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The authors present a review of past, present, and future blue/green laser technologies. They discuss: II-VI semiconductor materials and alternate approaches (including the use of SiC and III-V nitrides, e.g. GaN, AlN); laser diode design; multiple quantum well approaches; contacting to p-ZnSe; and remaining challenges. >

The first blue/green laser diode demonstrations reported in the summer of 1991 presented a hope that viable light-emitting devices based on the wide-bandgap II-VI compound were truly possible. In this paper we place those developments into the context of the 30-year research effort which has opened the possibility for the new laser diode, LED, and display devices operating in the short visible wavelengths. Active research interests worldwide have led to rapid progress in the field, culminating in the recent demonstration of room-temperature continuous wave diode lasers. >

The physical features and practical challenges for wide bandgap II-VI semiconductor blue-green light emitters, both LED's and diode lasers, are reviewed in this rapidly moving field. Fundamental issues of optical and transport phenomena in the polar ZnSe-based compounds are examined, together with their impact on expected device performance. Examples of current device performance in the quantum well emitters are given, which have culminated in the recent demonstration of a room temperature CW diode laser. >

The advantages of semiconductor lasers that produce shorter wavelengths and the underlying physics that make them difficult to achieve are examined. The breakthroughs that have brought a semiconductor laser within reach are described. These include advances in fabricating and doping the large bandgap II-VI semiconducting compounds ZnSe and ZnS on which the new lasers are based and the development of quantum well structures. Integrated design, initial performance, and epitaxial fabrication of the first blue-green emitting laser diode heterostructures are discussed. Some alternative approaches are briefly considered. The problem of developing a contact to connect the laser, or a blue-green light emitting diode based on the same technology, is examined. >

Publisher Summary This chapter presents the fundamental physics and the device-related scientific aspects of II-VI semiconductor visible light emitters. Excitons in these materials are relatively stable and play an important role in light emission. Doping and electrical contacts present special problems in these materials. Defect formation energies are relatively small and promote device degradation. The chapter also discusses a range of compounds from which present light-emitting structures are being fabricated, by focusing on the description and characterization of the electronic states that are central to the quantum well and heterojunction physics of ZnSe-based heterostructures. Doping and electron-hole transport of the wide band-gap II-VI semiconductors, both in thin films and in heterostructures are presented in the chapter along with an examination of the physics of light emission by II-VI heterostructures in the context of spontaneous and stimulated emission by Coulomb correlated quasi-2D (quasi- two dimensional) electron-hole pairs.

The recent proof-of-concept demonstration of a blue-green diode laser in 1991 [1],[2] relied in part on the useful optical properties of the (Zn,Cd)Se/ZnSe and (Zn,Cd)Se/Zn(S,Se) quantum wells. These included large exciton effects to room temperature and beyond [3], a particularly unusual circumstance in wide gap II-VI semiconductors, given the large exciton-LO phonon interaction. However, due to quantum confinement, the binding energy in a ZnCdSe QW can exceed the LO-phonon energy so that excitons can in principle dominate also spontaneous emission to device temperatures. So far, most of the characterization of the new diode lasers has been performed at T = 77K, a temperature where the 2D excitons are particularly robust. The role of excitons in providing gain has been argued by us to be a physical reality in the ZnCdSe/ZnSe system [4], a feature which takes additional potential significance in terms of future diode laser design and development.

The culmination in the development of wide bandgap II-VI compound semiconductors towards light emitting applications in the blue-green has been the recent demonstration of diode laser action by the 3M group [1] and the Brown-Purdue team [2]. In this paper we show different variations in the heterostructure design to produce high power, high efficiency laser action, which is especially clear at low temperatures. Our system is based on the (Zn,Cd)Se/ZnSe or (Zn,Cd)Se/Zn(S,Se) QWs the former of which had earlier been shown by us to show good electronic confinement [3], enhanced excitonic effects [4], and good optically pumped laser action, including cw operation at T=77K [5].

A “two-carrier” computer simulation showing the transient response of BARITT diodes to an applied current step is presented. The internal dynamics of the diode in terms of the time evolution of the electric field, electron and hole densities as well as both conduction and displacement currents are discussed. The dynamics of the solution reveals the formation of a low field region at the injecting contact, a result which is not obtainable from “one-carrier” models of BARITT diodes. A small signal analysis is performed on the steady state solution and demonstrates an increased differential negative resistance (DNR) for increased device length at a given doping, and an increased DNR coupled with a wider frequency bandwidth for a decrease in carrier lifetime at the injecting junction.

Diluted magnetic semiconductors (DMS) are mixed semiconducting crystals whose lattice is made up in part of substitutional magnetic ions. Cd1−xMnxTe and Hg1−xMnxTe are examples of such materials. Their structural and band parameters can be “tuned” by composition over a wide range. They can thus be exploited in situations completely similar to those involving Ga1−xAlxAs. Using molecular beam epitaxy, we have grown Cd1−xMnxTe superlattices with alternating Mn content, having up to 150 layers, with layer thickness ranging from 50 to 100 A. The superlattice structure is clearly revealed by transmission electron microscopy and by zone-folding of the phonon spectrum observed in Raman scattering. Photoluminescence observed on Cd1−xMnxTe superlattices is several orders of magnitude greater than that from a Cd1−xMnxTe film with uniform Mn content, or from bulk Cd1−xMnxTe specimens. The presence of localized magnetic moments in DMS results in a strong exchange interaction between these moments and band electrons. This in turn leads to gigantic Zeeman splittings of impurity states, exciton levels, Landau levels, and the bands themselves. Zeeman splittings as large as 20 meV (which in non-magnetic semiconductors would require unrealistic megagauss fields) are easily achieved in DMS in fields of several kilogauss. Since the magnitude of this exchange-induced splitting in DMS can be comparable to the binding energies and to the minigaps encountered in multiple quantum wells, DMS superlattices hold promise of a host of novel effects of both fundamental and applied interest.

