This paper reviews some of the progress that has occurred recently in both the theory and performance of resonant-tunneling diodes. It begins by describing the present physical understanding of the resonant-tunneling process. Recent experimental advances in resonant-tunneling oscillators are then discussed, including the demonstration of oscillations at frequencies up to 200 GHz. To better understand these results, a detailed analysis is made of the mechanisms that limit the speed of this device. This analysis includes a calculation of the quasibound state lifetime, a determination of the device capacitance from the electrostatic band-bending, and an estimate of the negative differential conductance using two different methods. On this theoretical basis, a device structure is analyzed that could oscillate up to 600 GHz.

This paper reviews our current research activities into resonant-tunneling hot electron transistors (RHETs) and resonant-tunneling bipolar transistors (RBTs) and describes our recent advances in these device technologies using InGaAs-based materials. The InGaAs-based RHET's common-emitter current gain at 77K is typically 17 (with a maximum of 25), which is about four times greater than that of a GaAs-based RHET. The collector current peak-to-valley ratio reaches 19.3 (with a maximum of 21.7). This is eight times that of the GaAs-based RHET. The InGaAs-based RBTs have been operated at room temperature with a current gain of 26 and a collector current peak-to-valley ratio of 2.3. The scattering parameters of the InGaAs-based RBT have been measured in a frequency range from 0.2 to 20.2 GHz. The cutoff frequency fT is measured as 12.4 GHz, indicating an emitter-to-collector delay of 12.8 ps. The delay due to the resonant-tunneling barrier used for this RBT is estimated to be 1.4 ps using an equivalent RBT circuit analysis.

In order to better understand the trapping and response time of resonant electron wave packets in a resonant tunneling structure, we examined the time-dependent transport of a spherically symmetric Gaussian wave packet through a model double-barrier potential. Within the effective mass picture, the wave packet is projected onto the complete eigenstates of the model potential. The wave packet is initially localized to the left of the barriers and allowed to propagate through the structure. At wave packet energies corresponding to the resonant energies, the probability transmitted to the right of the barriers exhibits a steplike behavior as a function of time. A computation is made of the quantum mechanical delay time, to~tclass, where to is the packet transit time and tclass is the transit time for a classical particle. An increase in this delay time is observed when the packet energy corresponds to the energy of the resonant levels, and a comparison is made to the decay time of a particle in the first resonant state of the well. The delay time increases with increasing barrier area and decreasing packet energy width.

We have calculated the tunneling of holes in single-barrier and double-barrier GaAs/AlGaAs diodes using a ten-band, empirical tight-binding model. Transfer matrices are used to model the wave-function on a layer-by-layer level allowing for a simple, transparent imposition of an electric field, and the mixing of heavy, light and splitoff valence band states. Transmission resonances have been found, and hole currents calculated at 77 degrees K. Mixing of different hole states has been examined as a function of aluminum fraction and thickness of the barriers and well.

The effect of impurities placed in double barrier resonant tunneling diodes, on the current voltage characteristics of the devices was experimentally determined. Four different double barrier structures were grown by Molecular Beam Epitaxy with n-type, p-type, undoped and highly compensated doping in the center of the well. Two additional samples were grown with and without n-type doping in the barriers. Resonant tunneling devices of various sizes were fabricated, and measured at 77 K. Systematic shifts in the peak current voltage and peak to valley ratio were observed for the devices with different dopant profiles. The shifts in peak current position for the devices with varied well doping are correctly predicted by a modified ballistic model which includes the effects of band bending due to ionized impurities in the well. The doped devices showed a systematic decrease in the peak to valley ratio which is not predicted by the ballistic model. A scattering assisted tunneling mechanism which increases the valley current is proposed. The magnitude of the calculated increase in valley current due to elastic scattering by impurities which were intentionally placed in the well, is consistent with the observed lowering of the peak to valley ratio.

