Philippe Lemasson

Thermogravimetric and electrical study of non-stoichiometric titanium dioxide TiO2−x, between 800 and 1100°C

By means of thermogravimetrie and electrical measurements, it has been possible to give accurate informations on the nature of the defects produced by reduction of TiO2 between 800 and 1100°C. For small partial pressures of oxygen Po2. interstitial titanium Ti4i prevails at temperature higher than 900°C. When PO2 increases, oxygen vacancies are produced at first in the doubly ionized form V″o and a progressive transition to singly ionized vacancies V″o can be assumed. The formation enthalpies associated with these defects as calculated from our experimental results are 10.1 eV for Ti4i, 4.6 eV for V″o and 3.6–4.0 eV for V′o. The electronic drift mobility μ is independent both of nature and concentration of the defects. The shape of its temperature dependence leads to conclude that the conduction in spite of the low μ value (0.06 cm2 V−1 s−1 at 1100°C) is of the classical type for wide band semiconductors and not a hopping process.

Growth by solid phase recrystallization and assessment of large ZnSe crystals of high purity and structural perfection

Some physical properties of ZnSe resulting from the ionicity of its chemical bond make it difficult its melt-growth, suggesting the use of low temperature growth. Among the classical techniques of low temperature growth, solution and vapor growth are shown to present some limitations. Solid state recrystallization is presented as an attractive alternative for the growth of large ZnSe crystals of high purity and structural perfection, as shown from photoluminescence and X-ray diffraction measurements.

Thermodynamic properties of titanium dioxide, niobium dioxide and their solid solutions at high temperature

Abstract By means of thermogravimetric measurements, it has been possible to obtain information on the nature of the intrinsic and extrinsic defects of TiO 2 , NbO 2 and solid solutions Nb y Ti 1− y O 2± x Pure TiO 2 is an oxygen-deficient oxide The main defects are oxygen vacancies, doubly ionized V .. o or singly ionized V . o , and interstitial titanium Ti 3 i NbO 2 is a metal-deficient oxide The main defects are neutral niobium vacancies. The solid solutions Nb y Ti 1− y O 2± x may be divided into two groups If y > 0 04, the behavior is analogous to that of NbO 2 ; with the same defects, but the width of the homogeneity range decreases with the titanium content and Nb 0 04 Ti 0 96 O 2 is a stoichiometnc oxide If y 0 995 Nb 0 005 O 2 In the oxygen-deficient domain, the main defects are assumed to be neutral or singly ionized oxygen vacancies In the metal-deficient domain, the main defects are metal vacancies V 4 Ti From these results we have deduced the nature of intrinsic defects in TiO 2 to be Schottky defects: 2 V .. O + V 4 T1 .

Photoelectrochemical determination of minority carrier diffusion length in II–VI compounds

Abstract The minority carrier diffusion length L of different II–VI compounds (n-ZnSe, n-CdTe, p-CdTe, p-ZnTe and n-CdSe) has been measured by a photoelectrochemical technique based on the Ga¨rtner model. The conditions used in this technique are defined accurately to avoid erroneous interpretations. Five experimental parameters which may influence drastically the measurement of L are pointed out. A linear relationship between L and free carrier density in CdTe and ZnSe has been found.

Diffusion length of minority carrier in n‐type semiconductors: A photoelectrochemical determination in aqueous solvents

Photoelectrochemical measurements with aqueous electrolyte‐semiconductor junctions under monochromatic illumination for five n‐type semiconductor compounds are presented: ZnSe, CdSe, CdTe (II‐VI) and GaP, InP (III‐V). The Schottky behavior of these junctions is demonstrated in the dark and under illumination. Only if conditions such as surface preparation, proper band bending, and doping level as well as energy of incident photons are achieved, the photocurrent versus potential characteristics are described using Gartner’s equation. Values of the diffusion length L of holes are determined by an extrapolation and a fitting method. They depend on the particular properties of the samples, particularly the concentration of free electrons. Results obtained are: ZnSe (7×1017 cm−3) L=0.025 μm; CdSe (2.5×1018 cm−3) L=0.124 μm; CdTe (8.5×1017 cm−3) L=0.14 μm; GaP (4×1016 cm−3) L=0.24 μm; InP (3×1017 cm−3) L=0.15 μm.

Analysis of photocurrents at the semiconductor—eletrolyte junction

Abstract The photocurrent vs potential characateristics for three different electrolyte—semiconductor junctions, representative of those generally found in semiconductor photoelectrochemistry, are analyzed in detail using parameters which define both the semiconductor and the electrolyte. It is shown that, in general, the behaviour of junctions including semiconductors with sufficiently wide energy pags and large free carriers densities, may be accurately described using the Gartner model in the potential region which does not include the onset of the photocurrent. In this case, it is the characteristics of the semiconductor which control the photoresponse and the electrolyte does not induce limiting steps in the charge transfer across the interface. If certain restrictive conditions are fulfilled concerning the relative orders of magnitude of the semiconductor space charge region, diffusion length of minority carriers and penetration depth of light into the semiconductor, the value of the flat band potential may be easily determined by ploting i 2 ph vs V and extrapolating to i 2 ph = 0. In the other cases, the relation established by Gartner between i ph and V has been verified provided that the free carriers density be sufficiently large.

Correlation between the non-stoichiometry of titanium dioxide and its photoelectrochemical behaviour

TiO2(rutile) has been reduced in well-defined conditions to give n-type TiO2–x semiconductors in which both the nature and concentration of the defects attributed to reduction are known. The influence of the value of x in TiO2–x on the efficiency of such a material when used as a photoanode is very marked and, in particular, there exists a value of x for which this efficiency is a maximum. The influence of x on the electrochemical behaviour of TiO2–x is only noticeable for electrodes whose active surface has been previously mechanically polished. Finally the broadening of the absorption spectrum towards the visible as x increases can be attributed to the existence of deep acceptor levels which are excited by the light, giving free holes that can be involved in the OH– oxidation process.

Room-temperature electroluminescence in the mid-infrared (2-3 μm) from bulk chromium-doped ZnSe

Electroluminescence associated with impact excitation or ionization of deep Cr(2+) impurity centers in bulk ZnSe is reported. A broad signal of mid-infrared luminescence between 2 and 3 microm is observed once the biased bulk ZnSe device runs into a nonlinear conduction regime. Optical powers in the nanowatt range have been measured at room temperature. The different mechanisms involved in this intracenter infrared light emission are discussed.

The semiconductor-electrolyte interface : Optical transitions in CdMnTe alloys

Abstract The junction p-Cd1−xMnxTe-electrolyte is studied under monochromatic illumination for the alloy composition range 0 ≤ x ≤ 0.7. Optical transitions in the semiconductor are evidenced by photocurrent measurements. In the composition range 0 ≤ x ≤ 0.45, the nature of the fundamental transition appears to be the same as in CdTe (valence band Γ → conduction band Γ ). For x > 0.45, the observed photocurrent p ak, pinned at a consta t energy value ca. 2.1 eV, may presumably be related to a transition Γ → d(Mn) level whereas the photocurrent edge associated to the forme transition increases linearly with x.

Mesures d'impédances en électrochimie des semi-conducteurs: Influence des états de surface

Abstract With a phase sensitive detector giving the resistive and capacitive components of an impedance, we show the influence of surface states attributed to an anion adsorption on the behaviour of a semiconducting tungsten trioxide electrode. A simple electrical representation gives a possibility of evaluating their density and of comparing the influence of the type of the anion present in the electrolyte on that number.