Mid-wave infrared lasers have been fabricated employing InAs/A1Sb superlattice cladding layers and multi-quantum well active regions consisting of Ga75In025Sb1InAs broken-gap superlattice wells and Ga75In025As023Sb,,77 barriers. Diodes demonstrated to date include lasers with emission wavelengths of 3.18j.tm at 255K, 3.40im at 195K, and 4.32p.m at 110K. Keywords: infrared, laser, diode, superlattice, multi-quantum well

A new class of quantum cascade lasers based on type-II quantum wells is analyzed. In these novel mid- and long-wavelength IR lasers, not only can a population inversion be easily created with a nearly 100% current injection efficiency, but also the nonradiative loss from the optical phonon scattering can be greatly suppressed. A general description of how the lasing threshold current depends on the injection, radiative, and inversion efficiencies is formulated to illustrate the expected improvements over the recently reported quantum cascade laser. Also, the features that distinguish quantum cascade lasers from traditional bipolar lasers are discussed in the contest of the carrier transport in equivalent circuit models to illustrate the advantages of quantum cascade lasers for high power mid- and long-wavelength IR source applications.

We have investigated the intersubband relaxation times in asymmetrical step quantum wells with a high width ratio and observed population inversion between the state in the step and the state in the well. Based on the observed population inversion a novel intersubband laser structure at 10 micrometer wavelength is proposed.

The coupling effects between quantum wells (QWs) on the small-signal response of unstrained multiple QWs (MQWs) are studied theoretically in a two-band-model approximation. A finite- well model and flat-band conditions were assumed. For any barrier thickness between QWs the gain spectrum showed many peaks located at heavy-hole and light-hole transitions. Therefore, the lasers under investigation are being treated as multiple-level systems. It has been shown that the dependence of this peak is a function of the barrier thickness. The traditional rate equations were reformulated and solved for the intensity modulation response. Results and discussion for the characteristics referred above of a GaInAsP-InP double quantum well laser are presented as a specific example. We conclude that, in contrast to the common belief, the coupling effects can enhance the modulation characteristics under certain conditions.

The effects of bandfilling and bandgap shrinkage in the femtosecond absorption saturation measurements of GaAs have been studied using femtosecond pulses generated from CPM dye laser and self-mode-locked Ti:sapphire laser. For exciting photon energy of 2 eV and carriers density of 1 by 10¹⁸ cm^-3, an optical induced absorption increase is observed and is attributed to the bandgap shrinkage. The dependence of the change of absorption coefficient on photon energy, temperature and excited carrier densities is discussed.

In this paper we show that strain is a useful effect and in addition to improving the performance of existing devices it may be used with greater functionality to demonstrate novel optoelectronic devices. We give as examples two such devices that we have conceived and demonstrated, one each in the two respective areas of strain, lattice-mismatch induced and thermal expansion coefficient mismatch induced. The higher performance and functionality in these devices demonstrate that strain engineered heterostructures are a very promising area for device research and development.

Time-resolved excite-probe measurements were performed on GaAs multiple quantum wells to determine the spin relaxation time as a function of well width (L) at room temperature, and as a function of temperature in the range of 10 to 150 K. These studies suggest that the room temperature spin relaxation time follows an exponential well width dependence, indicating that at room temperature spin flip is dominated by a combination of the D'Yakonov-Perel and the Bir-Aronov-Pikus mechanisms. The low temperature results suggest that a different mechanism, the exchange interaction, is dominant at low temperatures.

Vertical cavity surface emitting lasers (VCSELs) with very low threshold current and voltage of 340 (mu) A and 1.5 V is achieved. The molecular beam epitaxially grown wafers are grown with a highly accurate, low cost and versatile pre-growth calibration technique. One- hundred percent VCSEL wafer yield is obtained. Low threshold current is achieved with a native oxide confined structure with excellent current confinement. Single transverse mode with stable, predetermined polarization direction up to 18 times threshold is also achieved, due to stable index guiding provided by the structure. This is the highest value reported to data for VCSELs. We have established that p-contact annealing in these devices is crucial for low voltage operation, contrary to the general belief. Uniform doping in the mirrors also appears not to be inferior to complicated doping engineering. With these design rules, very low threshold voltage VCSELs are achieved with very simple growth and fabrication steps.