It is shown that the introduction of deep levels in the barriers of tight-binding superlattices can have profound effects on the electronic states. By appropriate choice of the quantum well thickness and of the location of the deep center within the barrier strong mixing between the defect and superlattice states takes place. Enhancement of the miniband widths by several orders of magnitude and the creation of new Bloch states within the band-gap of the superlattice are found. Possible implementations of these new structures by Molecular Beam Epitaxy are discussed.

A simple semiclassical treatment of the vertical transport in barrier-conductor structures is presented. The distribution function is constructed by fitting the solutions of the Boltzmann equation for the conductor parts with the barrier reflection and transmission probabilities. This semiclassical theory describes multiple reflection in a random phase approximation leaving out the fine structure associated with the quantum interference. As an application we analyze single and double barrier structures in detail. We study the high frequency behaviour of various diode structures. For the hot electron transistors (HET) we derive simple formulas for the base transport factor, transconductance and other elements of the ac-small signal equivalent circuit. The transistor model is also valid for the resonant hot electron transistor (RHET).

We have recently demonstrated novel high responsivity 8-12 μm detectors (comparable to HgCdTe) based on intersubband absorption and photoexcited tunneling in doped multiquantum well superlattices of GaAs/AlGaAs. These detectors have the potential advantage over HgCdTe of a more mature materials and processing technology and the possibility of direct integration with high performance GaAs FETs. In this talk we will discuss the detailed physics and device operation of these detectors, as well as our newest results indicated below.

Experimental observation of resonant tunneling of holes in Si1-xGex/Si structures is presented. A negative differential resistance with a peak to valley ratio in current of 2.2/1 has been obtained at 4.2°K in a double harrier structure consisting of Si barriers 6.0 nm wide and a 3.3 nm wide Si0.79Ge0.21 quantum well. In a sample with a 5.5 nm wide Si0.75Ge0.25 well and Si barriers 10 nm in thickness, five resonant levels were observed at low temperatures. The tetragonal strain in the Si1-xGex layers was measured by X-ray diffraction, and used to calculate the alloy composition. Cross-sectional transmission electron microscopy (XTEM) observations revealed defect free epitaxy and provided precise measurements of the quantum well and barrier dimensions. The peak resonance voltages were found to be in reasonahle agreement with calculated positions of heavy hole levels in the quantum wells. Evidence for band mixing of light and heavy holes was observed when a strong magnetic field oriented along the current direction produced large shifts in some of the resonant voltages.

The introduction of quantum wells as the active layer of double heterostructure (DH) lasers has considerably improved the threshold and modulation properties in the GaAs/AlGaAs material system. Improved performance has also been recently reported for several other material systems using quantum well lasers (QWL's). The main characteristics of QWL's can be qualitatively understood considering: (i) the balance between the reduction of the active volume and the lowering of the optical confinement factor (ii) the fact that the equivalent 3D carrier density at threshold is higher than in conventional DH lasers leading to significant population of higher-lying energy states and possible important leakage currents. Nevertheless quantitative agreement can only be obtained by introducing the more specific effects of the square 2D density of states and modified optical matrix elements. We show that the inclusion of these features allows us to fully understand the specific properties of QWL's like the recently observed large wavelength jumps under current injection and the anomalous temperature behavior of the threshold current of short cavity lasers.

The capture of electrons by quantum wells via phonon emission involves a complex interplay of the polar interaction, the electronic subband structure, and the mode patterns of the LO phonons. Simple theoretical models for these are described, and it is shown that alongside the more familiar electron resonances lie sharp phonon resonances which markedly affect the dependence of capture rate on well-width. The effect of these resonances is much weaker in intersubband transitions. It is pointed out that in practice the combination of layer fluctuations and randomized injection of carriers lead to a capture time of about 1ps in the AlGaAs/CaAs system which is only very weakly dependent on well-width. On the other hand the intersubband scattering rate is expected to be appreciably dependent on well-width, but the predicted magnitude is a few times larger than observed and the observed trend with well-width is weak.