A study of the internal dynamics and microwave properties of an X-band transferred-electron device having ohmic contacts is reported. Numerieal calculations of the electric field evolution and large-signal properties of the device have been carried out for frequencies throughout the 6-12-GHz range and for a wide range in ac voltage amplitude including amplitudes insufficient for space-charge quenching. An investigation is made of the influence of electron diffusion upon the field evolution and properties of the device. The large-signal admittance plane is determined and used to examine the behavior of the device in microwave circuits. A comparison is made between the calculated device properties and those of experimental devices. Circuit-controlled oscillation is found to be possible throughout the 6-12-GHz range for ac voltage amplitudes well below those required for space-charge quenching. At these amplitudes diffusion is shown to have a strong effect upon the electric field evolution and large-signal properties, and this effect is shown to bear significantly upon the behavior of the device in microwave circuits. Finally, the numerically calculated large-signal properties are found to agree reasonably well with experimental results.

Recent developments in acoustic surface wave (ASW) device applications are based on a coupling between an ASW and charge carriers in a semiconductor. The problems related to such devices are similar to those encountered in MOS and CCD work since the wave-charged particle interaction occurs within a Debye length of the surface. This paper presents a brief review of ASW-semiconductor devices.

A lumped equivalent circuit is used to represent the impedance presented to the device terminals for a waveguide-mounted Gunn oscillator. Analytic expressions are used for all the lumped elements. This circuit model is used to predict tuning curves which are compared to experimental tuning curves obtained for iris coupling, for full- and reduced-height waveguides, and for various post diameters. Iris-controlled nontunable modes are reported and explained by the model. The real part of the load impedance presented at the device terminals is found to agree with Copeland's prediction for optimum loading. Experimentally observed mode switching is explained with reference to variations in device parameters.

An analytic model for electronic tuning of an X-band waveguide transferred-electron oscillator is presented. The oscillator is electronically tunable by a varactor, and mechanically tunable by movement of a short circuit. The model is used to predict oscillation frequency, maximum electronic tuning range, and electronic tuning versus varactor bias voltage. Two different methods, the "zero reactance theory" and the Slater perturbation theory, are used to calculate the electronic tuning. The results of these calculations are compared to experimental results for two different oscillator configurations.

Substantial interactions between drifting carriers in a semiconductor and slow circuit waves on meander lines have been demonstrated for the first time. Electronic gains in the meander‐meander line signal of up to 40 dB/cm were obtained with Ge at 4.2°K. Although no net gain was achieved with respect to the ports of the meander lines due to the structure's high transmission loss of 20–30 dB, the results demonstrate the possibility of solid‐state slow‐wave amplification which has been the subject of many previous theoretical studies.

In 1885 Lord Rayleigh predicted the existence of acoustic surface waves (ASW) as a mode for which acoustic energy is confined to within one wavelength of the interface between a semi-infinite solid and vacuum. Since about 1968 renewed interest in this mode resulted from transducer and piezoelectric material developments with emphasis on applications to high frequency signal processing. More recently, it has been shown that ASW have application to rapid scan optical imaging. This is made possible by a configuration combining a piezoelectric substrate with a semiconductor such that photoexcited charge carriers interact with propagated ASW. The potential is for a new class of solid state optical imaging devices which are not restricted to silicon technology, can employ narrow bandgap semiconductors, and do not require elaborate interconnecting leads to read out the image. The image is formed on the semiconductor continuum and is scanned by a propagating ASW pulse. Existing devices have scan times oi the order of 5 sec, while proposed versions may reduce this by a factor of ten. The operation of these devices depends critically on two phenomena. Firstly, the photoexcited carriers must modify the ASW propagation when piezoelectrically coupled to the wave; these modifications include attenuation, impedance variations, and harmonic generation. Secondly, an additional wave is used to decouple the reading pulse from the carriers; this second pulse is called the scan pulse and is vital to the imaging schemes employing attenuation or harmonic generation. Several device configurations using bulk and thin film semiconductors have been reported and were discussed together with the experimental results. It was shown how positive and negative images and their Fourier transforms could be obtained.

A comparative evaluation of rf diode and magnetron sputtered ZnO-SiO2-Si structures is described and summarized. The comparison incorporates b oth electrical data and physical data derived from SEM, X-ray diffraction, and other analytical techniques. The interfacial trap and net effective charge densities at the Si-SiO2 interface, ZnO film morphology, ZnO film conductance, and the electrical and physical effects of post-deposition a nnealing are some of the topics examined in the comparison

A forward‐propagating surface‐acoustic‐wave signal interacts with a pump field applied to a nearby semiconductor. The result is the generation of a backward‐propagating ``echo'' whose envelope is the correlation of the signal and the pump. The echo is 25 dB down from signal for the pump power used.