The Colloid Chemical Approach to Nanostructured Materials

Wide-bandgap II–VI compounds are been applied to optoelectronic devices, especially light-emitting devices in the short-wavelength region of visible light, because of their direct gap and suitable bandgap energies. Many methods have been extensively applied to grow high-quality films and bulk single crystals from the vapor and liquid phases.

Wide-Bandgap II-VI Semiconductors: Growth and Properties

An important aspect of the large expansion in the development and production of solid-state devices has been the increasing demand for more sophisticated multilayered semiconductor crystals. During the last decade a new thin film growth technique, molecular beam epitaxy (MBE), has been developed to provide improved control over composition, thickness, and doping profile in the direction of growth on an atomic scale. Based on these unique features, molecular beam epitaxy offers the feasibility of tailoring the electronic properties of the crystal to its desired function by growing high-quality epitaxial thin films with predetermined compositional and/or doping profiles perpendicular to the growth surface. The attractive capabilities of MBE for large-area uniform growth with extreme control over microscopic dimensions suggest that both characteristic and reproducibility of microwave and optoelectronic device structures may be improved. It is the purpose of this article to review the basic process of molecular beam epitaxy as a crystal growth technique, to indicate present trends and material problems, and — at a time of continuing innovation — to show promise for future application. The emphasis of this review is on the growth and properties of crystalline III–V compound semiconductor thin films focusing mainly on GaAs and AlxGa1-xAs.

Molecular Beam Epitaxy of Iii-V Compounds

We initiated the present study to investigate experimentally a fundamental quanturn-mechanical effect—the resonant transmission of electrons through potential barriers in semicon- ductors. The phenomenon of such resonance is a familiar one. In the simple case of a double barrier, the incident electrons are able to tunnel through both barriers without attenuation if their energies coincide with the resonant energies.1 In the caSe of a series of equally spaced barriers, e.g., that of the KronigPenney model of a one-dimensional crystal, the electronic structure exhibits forbidden bands of attenuation that are separated by allowed bands where perfect transmission of electrons would occur.

Superfine structures of semiconductors grown by molecular‐beam epitaxy

The performance of quantum well infrared photodetectors (QWIPs) as compared to other types of semiconductor infrared (IR) detectors is examined utilizing the α/G ratio, where α is the absorption coefficient and G is the thermal generation. From the discussion results that long wavelength QWIP cannot compete with HgCdTe photodiode as the single device especially at higher temperature operation (> 70 K) due to fundamental limitations associated with intersubband transitions. However, the advantage of HgCdTe is less distinct in temperature range below 50 K because of the problems involved in an HgCdTe material (p-type doping, Shockley-Read recombination, trap-assisted tunneling, surface and interface instabilities). Due to the limited charge storage capacity of available readout circuits, a small photoconductive gain of QWIPs improves the noise equivalent difference temperature the sensor. The imaging performance of HgCdTe photodiodes and QWIP focal plane arrays can be said to be nearly equivalent when they are operated under their optimum conditions. Decision of the best technology is therefore driven by the specific needs of a system.

L.L. Chang

Papers

2.2 – Molecular-Beam Epitaxy†