We have modeled ultra-fast processes occurring during photoexcitation in AlGaAs/GaAs quantum wells using an ensemble Monte Carlo simulation. This simulation models the dynamics of quasi-two--dimensional electrons and holes confined within a single quantum well including the effects polar optical and transverse optical phonons, intervalley scattering, ionized impurities, and intercarrier scattering. The effect of nonequilibrium phonons and degeneracy are also included. We have used our simulation to compare to time resolved photoluminescence and transmission experiments in semiconductor quantum wells. For the latter experiments (W.H. Knox et al., Phys. Rev. Lett. 56, 1191 (1986)) performed at low excitation energies in the band, the results of our simulation show that electron-electron scattering is the dominant mechanism in accounting for the experimentally observed relaxation. While the electrons relax to form a thermalized distribution over 200 fs, the heavy holes generated at the same time relax within 50 fs, and the corresponding differential transmission spectrum is primarily determined by electron relaxation in the conduction band. Thermalization of the electrons with the cooler holes occurs over a longer time scale of 1 ps via inelastic intercarrier scattering.

Silicon p-n junctions and doping superlattices nipi's have been grown by molecular beam epitaxy (MBE) using an arsenic ion beam for n doping and a boric oxide source for p doping. The layers were characterised by secondary ion mass spectrometry (SIMS) and the electrical characteristics of nipi's and individual junctions were measured. The superlattices were evaporated through a silicon mask to create nini and pipi regions at the edges of mesas to which selective contacts of Au/Sb and PtSi were made to the n and p layers respectively. Our structures typically had from 10 to 20 periods with very thin or nonexistent i regions between the doped layers which were varied in the range 30 to 100 nm and doped to 5x1017-5x1018 at./cm3. The conductivity of the p layers was measured through split contacts after illumination with 632.8 nm laser pulses to determine the effective recombination lifetime. Lifetimes of 245 ms were measured at 85°K and factors limiting the achievement of longer lifetimes were investigated.

The electronic structure, its investigation by magneto-optical investigations and the infrared photoconductive properties of doping superlattices of PbTe are reviewed. Detectivity values of 1011 cm He1/2 W-1 are obtained at the peak wavelength of 5.8 μm at 90 K, which is close to the theoretical limit. The high quantum efficiency can be attributed to a lifetime enhancement due to the indirect energy gap in real space. The influence of blocking contacts is discussed.

We discuss new studies of a multiple quantum well hetero n-i-p-i structure which combines the advantages of multiple quantum wells and doping superlattices. The structure exhibits large changes in its absorption coefficient for intensities of only a few mW/cm2. We have used the spectral dependence of the nonlinear absorption coefficient to deduce photogenerated carrier lifetimes as a function of intensity.

AlGaAs doping superlattices ("nipi structures") have been successfully grown in 30% AlGaAs by MBE on both (100) and misoriented substrates. Tunable photoluminescence (PL) as a function of incident laser intensity has been observed in samples with intrinsic spacer layers of 100-500 Å over a temperature range from 2 K to 120K. Luminescence shifts as large as 230 meV were observed at 2 K as the incident intensity was varied by a factor of about 500. The undoped spacer thickness was found to play an important role in the tuning of the photoluminescnece, with the dependence of the peak energy on incident intensity becoming weaker for thicker spacer layers. This decrease in tuning rate can be associated with the reduced probability for tunneling transitions with increasing spacer thickness. In addition, samples grown on misoriented substrates showed higher tuning rates for the thicker spacers than samples grown on (100) substrates. This behavior has been attributed to a higher density of traps in the (100) oriented samples.

Dispersion and miniband structure of GaAs compensated n-i-p-i doping superlattices have been calculated by using matrix transfer method, multistep potential approach, and numerical calculations. The results are compared with energy levels calculated by use of harmonical-oscillator-type potential well. The dependence of the effective energy gaps of heavy and light holes on the period length L is also discussed.

