Semiconductor MQWs represent a new technology for opto-electronics. These MQWs have an electroabsorption effect approximately 50 times larger than conventional semiconductors. They are compatible with existing source and detector material systems and produce devices that are compact and high speed, which makes them useful for monolithic integrated optoelectronic devices.

In certain cases. GaAs/AlAs superlattices may be type II, electrons being spatially separated from holes. Thus. fundamental optical transitions are weak. Under longitudinal electric field. this separation may be reduced and optical transition probability increased. We report preliminary photoluminescence experiments which evidence this effect. A theoretical description using the envelope function approximation is given.

Large change in the field induced refractive index is achievable by the use of the MQW structures. The property of which can be advantageously utilized to realize high speed external modulators and switches. The GalnAsP/InP MQW structures was prepared by LPE method and the electric field induced absorption was measured. Reflection of light due to refractive index change as well as absorption coefficient change was observed for the first time. Intersectional optical switch based on this change was also fabricated.

Doping superlattices, composed of alternating n- and p-doped semiconductor layers, possibly with intrinsic regions in between ("n-i-p-i structures") exhibit novel electrical and optical properties. Their potential for a wide variety of new electro-optical and opto-optical devices is discussed and recent results are reported.

We present a theoretical survey of a set of electro-optic switches based on the quantum confined Stark effect, QCSE, and quarter wave stacks in GaAs/ AlGaAs semiconductors.

It is about 10 years since the first published report of laser action at room temperature in a quantum well structure, electrically pumped from a p-n junction(1). Research on the basicproperties of quantum well structures had been in progress pince about 1973, made possible by the developments of molecul4v,beam epitaxy (MBE) 0,3), and later of metalorganic chemical vapour deposition 04011CVD)k4), as techniques for epitaxial growth of thin layers of III-V semiconductors. In his review of 1975 Dingle(5) cited the ability to tune the laser emission wavelength by adjusting the well width, and the fact that the step-like density of states function should modify the gain characteristics compared with the bulk, as attractive features of a quantum well laser. An account,of laser action in such structures, achieved by optical pumping, was published in 19760). It is the purpose of this paper to assess the current capabilities of quantum well lasers, fabricated in the GaAs/AlGaAs materials system, and to review our understanding of their operation. In so doing we adopt a historical view and compare the device performances which have been achieved with the initial expectations, and discuss the reasons for the differences which have emerged. We also review attractive features of QW lasers which have come to light in the course of this research activity. This article falls into three main sections: a brief account of the initial expectations of quantum well lasers, a summary of the performance which has actually been achieved, and an account of our current understanding of the operation of quantum well lasers.

A model for the determination of the threshold current of quantum well laser is presented, using a no-k-selection rule for the gain calculation and taking into consideration propaga-tion and confinement properties of the waveguide. A graded refractive index separate confinement heterostructure (GRIN-SCH) was fabricated by molecular beam epitaxy. Threshold currents of 11 mA are obtained on ridg (3 x 200 4m) devices from a wafer for which the broad area threshold density is 380 A/cm². Gain measurements performed on these lasers show the good fit between experimental spectra and theoretical spectra obtained with the model.

The temperature sensitivity of threshold current of double quantum well lasers is studied both theoretically and experimentally over the temperature range 77 K to 300 K. It is found that these devices behave predictably up to 160 K but that the model used fails to describe the temperature sensitivity beyond 160 K. Lasers with reduced threshold gain have a lower temperature sensitivity around room temperature at the expense of an increased threshold current density.

High speed photoluminescence (PL) switching by electric field-induced carrier separation inside the Quantum Well (QW) at room temperature, combined with carrier escaping out from the well to the barrier layers is demonstrated to be free from carrier life time limitation. A new technique for evaluating radiative lifetime separated from over-all life time is also shown. Based on the experimental data, possible device structures with functions of carrier-injection and of field control for practical application will be discussed.

