InAsxPi.x/InP (10 period 50/100Å with x=0.25–0.79) pseudomorphically strained multiple quantum wells (SMQWs) were grown by gas source molecular beam expitaxy (GSMBE) at 470°C and characterized by cross-sectional transmission electron microscope (XTEM), double crystal x-ray diffraction (DCXRD), and optical spectroscopy. The structural analysis demonstrates that excellent control of the sharp interface and limited As-P interdiffusion can be achieved by GSMBE growth. XTEM images of these SMQWs display no misfit dislocations, and DCXRD scans reveal high order superlattice satellite peaks. Photoluminescence (PL) and transmission measurements were performed for all SMQWs to evaluate crystal quality. Only slight degradation in luminescence was observed as the As composition increased. Based on the three-band Kane model which includes the lattice strain, the transition energies of SMQWs were calculated using the conduction-band offset (Qc=δEc/δEg) as an adjustable parameter. The best fit of measured and calculated interband transition energies suggests that Qc is independent of As composition and is 0.70±0.05. Finally, a growth kinetics model based on the Langmuir equation was derived to realize the As/P incorporation ratio in the InAsP materials. Theoretical results show good agreement with experimental data.

We report the highly reflecting (>99.9%) In0.53Al0.14Ga0.33As/In0.52Al0.48As 30.5 pairs distributed Bragg reflector (DBR) and the In0.53Ga0.47As/ In0.52Al0.48As active cavity layer grown at high temperature by low pressure metal organic chemical vapor deposition with in-situ double beam laser reflectometry. One of the laser wavelengths selected for in-situ measurement was same as the DBR wavelength. The growth temperature was 750 °C. Good surface morphology of the multi-layer stacks was achieved by the temperature ramping of the InP buffer layer at the beginning of a multi-layer stacks. The width of stop band edge of the DBR reflectivity spectrum was found to be 1000Å.

The energies and resonance widths of single doped quantum wells consisting of Al-GaAs/GaAs with rectangular and annealing induced diffusion modified shapes are calculated under an uniform electric field using the stabilization method. The electronic structure is calculated without an electric field in the finite temperature density functional theory with exchange-correlation potential treated in the local density approximation. Our scheme for solving the Schrödinger and Poisson equations is based on the Fourier series method. The electric field is added to the self-consistent potential and energies are obtained as a function of the combined width of the well and barriers. This yields us the stabilization graph from which the energies and resonance widths at different field strengths are extracted using the Fermi Golden rule.

A self-consistent (8 × 8)-k · P model based on the Kohn-Sham density-functional formalism is used to calculate the quasi-particle wave functions and energy levels in n-type doped quantum wells. The self-energy from electron-electron, electron-impurity, and electron-phonon scattering was included in the calculation. The electronic intersubband absorption was calculated using a generalized self-consistent-field method including the vertex correction. The theoretical approach was applied to explain the variation of the intersubband transition peak energies as the temperature or the electron density is changed. Good agreement between the theory and experiment was achieved. The present theoretical model was also applied to design two-colors quantum well infrared photodetectors and was found to agree with the experimental measurements. Based on the numerical calculation, an approach to realize the three-colors infrared photodetectors in the triple-coupled quantum wells was proposed.

Using a linear response theory, interwell-tunneling absorption of infra-red photons is calculated in a double-quantum-well structure with a wide center barrier in tilted magnetic fields. Tunneling occurs between the ground sublevels of the two quantum wells, with an energy difference that is tunable. The absorption intensity depends strongly on the temperature and the carrier densities. The resonance width is narrow even at high temperatures due to the two-dimensional phase restriction for tunneling. In magnetic fields, both the in-plane and perpendicular components of the field sensitively control and tune the absorption lineshape in very different ways, affecting the absorption threshold, the resonance energy of absorption, and the linewidth.

We demonstrate long wavelength quantum well infrared photodetectors (QWIP) with GaAs quantum wells and GalnP barriers grown using gas-source molecular beam epitaxy. Wafers were grown with varying well widths. The optimum well width was 75 Å, which resulted in a detection peak at 13 μm and a cutoff wavelength of 15 μm Dark current measurements of the samples with 15 μm cutoff wavelength show low dark current densities. Preliminary focal plane array imaging is demonstrated.

A survey is given about the potential use of Si/SiGe heterostructures for applications in the mid-infrared spectral range. We discuss theoretical foundations and experiments of intersubband absorption in p-type Si/SiGe quantum wells and show that due to the complex valence-band structure, normal-incidence absorption can be observed. On the basis of these quantum wells, mid-infrared detectors were fabricated and characterized in terms of responsivity, dark current and detectivity. In asymmetric, compositionally stepped quantum wells second harmonic generation of CO2 laser radiation has been demonstrated.