The long electron-holes recombination lifetime in n-i-p-i doping superlattices results in an extremely high photoconductive gain. Also the detectivity is very high, due to low generation-recombination noise and low dark conductance. However, the photoresponse becomes non-linear at relatively low optical power and turns into a logarithmic power dependence as the recombination lifetime decreases exponentially. This draw-back can be overcome by externally adjusting the recombination lifetime. This can be achieved by an appropriatelY chosen resistor Rext between n- and p-type selective contacts which provides an external recombination channel. This improvement, however, is usually at the expence of responsivity and noise performance. Recently, we have found that high responsivity and detectivity can be maintained and at the same time the linearity range can be extended to many orders of magnitude in optical powerextif the photoconductive n-i-p-i detector is operated at reverse bias with high values of Rext. This advantage is due to the low n-p leakage currents which are nearly independent of the reverse bias within a wide range. We document this break-through by our recent photoresponse measurements under d.c. and high-frequency conditions. Extensions of the concept to build detectors with gain much larger than 10 at bit rates significantly above 1GHz will be discussed briefly.

We report the results of a systematic investigation of the effects of decreasing the AlAs layer thickness from 41Å to 5Å on the band alignment of GaAs-AlAs quantum wells in which the GaAs thickness was kept constant at nominally 25Å. Combining the techniques of photoluminescence and photoluminescence excitation spectroscopy we have mapped out both the direct r-related bandgap and the x-r bandgap as a function of AlAs thickness. We observe a reversal of the band alignment from the type II to the type I arrangement when the AlAs thickness is reduced below ~13Å In addition, we present further evidence which confirms that the type II emission process is related to the Xz-r pseudo-direct bandgap. In the structures with very thin (<10Å) AlAs layers we note a significant modification of the type I excitation spectra where the n=1 exciton peak can be hundreds of times stronger than the apparent absorption in the continuum region.

We have investigated the temperatdre dependence of the photoluminescence (PL) decay kinetics of a series of GaAs/AlAs quantum well structures where the GaAs thickness was kept constant at 25Å and the AlAs was varied between 41Å and 19Å. In these structures the band alignment is type II and the dominant photoluminescence process at 4K is due to recombination of excitons involving electrons confined at the AlAs X point and holes in the GaAs. At 4K on the low energy side of the zero phonon type II transition the PL decay is a single exponential over at least two decades. The time constant of this decay is a strong function of the AlAs layer thickness. The variation of this decay time is in line with a change in oscillation strength of the type II process due to the change in the mixing between the Xz(AlAs) electron states and the Γ (GaAs) electron states. At higher temperatures (T>15K) the photoluminescence intensity and the decay time decrease very rapidly with increasing temperature. This is due to the increased influence of non-radiative proceses as the type II excitons become delocalised.

We have measured the photoreflectance (PR) spectra at 300K and 77K of two strained layer <001> InxGall-xAs/GaAs (x≈0.12) multiple quantum wells (MQW) with nominal well (Lz) and barrier (LB) widths of 50A/100A and 30A/100A, respectively, as deduced from the growth conditions. Phototransmittance at 77K of the latter sample has been studied. In both samples we have observed a number of features in the PR spectra corresponding to miniband dispersion (coupling between wells) of both confined and unconfined (above the GaAs barrier) transitions. The coupling between wells leads to different transition energies at the mini-Brillouin zone center (Γ) and edge (π) along the growth direction. This is the first observation of unconfined features and miniband dispersion in this system. Even though our samples have fairly wide barriers (LB ≈100A) the coupling between wells is an important effect because of the relatively small confinement energies for x≈0.12. Using the envelope function approach we have calculated the various transition energies taking into account both strain and quantum well effects, including miniband disper-sion. Good agreement with experiment is found for a heavy-hole valence band discontinuity of 0.3±0.05 and LZ/LB = 52±3A/105±5A(x=0.11±0.01) and 32±3A/95±5A(x=0.12±0.01) for the two samples, respectively. The In composition and well/barrier widths are thus in good agreement with the growth conditions. Although the symmetric component of the fundamental light-hole to conduction band transition is a strong feature, the small observed amplitude of the antisymmetric component for both samples is evidence for the type II nature of the light-hole to conduction band transitions.

