HgTe-CdTe Superlattices Research Articles

Investigation of HgTe-HgCdTe superlattices by high-resolution X-ray diffraction

In this study, high-resolution diffraction has been used to investigate the strain state and uniformity of (001) and (112) oriented HgTe-CdTe superlattices grown by molecular beam epitaxy. A number of reciprocal space maps were taken over the surface of the grown wafer, and variations in the spread of lattice spacings and tilts were quantified and used to identify the presence of local defects. Though all growths were fully strained, those with a larger mismatch exhibited a greater spread of lattice tilts from the substrate to the superlattice layers in both orientations. Further, the variation in composition of the CdZnTe substrates resulted in a variation of strain over the surface of the superlattice, and evidence of areas of significant dislocation densities present in substrate layer corresponded with defect-rich areas in the superlattice layer.

Electron cyclotron resonance plasma etching of HgTe-CdTe superlattices grown by photo-assisted molecular beam epitaxy

In order to form HgTe-CdTe superlattice diode arrays, a well-controlled etch process must be developed to form mesa structures on HgTe-CdTe superlattice layers. Wet etch processes result in nonuniform, isotropic etch profiles, making it difficult to control etch depth and diode size. In addition, surface films such as a Te-rich layer may result after wet etching, degrading diode performance. Recently, a dry etch process for HgTe-CdTe superlattice materials has been developed at Martin Marietta using an electron cyclotron resonance plasma reactor to form mesa diode structures. This process results in uniform, anisotropic etch characteristics, and therefore may be a better choice for etching superlattice materials than standard wet etch processes. In this paper, we will present a comparison of etch processes for HgTe-CdTe superlattice materials using electron microscopy, scanning tunneling microscopy, surface profilometry, and infrared photoluminescence spectroscopy to characterize both wet and dry etch processes.

Investigation of monolayer roughness in HgTe-CdTe superlattices

Infrared photoluminescence (PL) measurements were performed on (2ID-oriented superlattices with energy gaps in the range 110–495 meV. Most of the samples with thinner HgTe quantum wells displayed two PL peaks separated by Δhω ≈ 30–65 meV (which generally increased with decreasing well thickness). Both peak energies (Ep) sometimes varied gradually with location on the surface, and in one case three peaks of approximately equal spacing were observed in some locations. The data are consistent with a model which assumes the presence of randomly distributed islands having well thicknesses varying by approximately one monolayer. We find that Ahco and the variations of the spectra with temperature agree well with calculations based on this simple model.

Reflectance and photoreflectance for in-situ monitoring of the molecular beam epitaxial growth of CdTe and Hg-based materials

Epitaxial growth of Hg-based semiconductors by molecular beam epitaxy (MBE) and metalorganic MBE (MOMBE) has progressed sufficiently to shift emphasis to the control of factors limiting the yield of both materials and devices. This paper reports on anex-situ study to evaluate the suitability of reflectance and photoreflectance (PR) asin-situ characterization techniques for the growth of CdTe and HgCdTe. Photoreflectance yields information about CdTe layers, with largest utility for doped and multi-layer structures. However, caution must be taken in interpretation of the spectra since the near-bandedge PR spectra consists of multiple transitions and the E1 transition energy is very sensitive to the sample history. Photoreflectance appears to be of limited utility for HgCdTe single layer growth with x<0.4. However, reflectance measurements of the E1 peak can be used to determine composition in HgCdTe single layers with an accuracy Δx = ±0.01, which can be useful for growth control. A tight binding model was used to calculate the E1 peak energy as a function of bandgap for HgCdTe and HgTe/CdTe superlattices. Comparisons are made with experimental observations. Surface interdiffusion in HgTe-CdTe superlattices was probed using reflectance measurements.

Band structure, magneto-transport, and magneto-optical properties of lnAs-Ga1-xlnxSb superlattices

We have theoretically and experimentally investigated the electronic properties of InAs-Ga1-xInxSb superlattices. It is found that a strong repulsion between the El and HI bands in superlattices with thin Ga1-xInxSb layers leads to dispersion relations that closely resemble those in HgTe-CdTe superlattices. Temperature-dependent magneto-transport and magneto-optical measurements on samples with a range of InAs layer thicknesses confirm several of the theoretically predicted consequences, e.g., the coexistence of two electron species in semimetallic superlattices and a very light electron cyctron mass in narrow-gap semiconducting samples. The electron mobility is found to be dominated by interface roughness scattering under nearly all conditions of interest. Implications for this system as an infrared detector material are discussed.

Monolayer thickness fluctuations in infrared photoluminescence for [211]-oriented HgTe-CdTe superlattices

We report the observation of multiple photoluminescence peaks due to monolayer thickness fluctuations in HgTe-CdTe superlattices. The spectra for seven different [211]-oriented superlattices with band gaps varying from 133 to 495 meV exhibit double peaks, and in nearly all cases the energy splitting corresponds to a difference of ≊1.5 ML in the quantum well thickness.