Tuning of the vacuum Rabi exciton-photon coupling in semiconductor quantum microcavity structures by external electric and magnetic fields is presented. Contrasting effects due to the two external perturbations are found, principally due to the decrease of exciton oscillator strength in electric field and increase of oscillator strength in magnetic field. The strong coupling limit is easily achieved in these high quality structures with line widths on resonance as small as 1 meV. In addition to the tuning phenomena, the effects of exciton Zeeman splitting on the spectra and unexpected linewidth narrowing phenomena on resonance are reported.

The possibility of quantum efficiency enhancement in GaAs/AlGaAs quantum well infrared photodetectors (QWIP) by means of waveguide propagation of radiation in superlattice is investigated in this paper. Epitaxial structures for photodetector manufacturing were grown by low pressure metal organic chemical vapor deposition (LP-MOCVD) on high doped (3 (DOT) 10¹⁸ cm^-3) n-type substrates. The use of these high doped conductive substrates allows us to achieve an abrupt change of refractive index on interface between superlattice and substrate. Due to this fact optical restriction of electromagnetic wave propagation along superlattice arises. A fine structure with peaks ((Delta) (lambda) equals 0.1 mkm) was found on the photosensitivity spectra of this QWIP ((lambda) _max equals 9 mkm). We consider this effect can be explained by arising of standing waves in volume of QWIP. It indicates one possibility of waveguide propagation of radiation in QWIP structures grown on high doped conductive substrates. The use of QWIP on conductive substrates allows us to increase a quantum efficiency and to simplify the technology QWIP-lines manufacturing.

We review results of logic and memory devices and circuits based on the negative differential resistance associated with resonant tunneling and interband tunneling effects. We have fabricated resonant interband tunneling field effect transistors on both InAs/GaSb/AlSb and InGaAs/InAlAs/InP material systems. A new exclusive-NOR device has also been demonstrated. Preliminary results of a FULL ADDER are shown. Static random access memory based on the bistability of two serially connected diodes is also achieved. We show simulations and compare our devices with other approaches and discuss important issues related to applications of resonant tunneling devices and circuits.

The influence of double-barrier quantum well asymmetry on the switching time of RTDs is studied. For circuit simulation purposes the RTD is modeled as a resistance in series with a parallel combination of a non-linear dependent current source and a non-linear capacitor. The current source embodies an analytic expression for the I(V) characteristics derived from basic principles. The capacitor embodies a C(V) equation derived from a self-consistent numerical two-band model simulation of the structure. It is found that the switching time is most sensitive to asymmetry on the emitter-side barrier, and that the smallest emitter barrier width structure exhibits the smallest switching time.

We simulate the current-voltage characteristics of an InGaAs/AlAs resonant-tunneling diode under dark and illuminated conditions. The current is given by a tunneling formula that has been generalized to allow for quantum mechanical effects in the contacts. The optically generated carriers effect on the current-voltage characteristic is included through the use of a rate equation. This method of determining the optical response is shown to be accurate at low intensity and useful for extracting the recombination lifetime. The existing simulator shows great promise as a design tool for optical RTDs and related devices.

We investigated the intersubband photocurrent as a function of the bias voltage and the incident wavelength in n-type photovoltaic GaAs/AlAs/Al_0.3Ga_0.7As double-barrier quantum well (DBQW) infrared detectors. The significant photovoltaic behavior of the detectors arises from a segregation of the dopant during the growth process. For an externally applied bias voltage, which compensates the internal space-charge field, the photocurrent exhibits a multiple sign change for varying incident wavelengths. This observation can be understood in the context of resonant coupling between the excited state in the GaAs quantum well and states, which are confined in the Al_0.3Ga_0.7As-region. This coupling leads to an enhancement of the tunneling rates through the AlGaAs barriers and to a partial localization of the above barrier states in the GaAs region.

Through the use of a vertically-integrated resonant tunneling diode heterostructure, we have examined the impact of lattice-mismatch on the electrical properties of the AlAs/In_0.53Ga_0.47As/InAs resonant tunneling diode (RTD). For strained-layers below the critical thickness, the current-voltage characteristics of the RTD track the bandgap change. In contrast, for strained layers greater than the critical thickness, the current-voltage characteristics are significantly degraded in the case of three-dimensional relaxation, but retain their characteristics in the case of two-dimensional relaxation.

