In this study, we report the segregation of magnesium in the grain boundaries of magnesium-doped cuprous oxide (Cu2O:Mg) thin films as revealed by atom probe tomography and the consequences of the dopant presence on the temperature-dependent Hall effect properties. The incorporation of magnesium as a divalent cation was achieved by aerosol-assisted metal organic chemical vapour deposition, followed by thermal treatments under oxidizing conditions. We observe that, in comparison with intrinsic cuprous oxide, the electronic transport is improved in Cu2O:Mg with a reduction of resistivity to 13.3 ± 0.1 Ω cm, despite the reduction of hole mobility in the doped films, due to higher grain-boundary scattering. The Hall carrier concentration dependence with temperature showed the presence of an acceptor level associated with an ionization energy of 125 ± 9 meV, similar to the energy value of a large size impurity−vacancy complex. Atom probe tomography shows a magnesium incorporation of 5%, which is substantially present at the grain boundaries of the Cu2O.

The electrical properties of ZnO mono-crystalline materials, either in the form of bulk crystals or epitaxial films, were investigated for a large range of un-intentional or intentional doping concentrations extending from 4.0×1015 cm−3 up to 1.3×1020 cm−3. Hall and resistivity measurements were carried out from 10 K to 300 K, yielding the temperature dependent carrier densities and carrier mobilities. This allowed for an unambiguous determination of the dopant ionization energies, taking into account the concentration of compensation centers. The ionization energy variation as a function of dopant concentration was found to follow Mott's law, being consistent with the hydrogenic behavior of all involved donors; an effective critical Mott's concentration for the insulator to metal transition was found to be around 4.2×1018 cm−3, while the apparent value of the isolated donor ionization energy was determined as being 60 meV.

We report on the electrical, optical and photoluminescence properties of industry-ready Al doped ZnO thin films grown by physical vapor deposition, and their evolution after annealing under vacuum. Doping ZnO with Al atoms increases the carrier density but also favors the formation of Zn vacancies, thereby inducing a saturation of the conductivity mechanism at high aluminum content. The electrical and optical properties of these thin layered materials are both improved by annealing process which creates oxygen vacancies that releases charge carriers thus improving the conductivity. This study underlines the effect of the formation of extrinsic and intrinsic defects in Al doped ZnO compound during the fabrication process. The quality and the optoelectronic response of the produced films are increased (up to 1.52 m Omega.cm and 3.73 eV) and consistent with the industrial device requirements.

Temperature-dependent Hall effect and resistivity measurements are performed on polycrystalline Cu2O:Sr thin films deposited on glass substrate by metal organic-chemical vapor deposition. Their electrical properties are studied as a function of the strontium content, as determined by wavelength dispersive X-ray spectrometry. These electrical transport measurements highlight a copper vacancy doping mechanism induced by strontium incorporation, leading to a large decrease of the resistivity as low as 1.2 Ω·cm and corresponding free carrier mobilities about 15 cm2·V–1·s–1 at room temperature. Moreover, in addition to the main copper vacancy acceptor level, found at EA1 = 278 ± 21 meV above the top of the valence band, a coexisting shallower acceptor level with an ionization energy of EA2 = 133 ± 15 meV occurs for a strontium content above 5% and is tentatively assigned to a large size impurity–vacancy complex.

(Ga,In)N/GaN multiple quantum well blue light emitting diodes (LEDs) grown on mesa‐patterned silicon substrates with improved electro‐optic characteristics are demonstrated. The active regions are grown on top of high‐reflectivity AlN/(Al,Ga)N distributed Bragg reflectors (DBRs). Due to efficient stress relaxation at the mesa edges, crack formation during growth or upon the post‐growth cooling‐down of the samples can be avoided. A large number of AlN/(Al,Ga)N bilayers in the DBR can be then included in the LED structures leading to strong enhancement of the LED device output power in spite of the presence of the absorbing silicon substrate at the LED emission wavelength. Photograph of a blue light emitting diode (I = 20 mA) grown on top of a high reflectivity distributed Bragg reflector on a mesa‐patterned silicon substrate.

