The low temperature optical absorption spectra of fully coherent strain-compensated InAsxP1−x/GayIn1−yP (0.15⩽x⩽0.27; 0.09⩽y⩽0.13) multilayers grown on InP(001) by organometallic vapor phase epitaxy were measured using a Fourier transform infrared interferometer. All spectra show well-resolved excitonic transitions. The analysis with a solution to the Schrödinger equation in the envelope function formalism using the Bastard–Marzin model allows to determine accurately that the strained conduction band offset δEc equals (776±4)x meV, or 77%±2% of the total strained band gap difference. The heterojunction alignment is of type I for both heavy- and light-hole bands but the light-hole confinement is very shallow. The heavy- and light-hole exciton binding energies obtained from the analysis are ≃9 and 6–7 meV, respectively.

Photoluminescence (PL) and optical absorption spectra from samples containing InAsP/InP and InP/InGaP quantum well stacks reveal the presence of band tails in the excitonic density of states. At low temperature, radiative recombinations from these band-tail states contribute to the energy separation (redshift) between PL and optical absorption peaks for the fundamental transitions. This band-tail localization contribution is evaluated quantitatively by using a potential fluctuations model [developed by D. Ouadjaout and Y. Marfaing, Phys. Rev. B. 46, 7908 (1992)] to analyze the line shape of the low temperature PL peaks. The residual energy separation, after removing the band-tail localization component, arises from thermalization processes alone, demonstrating the validity of the model used to evaluate the band-tail induced redshift. We also found that the free excitons’ effective temperature varies linearly with the optical absorption peak width, dominated by inhomogeneous broadening at low temperatures. This empirical relation suggests an explanation for the difference on the origin of the Stokes shift reported in the literature.

We present investigations of the temperature evolution of the photoluminescence (PL) and optical absorption spectra of a series of tensile-strained InGaP/InP multiple quantum wells (MQW) samples. As in previous work on compressive-strained InAsP/InP MQW, the results support the assignment of the low temperature PL transitions to recombination from excitonic band-tail states. The energy of such transition is redshifted with respect to the free exciton recombination energy. This results in a large apparent energy difference between PL and optical absorption peaks which is, at 6 K, about 5 meV in the coherently strained samples and less than 1 meV in the partially relaxed ones. The analysis of the low temperature PL line shape which we present enables the determination of the excitonic band-gap energy for all the samples. The difference between the energy of the optical absorption transition and the excitonic band-gap energy thus determined gives a measure of the Stokes shift arising from thermalization effects alone. The values of the Stokes shift thus obtained are in better agreement with the sharpness of the optical absorption transitions which indicate samples of high crystalline quality.

An analysis and discussion of the device physics for the quantum-confined Stark effect based on barrier height and band alignment considerations is presented. It identifies two important design principles for band structure engineering of the multi-quantum well stack: (1) Due to the counterbalance relationship between field-induced redshift and field-induced polarization of the quantum well eigenstates, design strategies must look to attain an optimal balance or compromise between a minimum drive field and maximum absorption coefficient change. This can be achieved with an appropriate choice of the valence band discontinuity. (2) In III–V semiconductors, the strong asymmetry in the field response of the conduction and valence band eigenstates is due directly to the asymmetry of the conduction and valence band effective masses. As a result, optimum device performance is obtained by using a heterostructure with a disproportionately large conduction band offset to compensate the effective mass asymmetry and balance the field-induced wave function leakage in the conduction band to that in the valence band. The relative wave function leakage between conduction and valence bands is compared by examining tunneling currents through the quantum well barriers as a function of the electric field and barrier height. For conduction and valence band effective masses of, respectively, 0.055 and 0.5 times the free electron mass, the optimal band alignment requires a conduction band discontinuity 3–9 times greater than the valence band discontinuity. Applying these design principles for high speed, low drive voltage optical modulators shows that the overall performance of these devices may be improved by using a combination of balanced band alignments and low valence band barriers. The low valence band barriers reduce the drive field required to operate the devices, which has direct effects upon the drive voltage, device capacitance, attenuation coefficient, and optical coupling and propagation losses. The analysis and discussion is supported by experimental modulation depth and drive field data obtained from strained-layer multiple quantum well InAsP/InP and strain-compensated InAsP/InGaP optical modulators fabricated with layers grown on InP(001) by metalorganic vapor phase epitaxy.

InGaAsP/InP and InGaAsP/InAsP multilayers were grown on InP(001) by low-pressure organometallic vapor phase epitaxy. Large growth rates of ≈0.4–0.6 nm s−1 and an increased element-V overpressure were used to limit the morphological evolution of the strained layers during growth and to compensate for the relatively high temperatures (≈630 °C) necessary for vapor phase epitaxy in a diffusion-limited regime. High-resolution x-ray diffraction and reciprocal lattice mapping analyses indicate fully strained multilayers of high crystalline quality. This structural information, combined with room-temperature photoluminescence (PL) measurements, allows us to determine accurately the thickness and the composition of the layers. Well-resolved excitonic transitions between the heavy- and light-hole valence bands and the conduction band are visible in the low-temperature optical absorption spectra for compressive InGaAsP/InP multilayers. The PL spectra for compressive InGaAsP/InP structures show sharp and intense transitions between the first confined levels in the conduction and the heavy-hole bands. The PL peaks for InGaAsP/InAsP heterostructures are slightly broader than for InGaAsP/InP multilayers due to the more complex (quaternary-ternary) interface but remain sharp and intense.