In strained Sia−xGex heterojunction internal photoemission (HIP) photodiodes the spectral response can be tailored over a wide wavelength range by varying the Ge content. In this paper heavily in situ boron doped SiGe layers with 30% Ge were grown by low pressure rapid thermal chemical vapour deposition (LP(RT)CVD). The detectors exhibit a cut-off wavelength of 8.5 μm. A delta-like P peak, incorporated by atomic layer deposition technique, shifts the cut-off to shorter wavelengths. This shift is related to an increase of the barrier height at the SiGe/Si interface caused by the narrow n+-doped layer in agreement with device simulation. In this way the trade off between critical film thickness for high Ge content film growth and absorption depth for proper detector response can be overcome.

We have investigated several approaches to improve the material quality of lattice-mismatched In.75Ga.25As grown by Molecular Beam Epitaxy (MBE) on (100) InP substrates. They include linear grading of In composition from lattice matched In.53Ga.47As to In.75Ga.25As in a 1 μm buffer layer grown at reduced substrate temperature, in combination with various in situ annealing and material regrowth steps. The material was used for fabrication of mesa-structure p-i-n photodetectors with 2.2 μm cutoff wavelength. The room temperature dark current density at 1 V reverse bias was approximately 2 mA/cm2 in all structures that were subjected to anneal and regrowth process, a factor of three improvement over reference samples which were not subjected to annealing and regrowth. The dark current density at 15 V reverse bias (10 mA/cm2 for the best devices) was at least two orders of magnitude lower in all annealed samples than in reference samples. These results suggest that the MBE grown material can be an attractive alternative to the vapor phase epitaxy (VPE) grown material which is commonly used for fabrication of these detectors.

P-type InGaAs/InP quantum-well infrared photodetectors operated at 4.55 μm require the growth of ultra-thin (10 Å) quantum wells. We report a study of interfaces in QWIPs grown by gas-source molecular beam epitaxy in which we optimized the group V source supply sequence so that a 6 K photoluminescence linewidth as narrow as 8.4 meV was observed from a structure with 10 Å wells. Analysis of the PL suggests that interface roughness was minimized. Cross-sectional scanning tunneling microscopy, double crystal x-ray diffraction, and cross-sectional tunneling electron microscopy confirmed that high-quality interfaces and uniform layers were obtained. Using the derived structural parameters, photocurrent spectral response was theoretically predicted for these QWIPs and then experimentally verified.

We have performed an optimization study of the mid-infrared photoresponse of p-type GaAs/AlGaAs Quantum Well Infrared Photodetectors (QWIPs) designed for normal incidence detection. In these p-type quantum wells, normal incidence absorption is allowed (by the dipole selection rules for optical transitions) especially for transitions from the heavy-hole ground state to the second light-hole state. Previous theoretical modeling predicted that this transition will produce the strongest bound-to-continuum infrared absorption when the second light-hole state is located very near the top of the GaAs quantum well. For AlGaAs barrier layers with 30% aluminum, our modeling showed that a well width between 45Å and 50Å would optimize the normal incidence photoresponse of this p-type QWIP. In this work, photore^oonse spectra are reported for well widths ranging from 40Å to 65Å. A series of samples were tudied in which only the GaAs well width was varied in two monolayer increments, from 11 to 20 monolayers. Photoluminescence and X-ray diffraction measurements were used to verify the composition, well width, and structural quality of each sample. This study verified that the spectral range of the normal incidence photoresponse is narrower, as predicted by theory, for well widths in which the second light-hole state approaches the top of the valence band well.

The CdxHg1−x Te compounds are well suited to the design of semiconducting devices incorporating an optical microresonator since they display a wide variation of bandgap and refractive index with composition x, while the lattice parameter remains practically unchanged. It is then possible to create a specific optical function by stacking high quality pseudomorphic layers on a crystalline substrate.

Microcavities resonating in the 3–5 μm range have been made by growing a lower Bragg mirror (10.5 periods) and a nominally undoped cavity medium containing a 50 nm active layer (CdTe-HgTe pseudo-alloy). The upper mirror is a gold layer deposited on the cavity. The emission of these Resonant Cavity Light Emitting Diodes has been observed in the 3–4.5 μm range up to room temperature. It coincides with the cavity resonance mode (linewidth 8 meV). With the addition of a ZnS/YF3 upper mirror, a Vertical Cavity Surface Emitting Laser at 3.06 μm has also been demonstrated.

The microcavity concept appears to be very useful for designing new devices for the 3–5 μm range.

The results of arsenic incorporation in HgCdTe (MCT) layers grown by molecular beam epitaxy (MBE) are reported. The incorporation into MBE-MCT was carried out by a technique called planar doping. Arsenic was successfully incorporated during the MBE growth or after a low temperature anneal as acceptors. These results are very promising for in-situ fabrication of advanced optoelectronic devices using HgCdTe material.