Strained (InGa)As single and multiple quantum wells, embedded in GaAs and (AlGa)As, have been grown by molecular beam epitaxy. The critical thickness for (InGa)As/GaAs was studied by photoluminescence (PL) and found to follow the theoretical expression proposed by J.W. Matthews and A.E. Blakeslee [J. Cryst. Gr. 2Z (1974) 118]. In the PL spectra of strained QW's the dominating narrow line (at 2 K and 77 K) is associated with the recombination of excitons characterized by the ground state levels of electrons and heavy holes. Photoconductivity spectra at 77 K of QW's in GaAs revealed three peaks which are due to allowed excitonic transitions involving ground states, excited states as well as hole states from the strain split off valence subband. This split off valence subband in (InGa)As is below the band edge of the unstrained GaAs. The transition energies of SOW and MQW:s have been analysed with the conduction band offset, ΔEc, and the energy gap of the strained (InGa)As as adjustable parameters. This has shown that the conduction-to-valence band offset ratio across the GaAs/strained (InGa)As interface is; ΔEc:ΔEv = 0.83(±0.06) : 0.17, and it slightly decreases for the (AlGa)As - strained (InGa)As interface depending on the Al and In concentrations. In addition we found that with reduced In-concentration the offset approached that for the GaAs/(AlGa)As heterojunction.

The linear polarization behavior of electron-heavy hole excitonic recombination is studied on two types of single square quantum wells (SSQW). The SSQW structures with the special feature that the heavy hole to light hole separation is near an optical phonon frequency of the structure (called "double resonance" type) show a strong linear polarization dependence on the incident energy. The "ordinary" type SSQW (i.e. not of the double resonance type) show little or no linear polarization. The double resonance samples also show reduction of the photoluminescence linewidth when the incident energy is near the light hole to electron energy separation. These observations indicate a strong modification of the confined heavy hole level due to the mixing between the heavy hole and light hole plus an optical phonon mode.

Far-infrared spectroscopy of electric subbands which are created by the spatial confinement of electrons in low-dimensional semiconductor systems is discussed. As examples, two-dimensional (2D) subbands in GaAs/Ga1-xAlxAs heterojunctions and 1D subbands in multiwire metal-oxide-InSb structures are reviewed.

Pearsall et al [Phys Rev Lett 58, 729 (1987)] have recently presented electroreflectance data on short-period Si/Ge (001) superlattices grown on (001) Si substrates. It is clear from the electroreflectance spectra, that structure exists which more than likely originates from the Si/Ge superlattices. In this paper, results of calculations based on empirical pseudopotentials with spin-orbit coupling are presented which demonstrate that the basic conjectures of Pearsall et al concerning the Si/Ge (4:4) superlattice are correct. New assignments are suggested for transitions which have been observed and predictions are made which can be used to test these assignments. The character of superlattice states close to the band edges are discussed in terms of their real-space charge density and their origin in wavevector space. In particular, a reversal of |mj| = 3/2 and |mj| I = 1/2 valence states with changing buffer layer composition is demonstrated in terms of the effects on subband energy levels, subband dispersions and polarisation-dependence of cross-gap transition probabilities.

A combined Brillouin-Raman study has been performed in Si-Si1-xGex and GaAs-Al As superlattices. The photoelastic constant of Si0.5Ge0.5 with respect to Si for several light wavelengths is measured for the first time by light scattering experiments. The frequencies, linewidths and relative intensities of the superlattice acoustic phonon modes are carefully investigated, especially for scattering wavevectors in the vicinity of Brillouin zone boundaries. The acoustic and optical absorptions are found to have particular effects on the frequencies and relative intensities for scattering wavevectors in these regions.