Photoluminescence (PL) and optical absorption studies have been performed on strained-layer InAsxP1−x/InP (001) (x<0.27) multiple quantum wells grown by low pressure metal-organic vapor phase epitaxy. The series contains samples with both coherently strained and partially relaxed multilayers, where the relaxation is characterized by misfit dislocations. The PL transition line shape at low temperature and at low excitation intensity as well as the evolution of its peak energy with temperature are characteristic of the recombination of band tail localized excitons induced by potential fluctuations. The redshift of the PL peaks relative to the absorption peaks is attributed to two factors: band tail localization and thermalization. The low temperature PL spectra were fitted with an analytical model for the emission line shape, proposed by Ouadjaout and Marfaing [Phys. Rev. B 46, 7908 (1992)]. This allowed us to quantify the PL peak redshift due to band tail localization. After accounting for this effect, the residual energy difference, which we define as the Stokes shift, shows a very strong correlation with the degree of structural relaxation in the multilayers measured by high resolution x-ray diffraction and transmission electron microscopy. This allows the separation of the strain release contribution from the thermalization processes responsible for the Stokes shift.

Strained-layer multiple quantum wells InAsP/InP and InAsP/InGaP optical modulators based on the quantum-confined Stark effect have been fabricated from layers grown by metalorganic vapor phase epitaxy on InP(001). The device layers have been characterized by complementary high resolution x-ray diffraction, transmission electron microscopy, optical absorption and photoluminescence analyses. The structural properties of the layers were deduced from the above data and an accurate determination of the band alignment of the heterostructures was made by performing multiple transition fits to the optical absorption spectra using the Marzin–Bastard envelope function model for strained-layer superlattices. The electric field-dependent redshift of the fundamental electron-heavy hole transition was measured by a photocurrent method and found to be enhanced for structures with lower valence band barrier heights. This observation leads directly to the conclusion that the overall performance of high speed, low drive voltage optical modulators may be improved by engineering the band alignment of the multiple quantum well stack towards structures with disproportionately large conduction band offsets. An optimization of the band alignment will permit more efficient optical modulation by reducing the drive field required to operate the device, which, in turn, can have direct effects upon the drive voltage, device capacitance, attenuation coefficient, and optical coupling and propagation losses.

Strategies designed to optimize the performance of the multi-quantum well stack in optical modulator devices based on the quantum-confined Stark effect can be derived from an observation that the effective mass of the active quantum well material is significantly smaller in the conduction band than that in the valence band. This suggests that significant improvements in the overall performance of the devices may be achieved by engineering the band alignment of quantum well and barrier to accommodate the difference in the effective masses.

Self-assembled InAs islands were grown by metalorganic vapor phase epitaxy on InP(001) and characterized by atomic force microscopy and transmission electron microscopy. The growth temperature (450–600°C), the InAs deposition time (3–12 s, using a growth rate of ∼2.3Å/s), and the growth interruption time (8–240 s) were varied systematically in order to investigate the effect of thermodynamic and kinetic factors on the structural properties of InAs/InP and InP/InAs/InP structures. It is found that the structural properties of islands vary widely with the growth conditions, ranging from very small (4–5 nm height, ∼30–60 nm in diameter) coherent islands at low temperature (450–500°C) to large (∼350 nm wide) plastically relaxed islands at high temperature (600°C). For a given deposition time, the height of the coherent islands increases markedly with the growth temperature while their diameter shows only a moderate increase. The growth interruption time also affects the formation and the evolution of islands, which clearly shows that these processes are kinetically limited. Coherent islands with structural properties suitable for use in optoelectronic devices are obtained from ∼2.4–4.8 monolayer thick InAs layers using a growth temperature of 500°C and a 30 s interruption time.

Strained-layer multiple quantum well (MQW) InAsP/InP optical modulators have been fabricated from layers grown by metal-organic vapor phase epitaxy. The devices are a series of p-i(MQW)-n photodiodes in which the active core regions consist nominally of 25 periods of 10 nm InAsP quantum wells of 4.4%, 10.0%, 15.6%, and 26.4% As composition separated by 10 nm InP barriers. Structural parameters for the samples were obtained using high-resolution x-ray diffraction rocking curves and transmission electron microscopy. The series contains samples with both coherently strained and partially relaxed multi-layers where the relaxation is characterized by misfit dislocations. The band offsets for the heterostructures were determined by fitting the energy positions of the optical absorption peaks with those computed using the Marzin–Bastard model for strained-layer superlattices [as in M. Beaudoin et al., Phys. Rev. B 53, 1990 (1996)]. The conduction band discontinuities thus obtained are linear in the As composition (7.56±0.08 meV per As % in the InAsP layer) at low and room temperature for As concentrations up to 39%, and up to 17% average relaxation. Comparisons between the coherently strained and partially relaxed samples demonstrated a broadening of optical transition linewidths due to relaxation which appears to be of minor consequence for optical modulator devices as the essential optical and electrical properties remain intact. The electric field-dependent red-shift of the n=1 electron-heavy hole transition was measured by a photocurrent method and found to be enhanced in structures with lower barrier heights.

The metalorganic vapor phase epitaxy of coherent self-assembled InAs islands on InP(001) is demonstrated. Samples are characterized using transmission electron microscopy and photoluminescence (PL) spectroscopy at 77 K. The deposition of ∼2.4–4.8 monolayers (ML) of InAs at 500°C followed by a 30 s growth interruption results in the formation of coherent islands whose average diameter is 30–35 nm with a standard deviation of 8 nm and whose areal density is (3–4)×1010 cm−2. The PL emission is centered at 0.79 eV and has a full width at half maximum (FWHM) of 90 meV. When the nominal deposited thickness is increased to ∼9.6 ML, the average island diameter increases to ∼120 nm while the areal density decreases to ∼109 cm−2. The resulting PL is then centered at 0.83 eV with a FWHM of 130 meV and also displays a peak at 1.23 eV which is attributed to an InAs wetting layer ∼2 ML in thickness.