In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.

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Fundamentals of zinc oxide as a semiconductor

Sensor technology is one of the most important key technologies of the future with a constantly increasing number of applications, both in the industrial and in the private sectors. More and more gas sensors are used for the control of technical processes, in environment monitoring, healthcare, and automobiles. Consequently, the development of fast and sensitive gas sensors with small cross sensitivity is the subject of intense research, propelled by strategies based on nanoscience and -technology. Established systems can be improved and novel sensor concepts based on bottom-up approaches show that the sensor properties can be controlled by molecular design. This Review highlights the recent developments and reflects the impact of nanoscience on sensor technology.

Metal and Metal Oxide Nanoparticles in Chemiresistors: Does the Nanoscale Matter?

Porous silicon in drug delivery devices and materials

Recent advances in nonsilica fiber technology have prompted the development of suitable materials for devices operating beyond 155 mu m The III-V ternaries and quaternaries (AlGaIn)(AsSb) lattice matched to GaSb seem to be the obvious choice and have turned out to be promising candidates for high speed electronic and long wavelength photonic devices Consequently, there has been tremendous upthrust in research activities of GaSb-based systems As a matter of fact, this compound has proved to be an interesting material for both basic and applied research At present, GaSb technology is in its infancy and considerable research has to be carried out before it can be employed for large scale device fabrication This article presents an up to date comprehensive account of research carried out hitherto It explores in detail the material aspects of GaSb starting from crystal growth in bulk and epitaxial form, post growth material processing to device feasibility An overview of the lattice, electronic, transport, optical and device related properties is presented Some of the current areas of research and development have been critically reviewed and their significance for both understanding the basic physics as well as for device applications are addressed These include the role of defects and impurities on the structural, optical and electrical properties of the material, various techniques employed for surface and bulk defect passivation and their effect on the device characteristics, development of novel device structures, etc Several avenues where further work is required in order to upgrade this III-V compound for optoelectronic devices are listed It is concluded that the present day knowledge in this material system is sufficient to understand the basic properties and what should be more vigorously pursued is their implementation for device fabrication (C) 1997 American Institute of Physics

The physics and technology of gallium antimonide: An emerging optoelectronic material

During the last years, ZnO thin films have been studied extensively due to their potential applications in e.g. piezoelectric and optoelectronic devices or photovoltaic cells. Ordered c-axis orientation of ZnO crystallites is desirable for applications where crystallographic anisotropy is a prerequisite such as for short-wavelength semiconductor diode lasers (SDLs), and piezoelectric surface acoustic wave or acousto-optic devices. Many works were dedicated to c-axis oriented ZnO thin films elaboration and the study of their properties, including physical and chemical methods. For instance, sol–gel processes are particularly well adapted to produce ZnO films in a simple, low-cost and highly controlled way. This review summarizes the main chemical routes used in the sol–gel synthesis of undoped ZnO thin films and highlights the chemical and physical parameters influencing their structural properties. In this process, the ZnO films synthesis includes three principal steps: (i) solution preparation, (ii) coating and (iii) heat treatment. For the first step, the particle formation is discussed including nucleation and growth, particle size, morphology and colloids stability. These three steps involve several parameters such as: (i) nature and concentration of precursor, solvent and additive, and solution aging time, for the chemical system, (ii) coating method, thickness and substrate for the coating step, and (iii) pre-and post-heat treatment for the last step. The influence of these steps and synthesis parameters on ZnO thin films orientation is discussed.

Sol–gel-deposited ZnO thin films: A review

The techniques of micromachining silicon are used for the manufacture of an anemometer with low electric consumption and great sensitivity. To reduce the energy consumption, a suspended membrane of silicon rich silicon nitride SiNx makes it possible to carry out the heat insulation between the heater and the substrate. Platinum (Pt) thin film (3000 A) with titanium (300 A) adhesion layer on SiNx/Si substrate is used for the hot resistor. Among the methods of Pt deposition tested, electron beam evaporation gives the best results for the temperature coefficient of resistance (TCR) of Pt. Its response time is about 6 ms. Sensitivity in laminar and turbulent flow range are respectively 4.80 mV/(m/s)0.45/mW and of 0.705 mV/(m/s)0.8/mW for about 20 mW power supplied. The experiments show that the temperature rise of the sensor is not sensitive to the ambient temperature. Moreover, sensor response shows no significant changes according to parallel or perpendicular orientation of the gas flow.