In this work, statistical formulations of the temperature dependence of ionized and neutral impurity concentrations in a semiconductor, needed in the charge balance equation and for carrier scattering calculations, have been developed. These formulations have been used in order to elucidate a confusing situation, appearing when compensating acceptor (donor) levels are located sufficiently close to the conduction (valence) band to be thermally ionized and thereby to emit (capture) an electron to (from) the conduction (valence) band. In this work, the temperature dependent Hall carrier density and Hall mobility data adjustments are performed in an attempt to distinguish the presence of a deep acceptor or a deep donor level, coexisting with a shallower donor level and located near the conduction band. Unfortunately, the present statistical developments, applied to an n-type hydrothermal ZnO sample, lead in both cases to consistent descriptions of experimental Hall carrier density and mobility data and thus do not allow to determine the nature, donor or acceptor, of the deep level. This demonstration shows that the emission of an electron in the conduction band, generally assigned to a (0/+1) donor transition from a donor level cannot be applied systematically and could also be attributed to a (−1/0) donor transition from an acceptor level. More generally, this result can be extended for any semiconductor and also for deep donor levels located close to the valence band (acceptor transition).

ZnO epilayers usually exhibit high n-type residual doping which is one of the reasons behind the difficulties to dope this material p-type. In this work, we aimed at determining the nature of the involved impurities and their potential role as dopant in ZnO thin films grown by metalorganic vapor phase epitaxy (MOVPE) on sapphire and ZnO substrates. In both cases, secondary ion mass spectroscopy (SIMS) measurements give evidence for a strong diffusion of impurities from the substrate to the epilayer, especially for silicon and aluminum. In the case of samples grown on sapphire substrates, aluminum follows Fick's diffusion law on a wide growth temperature range (800−1000°C). Thus, the saturation solubility and the diffusion coefficient of aluminum in ZnO single crystals have been determined. Furthermore, the comparison between SIMS impurity and effective dopant concentrations determined by capacitance-voltage measurements highlights, on one hand a substitutional mechanism for aluminum diffusion, and on the other hand that silicon acts as a donor in ZnO and not as an amphoteric impurity. In addition, photoluminescence spectra exhibit excitonic recombinations at the same energy for aluminum and silicon, indicating that silicon behaves as an hydrogenic donor in ZnO. Based on these experimental observations, ZnO thin films with a controlled n-type doping in the 1016−1019cm−3 range have been carried out. These results show that MOVPE growth is fully compatible with the achievement of highly Al-doped n-type thin films, but also with the growth of materials with low residual doping, which is a crucial parameter to address ZnO p-type doping issues.

Hall effect and capacitance-voltage C(V) measurements were performed on p-type GaN:Mg layers grown on GaN templates by molecular beam epitaxy with a high range of Mg-doping concentrations. The free hole density and the effective dopant concentration NA−ND as a function of magnesium incorporation measured by secondary ion mass spectroscopy clearly reveal both a magnesium doping efficiency up to 90% and a strong dependence of the acceptor ionization energy Ea with the acceptor concentration NA. These experimental observations highlight an isolated acceptor binding energy of 245±25 meV compatible, at high acceptor concentration, with the achievement of p-type GaN:Mg layers with a hole concentration at room temperature close to 1019 cm−3.

In this paper, the optical signatures of arsenic complexes in As-doped HgCdTe samples grown by molecular beam epitaxy are clearly identified using comparison between photoluminescence spectra, Extended X-Ray Absorption Fine Structure, and Hall measurements. The ionization energies of the different complexes are measured both by photoluminescence and Hall measurements.

The residual n-type conductivity of O-polar hydrothermally grown ZnO single crystals and the role of annealing on the transport properties are assessed by temperature dependent Hall measurements on a wide 20–800 K temperature range. A deep level lying 250 meV below the conduction band is responsible for the residual n-type conductivity of unannealed samples. After annealing, a shallow donor level with 25 meV ionization energy becomes responsible for the sample conductivity in the room temperature range. Thanks to high temperature Hall measurement, the coexistence of the deep and the shallow level has been demonstrated in the case of annealed sample.

Correlations between photoluminescence and temperature-dependent Hall measurements were carried out on unintentionally doped HgCdTe epilayers with cadmium composition of 32.7%. These films were grown by liquid-phase epitaxy and post-annealed under different conditions as follows: a p -type annealing was used to control the mercury vacancy concentration, and an n -type annealing under saturated mercury atmosphere was used to fill the mercury vacancies. Comparison of the results obtained by these two characterization techniques allowed us to identify the two acceptor energy levels of the mercury vacancy. Moreover, the “U-negativity” of the vacancy was evidenced: the ionized state V − is stabilized under the neutral state V 0 by the dominance of the Jahn–Teller effect over Coulombic repulsion. Finally, three epilayers with different cadmium compositions were also characterized to complete this study.