Theoretical formulations based on a coupled-mode theory and a supermode theory are developed for discussing transfer efficiency of guided electron waves in coupled quantum well structures. We apply a coupled-mode theory to electron waveguide couplers in the weak coupling regime and show that the transfer efficiency will not be greater than 100%. In the strong coupling regime, we can appropriately handle the electron waves in coupled quantum well waveguide structures by introducing a supermode theory, and demonstrate that the transfer efficiency is always less than 100%. We discuss the underlying physics in an attempt to gain an insight into the transfer efficiency related issues. Calculations for specific semiconductor quantum well structures such as GaAs/AlGaAs quantum wells are given for quantitative illustrations.

We propose and demonstrate the integration of a quantum well intersubband photodetector (QWIP) and a light emitting diode (LED) for making large two-dimensional focal plane arrays for thermal imaging applications. We combine the newly developed long wavelength infrared QWIP technology with the well-established near infrared LED technology both based on GaAs and related epitaxially grown alloys, such as AlGaAs and InGaAs.

In n-i-p-i structures with selective ohmic contacts to the n- and the p-layers both, carrier densities and electric fields, can be tuned over a wide range by applying moderate external voltages. The resulting absorption changes due to phase space filling and Franz-Keldysh effect, respectively, can be superimposed constructively by a suitable sample design. In contrast to optically excited n-i-p-i structures whose dynamic behavior is governed by the internal electron-hole recombination lifetimes (well up to milliseconds), the time constants for structures provided with selective n- and p-contacts are RC times given by the resistance of the doped layers including contact resistances, and by the capacitance of the interdigitated n- and p-layers. Although the areal capacitance of such n-i-p-i structures is relatively large compared to p-i-n structures, very short RC times can be achieved for sufficiently small devices as RC time constants scale basically quadratically with the device width. We have investigated the dynamical response on a series of n-i-p-i modulators grown by epitaxial shadow mask MBE with a width of the n-i-p-i region ranging from 100 micrometer down to 5 micrometer. For the smallest devices time constants as low as 1.5 ns have been measured. Modulation at up to 250 MHz has been demonstrated with a decrease in switching contrast from 2.2 at dc operation to 1.75 at 250 MHz. The voltage swing used in these experiments as only 3.7 V. We stress that these devices were not optimized. The switching time for the 5 micrometer sample was increased by about a factor 10 due to high contact resistances. The switching contrast can be enhanced easily and with no penalty on the high frequency response by using n-i-p-i structures with a larger number of periods, as the RC time constant is independent of the number of periods. Thus, with a suitable design operation in the multi-GHz range as well as an improved switching contrast should be possible.

We report on a novel electro-optic modulator structure based on the two-dimensional Franz- Keldysh effect (2D-FKE) in multiple quantum well (MQW) structures. Due to the increased electron-hole interaction in these quasi-two-dimensional systems, strong excitonic resonances are observed even at room temperature. If an electric field is applied parallel to the layers of a MQW structure, very low electric fields (10 - 30 kV/cm) are sufficient to cause field ionization of the excitons, because of their weak in-plane confinement. Large absorption changes as high as 7000 cm^-1 with field changes of only 30 kV/cm have been observed in GaAs/AlGaAs-MQWs. In addition, an increase of the absorption below and oscillations of the absorption coefficient above each subband transition are obtained due to the two-dimensional Franz-Keldysh effect. These features have been applied in our novel electro- optic modulator structure. Using interdigitated metal-semiconductor-metal (MSM) contacts, high in-plane electric fields can be generated with moderate voltages. Furthermore the low capacitance of these MSM structures is particularly favorable for high speed applications. In a MSM-modulator structure, consisting of 75 GaAs/AlGaAs quantum wells with a distributed Bragg-reflector (DBR) below the MQW-layers, a maximum contrast ratio of 5:1 without using any cavity effects has been achieved with a voltage swing of 20 V.

