A. Lusson

Electrical activity of nitrogen acceptors in ZnO films grown by metalorganic vapor phase epitaxy

The electrical activity of nitrogen as an acceptor in ZnO has been investigated in two ways. First, nitrogen was introduced by means of diallylamine during metalorganic vapor phase epitaxy (MOVPE) yielding incorporation of nitrogen in the range 1016–1021 cm−3. This led to significant compensation of the natural donors with a minimum electron concentration of 5×1014 cm−3. Second, diffusion of nitrogen was carried out on undoped MOVPE layers under high pressure conditions stemming from the decomposition of NH4NO3. Conversion to p-type conductivity was observed in a systematic way with measured hole concentrations up to 6.5×1017 cm−3.

One step electrodeposition of CuInSe2: Improved structural, electronic, and photovoltaic properties by annealing under high selenium pressure

Films of Cu–In–Se alloys can be electrodeposited in a wide range of controlled composition. Annealing treatments under Se pressure transform these precursor films in large grain CuInSe2 films with improved electronic properties. These modifications are shown to depend on the Se pressure imposed during the treatment allowing a certain tailoring of the electronic properties of the films. The properties of electrodeposited/selenized films are presented as obtained from luminescence measurements, Hall effect, and photoelectrochemical characterization. An efficiency of 6.5% (total area, without antireflecting coating) is reported for the best CuInSe2/CdS/ZnO solar cell. An analysis of the device is also presented where limitations by interface recombination are shown to be the dominant loss mechanism.

Effects of intentionally introduced hydrogen on the electrical properties of ZnO layers grown by metalorganic chemical vapor deposition

Unintentionally doped ZnO layers grown epitaxially on a sapphire substrate have been exposed either to a hydrogen or deuterium plasma. Secondary ion mass spectroscopy measurements performed subsequently showed a rapid diffusion of hydrogen in these layers. Furthermore, the presence of hydrogen in the ZnO samples is found to be responsible for nearly a factor of 3 increase in the free electron concentration. This effect is attributed to the hydrogen passivation of compensating acceptor impurities present in the as-grown ZnO layers.

Temperature dependence of the fundamental absorption edge of mercury cadmium telluride

We have investigated the temperature dependence of the fundamental absorption edge of a series of Hg1−xCdxTe alloys (with composition x ranging from 0.5 to 1). Analyzing our data in the light of the three‐dimensional theory of direct‐allowed excitons, we find precise values for the fundamental Γ8‐Γ6 interband transition energy in a temperature range extending from 0 to 300 K. All experimental results, including previous data for HgTe and mercury‐rich Hg1−xCdxTe alloys, are well accounted for using a simple empirical formula: Eg (eV)=−0.303(1−x)+1.606x−0.132x(1−x)+[6.3(1−x) −3.25x −5.92x(1−x)]10−4T2/[11(1−x)+78.7x+T]. This expression, which is valid for all compositions 0≤x≤1 and temperatures 0≤T≤500 K, predicts an alloy composition such that the band‐gap energy is temperature independent: We find x=0.505. Finally, it can be used for technological application purpose (far‐infrared detection as well as optical‐fiber communications performed at realistic values of the temperature) and gives accurate values f...

Use of ZnO thin films as sacrificial templates for metal organic vapor phase epitaxy and chemical lift-off of GaN

Continued development of GaN-based light emitting diodes is being hampered by constraints imposed by current non-native substrates. ZnO is a promising alternative substrate but it decomposes under the conditions used in conventional GaN metal organic vapor phase epitaxy (MOVPE). In this work, GaN was grown on ZnO/c-Al2O3 using low temperature/pressure MOVPE with N2 as a carrier and dimethylhydrazine as a N source. Characterization confirmed the epitaxial growth of GaN. The GaN was lifted-off the c-Al2O3 by chemically etching away the ZnO underlayer. This approach opens up the way for bonding of the GaN onto a support of choice.

Hydrogen-acceptor interactions in diamond

Abstract Hydrogen-acceptor interactions are investigated in boron-doped diamond through deuterium diffusion experiments followed by SIMS measurements and through infrared absorption spectroscopy. From deuterium diffusion, we show that BD interactions can be properly demonstrated in low compensation B-doped homoepitaxial layers. However, the presence of defects in such layers strongly affects this interaction. The degree of passivation of boron acceptors by deuterium depends on the diffusion temperature. At 550°C or below, the B and D concentrations exactly match giving rise to a complete disappearance of the absorption bands related to the electronic transitions of neutral boron acceptors. Under thermal annealing above 500°C, (B,D) pairs dissociate and neutral boron acceptors recover. At deuterium diffusion temperatures of 700°C, the B passivation is absent.

Growth of large, high purity, low cost, uniform CdZnTe crystals by the “cold travelling heater method”

Abstract The cold travelling heater method (CTHM) has been used to grow CdZnTe crystals (x Zn = 0.04 and 0.20) of 2 inch diameter, which is the largest diameter ever used in THM. A simple model confirms that convection is the dominant mechanism of matter transport in THM, justifying the use of the accelerated crucible rotation technique (ACRT), in which is imposed a forced convection regime, to enlarge the size of the crystals and increase the growth rate. In order to obtain single crystals, solid state recrystallization (SSR) has been applied for the first time to CdTe. Ingots of excellent axial and radial uniformity have been obtained by CTHM, as well as crystals purer than the Bridgman grown ones, from 5N elements as source material at ten times lower price than the 6N ones classically used for Bridgman growth. SSR crystals have been found also of lower purity than the CTHM ones. It follows that contamination in CdTe growth is expected to occur not only from the starting elements but particularly from the high temperatures used. The main agents of high temperature contamination are shown to be Li and Cu. Some mosaic structure of the crystals comes from the off-stoichiometric THM growth conditions and could be avoided by SSR annealing under Cd vapour pressure.

Systematic photoluminescence study of CdxHg1−xTe alloys in a wide composition range

A systematic photoluminescence study of CdxHg1−xTe has been carried out versus solid composition to follow step by step the evolution of the optical radiative mechanisms involved. Starting from Cd-rich samples, the recombination of bound excitons and intrinsic excitons trapped in potential fluctuations related to statistic alloy disorder has been observed over a large range of composition (0.2 < x < 1). The localization energy of excitons has been estimated from the thermal behaviour of luminescence; it exhibits a compositional dependence proportional to the alloy disorder function x(1 − x) and it amounts to about 10 meV for x ≈ 0.5.

Strain-Fields Effects and Reversal of the Nature of the Fundamental Valence Band of ZnO Epilayers.

We examine the influence of strain fields in ZnO epilayers. We show that a reversal of the nature of the fundamental valence band can be observed similarily to what was reported in GaN epilayers.

Some aspects of the MOCVD growth of ZnO on sapphire using tert-butanol

The growth of ZnO on (0001) sapphire substrates using metalorganic chemical vapor deposition is reported. Diethylzinc and tertiarybutanol were used, respectively, as zinc and oxygen sources. Growth conditions are detailed such as the substrate temperature and the precursors partial pressures. The influence of the cleanness state of the MOCVD silica reactor is emphasized, since it modifies both layer quality and crystalline orientation, and since it also affects growth process steps like sapphire thermal treatment and buffer layer deposition. ZnO epitaxial layers are characterised by scanning electron microscopy (SEM) to assess the surface orientation and morphology, X-ray diffraction (XRD) and photoluminescence (PL). This last technique demonstrates the high optical quality of the ZnO epilayers.