L’oxyde de zinc ZnO, est un semiconducteur II-VI très prometteur pour les applications en opto-électronique dans le domaine UV, notamment pour la réalisation de dispositifs électroluminescents (LED). Les potentialités majeures du ZnO pour ces applications résident notamment dans sa forte liaison excitonique (60 meV), sa large bande interdite directe (3.4 eV), la disponibilité de substrats massifs de grand diamètre ainsi que la possibilité de réaliser des croissances épitaxiales de très bonne qualité en couches minces ou nano structurées (nanofils). Néanmoins, le développement de ces applications est entravé par la difficulté de doper le matériau de type p. L'impureté permettant d'obtenir une conductivité électrique associée à des porteurs de charges positifs (trous), et donc la réalisation de jonctions pn à base de ZnO, n'a pas encore été réellement identifiée. C'est pourquoi une des étapes préliminaires et nécessaires à l'obtention d'un dopage de type p fiable et efficace, réside dans la compréhension du dopage résiduel de type n, ainsi que des phénomènes de compensation et de passivation qui sont mis en jeu au sein du matériau. La maîtrise de la nature des contacts (ohmique ou Schottky) sur différentes surfaces d'échantillons de ZnO nous a permis dans ce but de mettre en œuvre à la fois des mesures de transport (résistivité et effet Hall) et des mesures capacitives (capacité-tension C(V), Deep Level Transient Spectroscopy (DLTS) et Spectroscopie d'admittance).Dans un premier temps, nous avons donc cherché à comprendre de manière approfondie les propriétés électriques du ZnO massif. Nous avons ainsi étudié le rôle des défauts profonds et peu profonds sur la conductivité des échantillons, aux travers de différents échantillons massifs obtenus par synthèse hydrothermale ou par croissance chimique en phase vapeur. Nous avons également étudié l'impact de la température de recuits post-croissance, sur les propriétés de transport des échantillons. A la lumière des résultats obtenus sur le dopage résiduel de type n des échantillons de ZnO massifs, nous avons ensuite procédé à différents essais de dopage de type p du ZnO par implantation ionique d'azote et par diffusion en ampoule scellée d’arsenic. L'impureté azote a été choisie dans le cadre d'une substitution simple de l'oxygène qui devrait permettre de créer des niveaux accepteurs dans la bande interdite du ZnO. Nous avons également étudié l'impureté arsenic, qui selon un modèle théorique peut former un complexe qui permet d'obtenir un niveau accepteur plus proche de la bande de valence que le niveau. Outres les études réalisées sur les échantillons de ZnO massif et les essais de dopage de type p, nous avons également étudié les propriétés électriques d'échantillons de ZnO monocristallins sous forme de couches minces obtenues par croissance en phase vapeur d’organométalliques, dopées intentionnellement ou non. Les corrélations entres les mesures SIMS et C(V) nous ont permis notamment de mettre en évidence une diffusion et un rôle très importante de l'aluminium sur les propriétés électriques des couches minces de ZnO épitaxiées sur substrat saphir.Dans le cadre de cette thèse nous avons réussi à clarifier les mécanismes du dopage de type n, intentionnel ou non intentionnel, dans le ZnO monocristallin. Nous avons également identifié les impuretés et les paramètres de croissance importants permettant d'obtenir un dopage résiduel de type n le plus faible possible dans les couches épitaxiées. Cette maitrise du dopage résiduel de type n est une étape préliminaire indispensable aux études de dopage de type p car elle permet de minimiser la compensation des accepteurs introduits intentionnellement. Cette approche du dopage sur des couches minces de ZnO dont le dopage résiduel de type n est très faible apparait comme une voie très prometteuse pour surmonter les problèmes d'obtention du dopage de type p.