ZnSe-ZnTe superlattices have generated a great deal of interest in the optoelectonics arena particularly as visible light emitters. There is a large research effort particularly by the Japanese to make blue light emitting diodes (LEDs) with ZnSe-ZnTe superlattices. Successful growth of these superlattices has been achieved with molecular beam epitaxy (MBE). Photoluminescence from this system suggests that the valence band offset between ZnSe and ZnTe is about 1.0 eV. We have performed band structure calculations based on k•p theory to study the dependence of the valence band offset and the band gap on strain. Based on the assumption that the photoluminescence from the superlattice is due to the emission from a Tel bound exciton in ZnSe, we have fit the experimental photoluminescence data with k•p theory to obtain the best value of the valence band offset. The value we find is 0.97 ± 0.10 eV. Alternatively, assuming that the photoluminescence is due to band to band transitions, we obtain a valence band offset of 1.20 ± 0.13 eV. We have also calculated the superlattice band gap as a function of the constituent material layer thicknesses for the valence band offsets quoted. We expect these calculations to play an important role in gaining an understanding of the ZnSe-ZnTe superlattices.

Electric polarization fields can be generated by lattice-mismatch induced strain in strained-layer superlattices grown from piezoelectric constituent materials such as III-V semiconductors. The orientation of the polarization fields depends on the superlattice growth axis. For the most commonly studied case of a [100] growth axis, no piezoelectric fields are generated in superlattices made from zincblende-structure constituents. However, for any other growth orientation, these piezoelectric fields do occur. The [111] growth axis presents a particularly interesting case. For this orientation, the polarization fields are parallel to the growth axis. They have opposite polarity in the two constituent materials and can generate large (greater than 105 V/cm) internal electrical fields which also have opposite polarity in the two constituent materials. These internal electric fields strongly modify the electronic structure and optical properties of the superlattice. For example. they change electronic energy levels and wavefunctions and therefore optical transition energies and oscillator strengths. We consider cases where the internal fields are modulated by photoabsorption (because the photogenerated carriers screen the fields) and by an externally applied electric field. Large nonlinear optical and electro-optical coefficients occur in [111] growth-axis strained-layer superlattices because of the internal piezoelectric fields.

A new class of nonlinear optical materials, synthetic materials based on asymmetric QW structures and superlattices, possess large optical nonlinearities and can be engineered for a wide range of very interesting potential applications.

A discussion of the exciton regime of quantum dots is presented. We predict the possibility of enhancement of the optical nonlinearities due to exciton quantization. Applications to the dynamic Stark effect and two-wave mixing processes are described.

Numerical and analytical calculations for the time course of a simple GaAs/AlGaAs multi-quantum well avalanche photodiode are presented. The numerical calculations are based on an ensemble Monte Carlo calculation. The numerical results are obtained by an iterative method in which the parent electrons, those generated from photon absorption, are first simulated yielding the daughter electron and hole distributions. The daughter hole distribution is then simulated based on the time and spatial location of each particle's birth obtained from the previous simulation. The analytical formulation is based on the model of a marked filtered Bernoulli branching process. The analytical results are obtained from a generalized version of previously derived equations for the staircase avalanche photodiode. Both analytical and numerical results are presented for single-carrier initiated, single-carrier multiplication (SCI-SCM) devices. A comparison between the two calculations is addressed. Only simulation results are presented for single-carrier initiated, double-carrier multi-plication (SCI-DCM) devices.

We present the results of measurements, by means of a grating coupling method, of the index of refraction of several bulk materials and of slab waveguides of the GaAs/AlGaAs system. Preliminary results show good agreement with literature values obtained by other methods.