Molecular beam epitaxy (MBE) offers benefits such as the capability for growth of compositionally-tailored heterostructures and in-situ doping of HgCdTe alloys. These capabilities were applied to the growth of long wave infrared unispectral focal plane arrays (FPA) with 480×4 elements. The detectivity (D*) map of the FPA demonstrated performance that was higher than the specification value, with no defective channels. Two-color detectors with the n-p-n architecture, for the simultaneous detection of two closely spaced bands in the midwave infrared spectrum were also grown by MBE. These devices exhibited sharp turn-off and turn-on in both bands. The quantum efficiency was greater than 70% and average R°A values exceeded 1×106 Ω-cm2 in both bands. These result demonstrate that high performance HgCdTe devices can be grown by MBE.

Atomic hydrogen is shown to be efficacious for cleaning CdTe and HgCdTe substrates for subsequent CdTe growth by molecular beam epitaxy. While single crystal ZnTe and CdTe growth was obtained on Si substrates that underwent an ex-situ HF-based etch, only polycrystalline CdTe or ZnTe could be obtained on surfaces cleaned using an atomic hydrogen source. This result is possibly related to gas-phase transport of Te to the Si surface.

A major advantage of vapor phase epitaxial growth techniques is their flexibility to produce Hg1−xCdxTe layers with difFerent compositions from one run to the next, as well as the flexibility to produce compositional heterostructures of Hg1−xCdxTe in one process step. To take full advantage of this flexibility, reliable, automated control must be introduced. To this end, a phase-modulated spectroscopie ellipsometer (SE) has been implemented for use as a contactless wafer state sensor. In this work SE was used to monitor in real-time the stoichiometry of epitaxial Hg1–4CdxTe during growth by molecular beam epitaxy (MBE). SE has provided valuable information about the MBE growth process, by revealing even small fluctuations in x (± 0.002). In particular, SE has measured the compositional profiles of both LWIR/MWIR and MWIR/LWIR interfaces. Distinct profiles were revealed for interfaces created by abrupt changes in the CdTe effusion cell set-point and for interfaces created by ramping the cell temperature linearly. Ramping results in a smoothly graded interface, whose thickness may be pre-determined, though typically 2000 Å. An abrupt set-point change results in a sharper transition (∼300 Å) followed by oscillations in composition associated with the settling time of the cell (∼1500 Å). The thickness of a CdTe passivation layer grown on a LWTR layer was determined. The current status of the SE will be reported through other illustrative examples which demonstrate its utility as a diagnostic tool and as a sensor for realtime, feed-back control of the MBE process.

High density plasma etching of Hg1−xCdxTe in CH4/H2/Ar chemistry is examined using mass spectroscopy with careful surface temperature monitoring. The dominant etch products are monitored as a function of surface temperature (15–200°C), ion energy (20–200 eV), total pressure (0.5–5 mTorr), microwave power (200–400 W), and flow fraction of methane in the etch gas mixture (0–30%). In addition, observations are made regarding the regions of parameter space which are best suited to anisotropie, low damage etch processing. These observations are compared with previous results in the form of scanning electron micrographs of etched features for anisotropy evaluation and Hall effect measurements for residual damage. Insights to the overall etch mechanism are given.

We present preliminary results on HgCdTe(211)B layers grown on CdTe(211)B/ZnTe(211)B/Si substrates by molecular beam epitaxy. As-grown layers show excellent n-type characteristics as measured by Hall effect. Hall mibilities higher than 1×10 cm2/v-s have been measured at 40K for Cd composition ∼24% with doping level ∼ 3×1015 cm−3. Obtained RHEED patterns during the growth shows very smooth surface morphology.

Raman study of as-grown and H2O2-etched surfaces of Zn0.1Cd0.9 Te single crystal has been performed. A distribution of Te precipitates on the surface of as-grown ZCT, which increases after etching, has been encountered. With high irradiation powers, due to the oxidation of the surfaces, new bands of TeO32− are evolved. A downward shift in the peak position, as well as a halfwidth broadening, of all Raman modes has been observed with increasing laser power. The phenomenon, due to the formation of insulating oxide of tellurium in a dispersion of Te precipitates and vice-versa, has been attributed to the quantum confinement of phonons.

Spectroscopie ellipsometry has been used to monitor optical characteristics of HgCdTe surfaces during plasma etching in an electron cyclotron resonance reactor. Commonly used process conditions were found to induce changes in the ellipsometric parameters Δ and φ. A model was constructed to account for these changes in terms of process-induced roughness and mercury depleted sub-surface layers An independent characterization of the near-surface region was earned out ex situ after etching using Auger spectroscopy and x-ray photoelectron spectroscopy. Plasma process parameters were varied to isolate their influence on surface conditions and a set of parameters is given for which changes are minimized.