There is increasing interest in visible wavelength optical modulators compatible with storage media such as LiNbO₃ and organic polymer materials. Multiple quantum well (MQW) structures with AlGaAs QWs allow the fabrication of optical devices operating at much shorter wavelengths than conventional GaAs QW devices which operate in the vicinity of 860 nm. We show bias-dependent photocurrent (PC) measurement results from p-i-n MQW samples containing 10 nm Al_xGa_1-xAs QWs with x-values ranging from 0 to 0.54 and corresponding wavelengths ranging from 860 to 570 nm. The barriers in each sample are 5 nm thick and have 30% more Al than the wells. The measured PC is proportional to the photon flux absorbed in the sample and, therefore, can be used to determine the quality of the material for absorption modulation. PC characteristics comparable to those of a GaAs/Al_0.30Ga_0.70As MQW sample are obtained for wavelengths as short as 640 nm (QW x- value as high as 0.38). These spectra have sharp exciton features that maintain their general peak shape and steeply falling lower energy edge under applied reverse bias. Shorter wavelength samples, 610 nm (x equals 0.46) and 570 nm (x equals 0.54), do not show good modulation characteristics, but may potentially be improved by further optimizing the structure design and growth conditions. With the same MQW structures and growth conditions used to produce the above samples, optical modulators operating at wavelengths as short as 640 nm can be fabricated.

In this paper the epitaxial growth of n-i-p-i-InAs structures for infrared photodetectors with sensitivity in region (3-5) mkm grown by low pressure metal organic chemical vapor deposition (LP-MOCVD) have been represented. Experimental results: spectra absorption, responsivity of these photodetectors were demonstrated. The extension of photosensitivity and absorption spectra in long wave region up to 4 mkm have been found in comparing with common photodetectors based on bulk InAs. It was shown experimentally that n-i-p-i InAs structures are very attractive for manufacturing of high performance photodetectors in ave region (3-5) mkm on its base.

We use capacitance and photoluminescence spectroscopy to study the energy splitting of electron and hole states in InAs self assembled quantum dots embedded in GaAs bulk material. In our photoluminescence spectra, measured with high excitation, we observe five peaks below the wetting layer transition which we attribute to electron hole recombination from quantum dot levels of the same quantum number. Resonant excited photoluminescence experiments show clearly the existence of phonon enhanced carrier relaxation if the energy splitting between two different quantum dot levels matches a multiple of the available phonon energies. Therefore a maximum in the intensity of the resonantly excited photoluminescence does not necessarily occur when most of the dots are pumped resonantly into an excited state, the main criterion, however, is that the energy distance between the pumped levels and the levels below matches a multiple of the available phonon energies.

The potential for hybridization of compound semiconductors with silicon optoelectronics has led to significant efforts in optimizing the heteroepitaxy on Si substrate. Recent experiments show that the epitaxial quality of ZnS/Si (approximately 0.4% mismatch) is not much qualitatively different from that of ZnSe/Si (approximately 4.4% mismatch). This indicates that in semiconductor growth on silicon the formation of interfacial chemical compound plays a more dominant role than the lattice mismatch. In this paper, several ordered structures and disordered pseudobinary alloys are studied in the interface region of ZnS/Si and ZnSe/Si superlattices by using tight-binding calculations. For the ZnSe/Si system, we allow the lattice parameters at interface are relaxed to obtain the energetically most stable structure. The present study gives evidence that ordering induces a preference for lowing density of interface states. In addition, the band offset and layer thickness dependence of the band gap are also calculated.

The results of the investigation of two-dimensional electron gas (2DEG) using millimeter spectroscopy technique are presented. The measurements in the millimeter region allow us to register cyclotron resonance (CR) and Shubnikov-de-Haas (SdH) oscillations of electron gas in the same time. This spectral region is optimal to study the electron heating effects also. The main attention is focused on the investigations of the magnetospectra of photocurrent, photoconductivity, transmission and differential transmission of the electron system in 2DEG of GaAs/GaAlAs heterostructures using cyclotron resonance (CR) technique. The investigations of GaAs/AlGaAs heterostructures by photoconductivity technique have shown that the magnetospectra have the complex structure (even with the change of the photocurrent polarity). The magnetospectra depend on frequency and polarization of the incident radiation. The radical changes of the CR-line and the SdH oscillations at the reversion of the magnetic field direction are discovered. The discovered effects are very promising for the development of heterostructures diagnostics methods. These effects reveal a possibility to elaborate solid state detectors to analyze the parameters of millimeter and submillimeter waves. In the study of GaAs/AlGaAs heterostructures with high mobility in pulsed electric field up to 100 V/cm we have discovered an increase of transmission in the center of the CR line. The connection of this phenomenon with collective effects such as the hot electron resonant bunching in the momentum space at non-elastic scattering by optical phonons is discussed.