.95 and maximum value of 2.1. Thicknessprofile shows quite good uniformity of the film having minimum thickness value of 30.1 nm and maxi-mum value of 34.5 nm. Maximum conductivity value obtained was 4.63 X 1 -cm 1 for the film annealedat 350 3C. Maximum carrier density of 2.20 10 17 cm was deduced from the Hall measurement andFourier transform infrared spectroscopy clearly reveals major peak at 407 cm 1 in the spectra associatedwith the ZnO bond. 2009 Elsevier B.V. All rights reserved. 1. IntroductionTransparent conducting oxide (TCO) films are of prominentinterest due to their potential in optoelectronic devices such astouch panels, flat panel displays (FPD), and thin film solar cells[1,2]. For display applications, TCO films of high optical transmit-tance in the visible region and high electrical conductivity are re-quired. Indium tin oxide (ITO) has been commercially used forthe flat panel displays [3,4]. However, impurity-doped zinc oxides(ZnOs) serves as an alternative for ITO-based TCO because of itsadvantages of low cost, resource availability and nontoxicity. Zincoxide is accepted as wide band gap semiconductors as an alterna-tive to GaN because it has band gap of 3.37 eV at room tempera-ture, large free exciton binding energy (60 meV), and highmechanical and thermal stabilities. With enhancement in the qual-ity andcontrolof conductivityinbulkZnO [5] foundits potentialasa material for short wavelength light emitters and transparentelectronics [6,7].Generally, the undoped ZnO is showing a n-type conductivity.Particularly, an Al doped ZnO transparent conductive oxide (TCO)filmhashightransmittanceinthevisibleregionwithalowresistiv-ity and by doping it with aluminum its electrical properties can beimproved[8,9].Asaresult,theAldopedZnOshowsexcellenttrans-parency over the entire visible spectrum and has better transportproperties than tin oxide due to higher electron mobility. Thereare several methods used for the fabrication of ZnO thin films suchas pulsed laser deposition [10–12], sputtering [13,14], molecularbeam epitaxy [15], and sol–gel process [16], etc. Among thesemethods, the sol–gel technique has several advantages over theothers such as low cost, simplicity; homogeneity and large areafilms can be prepared at lower temperature.Many factors can affect the properties of aluminum doped ZnOfilms prepared by sol–gel method. Among them, Aluminum com-position and annealing temperature have significant effect on theoptical and electrical properties of the film. Present work reportsaluminum-doped zinc oxide films deposited on silicon substrateusing sol–gel spin coating process. We have already reported effectof aluminum doping on the properties of the films [9]. Here, wehave investigated effect of annealing temperature on the Alumi-num doped ZnO films prepared by sol–gel method on the siliconsubstrate by considering 1.5 at.% of aluminum. The main goal ofthe paper is to investigate effect of annealing temperature on theelectrical and optical properties of the aluminum doped ZnO filmsby keeping aluminum concentration fixed.

UV emission on a Si substrate: Optical and structural properties of γ‐CuCl on Si grown using liquid phase epitaxy techniques

AbstractConsiderable research is being carried out in the area of wide band gap semiconductor materials for light emission in the 300–400 nm spectral range. Current materials being used for such devices are typically based on II–VI and III‐nitride compounds and variants thereof. However, one of the major obstacles to the successful fabrication of III‐N devices is lattice mismatch‐induced high dislocation densities for epitaxially grown layers on non‐native substrates. γ‐CuCl is a direct bandgap material and an ionic wide bandgap I–VII semiconductor with a room temperature free exciton binding energy of ∼190 meV (compared to ∼25 meV and ∼60 meV for GaN and ZnO, respectively) and has a band gap of 3.4 eV (λ ∼ 366 nm). The lattice constant of γ‐CuCl (0.541 nm) is closely matched to that of Si (0.543 nm). This could, in principle, lead to the development of optoelectronic systems based on CuCl grown on Si. Research towards this end has successfully yielded polycrystalline γ‐CuCl on Si(100) and Si(111) using vacuum‐based deposition techniques [1]. We report on developments towards achieving single crystal growth of CuCl from solution via Liquid Phase Epitaxy (LPE) based techniques. Work is being carried out using alkali halide flux compounds to depress the liquidus temperature of the CuCl below its solid phase wurtzite‐zincblende transition temperature (407 °C [2]) for solution based epitaxy on Si substrates. Initial results show that the resulting KCl flux‐driven deposition of CuCl onto the Si substrate has yielded superior photoluminescence (PL) and X‐ray excited optical luminescence (XEOL) behavior relative to comparitively observed spectra for GaN or polycrystalline CuCl. This enhancement is believed to be caused by an interaction between the KCl and CuCl material subsequent to the deposition process, perhaps involving a reduction in Cl vacancy distributions in CuCl. This paper presents a detailed discussion of a CuCl LPE growth system as well as the characterization of deposited materials using X‐ray diffraction (XRD), room temperature and low temperature PL, and XEOL. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron Emission from Diamond (111) p+-i-n+ Junction Diode

AbstractWe successfully observed electron emission from hydrogenated diamond (111) p+-i-n+ junction diodes. Here, p+- and n+-layers mean that the boron and phosphorous impurity concentrations in these layers are around 1020 cm-3. Then the p+-layer on top of the diode suppresses electron emission from the top-surface area. The heavily doped layers also play an important role to obtain high diode and emission currents. The emission started when the applying bias voltage was equal to the built-in potential, and the emission current reached to over 1 μA at room temperature operation. With taking into account our previous photoemission yield spectroscopy results and with the very high binding energy of free excitons of 80 meV in diamond, we suggested that the electron emission was derived from free excitons generated in the i-layer of the diodes.