Anemometer with hot platinum thin film

ZnO thin films were deposited on sapphire, glass and silicon substrates by r.f. magnetron sputtering using metallic zinc target. A systematic study has been made of the influence of substrate temperature on the film structural properties. They exhibited a c-axis orientation of below 0.5° full width at half maximum of X-ray rocking curves, an extremely high resistivity of 1010 Ω cm and an energy gap of 3.3 eV at room temperature. It was found that a substrate temperature of 100 °C and target/substrate distance about 50 mm, very low gas pressures of 3.35×10−3 Torr in argon and oxygen mixed gas atmosphere giving to ZnO thin films a good homogeneity and a high crystallinity.

Structural properties of zinc oxide thin films prepared by r.f. magnetron sputtering

Metal organic chemical vapor deposition (MOCVD) has been investigated for elaboration of Bi 2 Te 3 and Sb 2 Te 3 using TMBi (Trimethylbismuth), TESb (Triethylantimony) and DETe (Diethyltellurium) as metal–organic sources. Their thermoelectric and physical properties were studied versus growth conditions. The MOCVD elaboration of Bi 2 Te 3 and Sb 2 Te 3 was carried out in an horizontal reactor for a temperature varying from 400 to 500°C, a total hydrogen flow rate D T varying from 3 to 6 l mm −1 and ( R VI/V ) ratio ranging from 1.5 to 15. The thin films were deposited on pyrex and silicon substrates. The partial pressure of the V element varied between 0.5 10 −4 to 2 10 −4 atm to obtain high growth rate for micro-peltier applications. The cristallinity was investigated by X-ray diffraction and we observed a typical preferential c -orientation. The SEM micrographs show the layers quality and confirms the hexagonal structure. The microprobe data indicate that the stoichiometry of Bi 2 Te 3 and Sb 2 Te 3 is constant for all thickness of the epitaxial films (0.3–7 μm). The films are always n-type conduction for Bi 2 Te 3 and p-type for Sb 2 Te 3 . Seebeck coefficient and the minimum values of the resistivity were found close to −210 and +110 μV K −1 , 9 and 3.5 μΩ.m for Bi 2 Te 3 and Sb 2 Te 3 , respectively. Electrical measurements (mobility and carrier density) were performed by Van der Pauw method. For the two materials, the best values of thermoelectrical properties were obtained at a growth temperature closed to 450°C and a VI/V ratio varying from 2 to 8. The thermoelectric properties of the two materials stay constant when the growth rate is increasing to value higher than 1.5 μm h −1 . This result is very interesting for thick film applications. The previous objective of these experimental results has been to perform the thermoelectric properties of n- and p-type films by establishing first suitable deposition conditions and the elaboration of ternary alloys is now possible.

Growth of Bi2Te3 and Sb2Te3 thin films by MOCVD

This paper describes an application of Quartz Crystal Microbalance (QCM) used as humidity sensor. Moisture apparition is detected by using a QCM associated with a Peltier module. When water condensation produced by the Peltier cooling appears on the QCM, a change of mass on the crystal sensitive surface results in the decrease of the resonant frequency. If we measure the delay time between the beginning of Peltier supply and the apparition of water condensation on the quartz, we determine the relative humidity and the condensation velocity. A study of thermal transfer is first presented. Relative humidity measurements are then realised in a climatic chamber. A theoretical approach is finally compared with the experimental results.

Frédérique Pascal-Delannoy

Papers

Anemometer with hot platinum thin film

Structural properties of zinc oxide thin films prepared by r.f. magnetron sputtering

Growth of Bi2Te3 and Sb2Te3 thin films by MOCVD

Quartz Crystal Microbalance (QCM) used as humidity sensor

Porous silicon layers used for gas sensor applications