Narrow-gap HgTe-CdTe superlattices

We review recent progress toward a comprehensive understanding of electronic processes in HgTe-CdTe superlattices. The unique band structure is analyzed, and its incorporation into new theories for transport and magneto-optical properties is discussed. The calculations are compared with recent experimental results, including the observation of enhanced electron mobilities in modulation-doped and setback-modulation-doped structures. Many of the intentionally doped superlattices with strong three-dimensional dispersion display prominent quantum Hall features, in some cases at magnetic fields for which there is no energy gap between the minibands for successive Landau levels. The band structure theory has been extended to the case of HgTe-CdTe quantum wires. It is predicted that due to the small free carrier masses, lateral confinement effects are far larger than those observed in wide-gap systems. A number of new phenomena are expected in narrow-gap nanostructures.

Band gap uniformity and layer stability of HgTe-CdTe superlattices grown by photon-assisted molecular beam epitaxy

Two material properties important to the application of HgTe/CdTe superlattices for device fabrication are band gap uniformity and thermal stability. In this paper, we present the results of an infrared photoluminescence study of (211)B HgTe/CdTe superlattices grown by photon-assisted molecular beam epitaxy which show that cut-off wavelength uniformity can be controlled to a level commensurate with the demands of advanced infrared detector fabrication. Infrared photoluminescence and transmission electron microscopy were also employed to demonstrate that (211)B HgTe/CdTe superlattices are less prone to interdiffusion than previously believed.

Novel magnetotransport and magneto-optical processes in semimetallic HgTe-CdTe superlattices

Recent observations of novel electronic processes in semimetallic HgTe-CdTe superlattices are reviewed, in the context of distinctive features in the superlattice band structures. It is demonstrated that many of the observed phenomena are either directly or indirectly related to an anticrossing of the HH1 band, which is nearly dispersionless in the growth direction kz, and the E1 band, which has strong dispersion in kz. For example, it has recently been demonstrated that there is a critical magnetic field above which one can magnetically activate a bipolar plasma whose density increases linearly with B-Bcrit. A second example is the observation of broad quantum Hall plateaux in superlattices with strong 3D dispersion, which contradicts the previous theoretical understanding of how dimensionality influences the quantum Hall effect. Other topics include recent technological advances towards more sophisticated structures (e.g. controlled modulation doping, set-back doping and submicron lateral patterning), comprehensive new theoretical descriptions of magnetotransport and magneto-absorption in HgTe-CdTe superlattices, and novel processes predicted to occur in HgTe-CdTe quantum wires.

Free-carrier magneto-absorption in HgZnTe-CdTe and HgTe-CdTe superlattices

The authors report transport and far-infrared magneto-optical measurements in n-type HgZnTe-CdTe and HgTe-CdTe superlattices over a large temperature range (1.5-300 K). The data show unambiguously that the HgZnTe-CdTe superlattices are semimetallic at low temperature and are degenerate intrinsic semiconductors for T>100 K, in good agreement with the theory which predicts a semimetal-semiconductor transition induced by temperature. Influence of magnetoplasma effects is finally discussed.

HgTe-CdTe superlattices for infrared detection revisited

Selected properties of HgTe-CdTe superlattices are re-examined in light of the new consensus that the valence-band offset is large. We conclude that while the cutoff wavelength for infrared detectors remains easier to control in superlattices than in the corresponding Hg1−xCdxTe alloy, the advantage is less than was predicted earlier assuming a small offset. The reduction of tunneling noise and minority carrier collection efficiency are discussed on the basis of revised electron and hole masses in the growth direction.

Properties of (211)B HgTe–CdTe superlattices grown by photon assisted molecular-beam epitaxy

The results of detailed optical, electrical, and structural studies performed on a series of (211)B oriented HgTe–CdTe superlattices grown by photon assisted molecular-beam epitaxy are reported. Higher order optical transitions in infrared (IR) photoluminescence (PL) spectra were observed for the first time in HgTe–CdTe superlattices. Low residual carrier concentrations were measured in undoped superlattices. Acceptor-bound excitonic transitions were observed in IR PL spectra of an arsenic doped superlattice for the first time. Excess carrier lifetimes of several hundred nanoseconds were measured, approaching that observed in the corresponding alloy. These results represent significant improvement in the electrical quality of HgTe–CdTe superlattices, and is another step towards the realization of a superlattice-based IR device technology.