Several workers have shown that the quality of MBE material for AlxGa(1-x)As-GaAs semiconductor laser diodes can be improved by incorporating additional layers into the laser structures. One method is the introduction of superlattice prelayers of AlxGa(1-x)As-GaAs bcfore the growth of the active region. This has been showp to improve the optical qualitykl) and to reduce tb.e interface recombination velocities l2) and has given laser diodes of high performance 0). A related technique to using superlattice prelayers is to use extended regions of superlattices as parts of the laser structure. The superlattices can be of AlxGa(l_x)As-GaAs or cap be all-binary structures of AlAs-GaAs. These have been applied to the cladding regions (4-0), the barrier regions k5,7) and the active regionko), though only with a superlattice of AlxGa(l_x)As-GaAs. Although these structures have in many cases given rise to laser diodes having good performance, the characteristics of the superlattices and the nature of their influence on device performance are by no means well understood. In particular it is not clear whether the good performance is due simply to improvements in morphology and interface properties or whether there are modifications to the physics of the gain generating process in the device. To get a better understanding of the operation of these devices we have explored the use of all-binary superlattices for the cladding, waveguide and barrier regions of laser diodes using GaAs quantum wells and we have compared them with similar structures grown using alloys in place of superlattices. We have made alloy devices with and without an all-binary prelayer. We also report the use of binary superlattices in the active region of a conventional laser, we believe, for the first time. We have investigated the threshold current, and its temperature dependence, of 50pm wide oxide stripe laser diodes embodying these structures. We have also made broad area devices of differing lengths in order to examine the gain-current and loss characteristics. Finally we have investigated the spontaneous emission spectra of light emitted through a narrow window in the top contact of superlattice and alloy devices. These investigations have shown that all-binary superlattices give lasers as good as the best comparable conventional alloy quantum well lasers as far as the threshold current is concerned, and have a weaker temperature dependence. We have also seen that the gain-current relationship shows similar saturation behaviour to normal alloy barrier quantum well devices. The spontaneous emission spectra indicate that the description of binary superlattices in terms of the average effective alloy composition is not adequate for an accurate modelling of the device.

The electronic states of a short asymmetric GaAs/AlAs superlattice have been studied by monitoring the optical transitions involving the superlattice resonances of such a structure embedded in a GaAs p-n junction. Three electroluminescence features arise which involve the superlattice states, one very strongly dependent on the bias applied to the p-n junction. The intensity of the emission involving superlattice states increases relative to that from the bulk when a region of negative differential resistance in the I-V characteristic is crossed. Photoconductivity measurements show four main features, three of which can be associated with the emission. Field dependences of these transitions are presented, and some initial conclusions drawn concerning the nature of the electronic states involved.

The optical oscillator strengths between valence bands and conduction band minmum in (GaAs)(AlAs)n, (GaAs)n/(AlAs)i and (GaAs)i/(AlAs)n (n =1≈10) superlattices are estimated by means of an improved tight binding method in which the overlap integrals up to the second nearest neighbor atoms, including new parameters, are explicitly taken into account. Kroemer's rule of band offset values is employed in order to investigate the influence of tetragonality of [001] superlattice structure on the optical polarizations of oscillator strength with and without the existence of spin-orbit interactions. It is indicated that the superlattices allow us to tailor band structure. The examples studied here show that this tailoring can result in a considerable sacrifice of optical oscillator strength. However the possibility remains of designing another superlattice structures without sacrifice of oscillator strength.

We present a calculation which shows that the optical crossection of the quantum well intersubband transition may be enhanced by a factor of fifty. This allows us to speculate on a design for a fast electro-optic modulator with high efficiency, for say carbon dioxide lasers. The enhancement ought also to have relevance for detectors but we do not consider the transport aspects of that problem.

We examine the structural and electronic properties of heterostructures grown out of lattice mismatched III-V compound semiconductors, with emphasis on their ability to serve as reliable materials for device applications. We also give a brief review of the most striking experimental results reported in the literature, in order to illustrate the diversity of interesting and specific features displayed by thin biaxially strained layers.