Although zinc oxide is a promising material for the fabrication of short wavelength optoelectronic devices, p-type doping is a step that remains challenging for the realization of diodes. Out of equilibrium methods such as ion implantation are expected to dope ZnO successfully provided that the non-radiative defects introduced by implantation can be annealed out. In this study, ZnO substrates are implanted with nitrogen ions, and the extended defects induced by implantation are studied by transmission electron microscopy and X-ray diffraction (XRD), before and after annealing at 900^{\circ}C. Before annealing, these defects are identified to be dislocation loops lying either in basal planes in high N concentration regions, or in prismatic planes in low N concentration regions, together with linear dislocations. An uniaxial deformation of 0.4% along the c axis, caused by the predominant basal loops, is measured by XRD in the implanted layer. After annealing, prismatic loops disappear while the density of basal loops decreases and their diameter increases. Moreover, dislocation loops disappear completely from the sub-surface region. XRD measurements show a residual deformation of only 0.05% in the implanted and annealed layer. The fact that basal loops are favoured against prismatic ones at high N concentration or high temperature is attributed to a lower stacking fault energy in these conditions. The coalescence of loops and their disappearance in the sub-surface region are ascribed to point defect diffusion. Finally, the electrical and optical properties of nitrogen-implanted ZnO are correlated with the observed structural features.

Photoluminescence and temperature-dependent Hall measurements of nonintentionally doped HgCdTe epilayers were compared. These films were grown by liquid phase epitaxy and postannealed under different conditions as follows: a p-type annealing was used to control the mercury vacancy concentration and a n-type annealing under saturated Hg atmosphere was used to fill the mercury vacancies. The comparison of the photoluminescence measurements with Hall effect measurements allows us to identify the two acceptor energy levels of the mercury vacancy and to evidence its “negative-U” property corresponding to a stabilization of the ionized state V− of the mercury vacancy compared to its neutral state V0.

A chemically assisted vapour phase transport (CVT) method is proposed for the growth of bulk ZnO crystals. Thermodynamic computations have confirmed the possibility of using CO as a sublimation activator for enhancing the sublimation rate of the feed material in a large range of pressures (10−3 to 1atm) and temperatures (800–1200°C). Growth runs in a specific and patented design yielded single ZnO crystals up to 46mm in diameter and 8mm in thickness, with growth rates up to 400μm/h. These values are compatible with an industrial production rate. N type ZnO crystals (μ=182cm2/(Vs) and n=7 1015cm−3) obtained by this CVT method (Chemical Vapour Transport) present a high level of purity (10–30 times better than hydrothermal ZnO crystals), which may be an advantage for obtaining p-type doped layers ([Li] and [Al] <10+15cm−3). Structural (HR-XRD), defect density (EPD), electrical (Hall measurements) and optical (photoluminescence) properties are presented.

Full maximum entropy mobility spectrum analysis was carried out on the basis of temperature and magnetic-field-dependent Hall measurements to assess the transport properties of homoepitaxial metal organic vapor phase epitaxy zinc oxide layers. Two different conductivity channels were clearly identified and the channel with higher mobility and higher carrier concentration is associated with the epitaxial layer. Hydrogen impurity acting as residual donor and as a passivating species for acceptors is proposed to explain the higher carrier concentration and mobility in the epilayer. In contrast to heteroepitaxial layers, no conduction channel is observed from the substrate to epilayer interface. (C) 2010 The Japan Society of Applied Physics

ZnO homoepitaxial growth is shown to change the luminescent properties of ZnO in a substantial way. Temperature dependent (4–300K) photoluminescence properties of the films grown by metal organic vapor phase epitaxy or by liquid phase epitaxy are compared to those of the hydrothermal ZnO substrate used for the growth. The intensity ratio between the free exciton and the donor bound exciton strongly increases, following homoepitaxial growth. The activation energy of the band edge emission intensity also increases from 13meV, which corresponds to the donor bound exciton localization energy, up to a value of 60meV, which corresponds to the free exciton binding energy. This indicates that homoepitaxial growth favors free exciton emission instead of donor bound exciton emission.

The spectroscopic properties of ZnO epilayers grown by molecular beam epitaxy are investigated. Three samples are compared: a homoepitaxial layer grown on a Crystec substrate, a sample directly grown on c-sapphire and a layer grown on c-sappire using an MgO buffer. In the latter case, in spite of the high dislocation density, temperature-dependent photoluminescence measurements show only a small decrease of the luminescence intensity between 4 and 300K, with an activation energy of 108meV. Cathodoluminescence presents an inhomogeneous emission on the micrometric scale: a stronger emission is measured in small nanometric areas. A tentative explanation of this behavior is proposed. The optical properties of these three samples are also compared to those of two-dimensional homoepitaxial layers and ZnO nanowires grown on sapphire and p-type GaN by metal organic vapor-phase epitaxy.