The internal strain resulting from lattice mismatch between thin active layers and a thick substrate can confer considerable advantages to long wavelength laser characteristics. With sufficient built-in strain, the highest hole band has a low effective mass and is well separated from the lower bands. The low effective mass reduces the areal carrier density needed for population inversion and leads to the virtual elimination of two important loss mechanisms: Auger recombination and intervalence band absorption. We consider a separate confinement heterostructure laser with strained InGaAs wells, grown on an InP sub-strate. This structure promises easier growth and greater stability than structures considered earlier. The valence band structure is calculated using the Luttinger-Kohn 6x6 Hamiltonian in the axial approximation. The threshold current density is of 220Åcm^-2 at room temperature, nearly an order of magnitude lower than conventional 1.55μm lasers. Furthermore, we predict an increased temperature stability, of advantage for long wavelength optical communication applications.

A detailed experimental and theoretical study of the optical transitions in strained quantum wells has been carred out. The quantum wells are grown so that the well region is under biaxial compressive (InvGai As/A1 Gai As system) or under biaxial tensile (GaAs/ InGaAlAs or GaAsP/A1GaAs systems)ltrainY. lfiYs allows the possibility of reversing the light hole-heavy hole ordering. Theoretical studies are based on solving the Kohn-Luttinger Hamiltonian for the hole state after taking account of the strain via a deformation potential formalism. Experimental data were obtained from low-temperature absorption measurements. Emphasis has been given, in particular, to quantum well structures with biaxial tensile strain in the well region. In this structure, by a careful selection of strain, coincidence of light hole (LH) and heavy hole (HH) states has been achieved and this has consequently enhanced absorption coefficient by a factor of about 2. Theoretical studies on the GaAs (well)/InGaAlAs (barrier) and GaAsP/AlGaAs structures confirm this enhancement is due to increase in the oscillator strength due to the merger of the HH and LH excitons and an increase in joint density of states as a result of large in-plane hole masses. This behavior opens up the possibility of artificially enhancing the quantum efficiency of high frequency photodiodes or of fabricating modulator with high on/ off ratio.

The natural lineshape of luminescence from a quantum well shows Gaussian broadening if its interfaces are not ideally abrupt. The results of a comparative study of the model quantum well system AlGaAs/GaAs/AlGaAs grown by molecular beam epitaxy with and without interruption of the growth at the interfaces is presented. In addition a detailed lineshape theory is outlined, allowing for a quantitative determination of the interface roughness distribution function. We find this function to depend in a delicate way on growth rates temperature, interruption time and chemical compositon of the growth surface. Roughness reduction upon growth interruption is analyzed in detail. For specific growth conditions and interruptions of 2 min at both interfaces formation of up to 7 gm large interface islands differing by a one monolayer step (2.8 Å) are observed. Consequently such quantum wells have a columnar structure, which can be directly visualized using cathodoluminescence imaging. Strong reduction of island size indicating transition from planar growth to three-dimensional growth is observed by CLI upon an increase of growth temperature from Tg = 600°C to 660°C.

The recent development of optoelectronic devices based on quantum wells should permit monolithic integration of active and passive optical components within the same epitaxial layers. A planar process using multiple quantum well (MQW) layers to realise this objective is described. Nonlinear optical processes will be present in such structures, and their significance is discussed. Carrier lifetime measurements performed on GaAs/AlGaAs multiple quantum well structures are reported.

Synthetically modulated structures offer the capability of tunability of bandgap and other material parameters. In particular, the GaAs/AlGaAs and Ino.53Ga0.47As/ Ino.52A10.48^As systems offer these advantages in the infrared and optical communication wavelength ranges. We report here the properties of superlattices and the properties of avalanche and p-i-n photodiodes made with thes materials. The multiquantum well struc-tures (MQW) have L_Z and L_B varying from 25-500Å. Impact ionization phenomena in the different structures have also been determined from carrier multiplication and noise measurements and have been analyzed by Monte Carlo techniques. Techniques for obtaining enhanced optical absorption in very high-speed photodiodes are demonstrated.

GaInAs/InP and GaInAsP/InP structures have been grown at conditions optimized for laser fabrication. Across 2" wafers a homogeneity in thickness, composition and doping was achieved to better than 2 % at a total pressure of 20 mbar and high gas flow rates. The abruptness of the transition for quantum wells ranging from 0.5 to 50 nm in width is on the monolayer level. Photoluminescence line shifts (2K) are among the highest observed so far (max. 528 meV); the line widths are very small (i.e. 2.2 meV for 20 nm wells).