Quantum Hall effect and setback modulation doping in HgTe–CdTe heterostructures

HgTe–CdTe superlattices with electron concentrations up to 3×1017 cm−3 have been grown by photoassisted molecular‐beam epitaxy with controlled indium doping of the CdTe barriers. Magnetotransport measurements indicate that the low‐temperature electron mobility is relatively independent of donor concentration at large ND, in contrast to the Hg1−xCdxTe alloy system in which μn strongly decreases with ND. Also studied are the first Hg‐based heterostructures with setback doping, i.e., with an undoped spacer layer between the donors in the middle of the barriers and the electrons in the quantum wells. A 43 Å setback is found to produce a factor of 2 mobility increase over any measured previously in heavily doped samples, and an 81 Å setback leads to further enhancement of μn. All superlattices with doping levels ≥8×1015 cm−3 display the quantum Hall effect. Quantization of the Hall conductivity in multiples of ≊NWe2/h indicates participation by nearly all of the NW periods in the superlattice, implying that the controlled doping is extremely uniform.

Electronic properties of strained (001) HgTe-CdTe superlattices

The electronic structure of strained (001) HgTe-CdTe superlattices is studied for (2, 2) ⩽ (n, m) ⩽ (20, 20), where n(m) is the number of monolayers of HgTe (CdTe). The effects of the valence band offset (VBO) are also considered. The calculation is based on an empirical tight binding (ETB) model together with a surface Green function matching analysis.

Magnetic generation of electrons and holes in semimetallic HgTe–CdTe superlattices

We predict theoretically, on the basis of field‐dependent band structure considerations, that it should be possible to generate a high‐density electron‐hole plasma in semimetallic HgTe–CdTe superlattices simply by applying a magnetic field. When the field is above a critical value, minority carriers should coexist with majority carriers in the zero‐temperature limit, with both densities increasing as B−Bcrit. Experimental confirmation of this unique effect is provided by low‐temperature magneto‐optical data showing the emergence of minority holes in two n‐type superlattices whenever B≥Bcrit.

Programs for modeling waves in quantum-size microstructures

This paper describes a software package QUANTUM which is used in Novosibirsk State University since 1985. QUANTUM was designed to solve different problems in quantum mechanics, solid state physics, semiconductor theory and wave theory. In particular, effects like semiconductor-semimetal transitions, light hole band anti-crossing for HgTe-CdTe superlattices, Stark ladder formation and resonance tunneling of lectrons and holes in GaAs-AlAs superlattices under external bias are studied by means of QUANTUM.

Electron mobilities and quantum Hall effect in modulation-doped HgTe-CdTe superlattices.

Photoassisted molecular-beam epitaxy and controlled modulation doping have been used to grow HgTe-CdTe superlattices with n-type carrier concentrations of up to 3\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. It is found that in contrast to ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Cd}}_{\mathit{x}}$Te alloys where the electron mobility decreases strongly with donor concentration, ${\mathrm{\ensuremath{\mu}}}_{\mathit{n}}$ in the modulation-doped superlattices is nearly independent of ${\mathit{N}}_{\mathit{D}}$ at large ${\mathit{N}}_{\mathit{D}}$. We also discuss an observation of the quantum Hall effect associated with carriers distributed throughout the interior of a HgTe-CdTe superlattice. Whereas previous reports of quantized steps in the Hall conductivity have involved a small number of conduction channels (hence a small fraction of the superlattice periods), we observe plateaus at multiples of \ensuremath{\approxeq}200${\mathit{e}}^{2}$/h in a number of 200-period superlattices with high doping levels. This indicates participation by nearly all wells in the superlattice, and implies that the controlled doping is extremely uniform.

Magnetic activation of bipolar plasmas in HgTe-CdTe superlattices.

It is shown theoretically that in semimetallic HgTe-CdTe superlattices, there is a critical magnetic field above which minority carriers with density proportional to B-${\mathit{B}}_{\mathrm{crit}}$ are expected to coexist with majority carriers in the zero-temperature limit. Experimental confirmation of the magnetically activated bipolar plasma is provided by low-temperature magneto-optical data showing the emergence of minority holes in an n-type superlattice whenever Bg${\mathit{B}}_{\mathrm{crit}}$.

Interface roughness limited electron mobility in HgTe-CdTe superlattices

We demonstrate that interface roughness is the dominant low-temperature scattering mechanism for electrons in HgTe-CdTe superlattices with thin wells. Not only do the experimental mobilities follow the expected d6W dependence, but the observed temperature dependence is accurately reproduced by theory when the treatment of interface roughness scattering is generalized for narrow-gap superlattices. The fits to data yield roughness correlation lengths in the range 60–200 Å.

Theory for electron and hole transport in HgTe–CdTe superlattices

We present results of the first detailed theory for electron and hole transport in HgTe-CdTe superlattices. The calculation incorporates the superlattice band structure in full generality, and also treats multi-well scattering and screening processes which have been ignored in previous theories. It is predicted that whereas the electron and hole mobilities should be nearly equal at low temperatures, the hole mobility falls far below the electron value at somewhat higher temperatures due to the extreme nonparabolicity of the valence band. This prediction is entirely consistent with experimental results reported previously. Excellent quantitative agreement with the data over a broad temperature range is achieved if interface roughness scattering is considered in addition to ionized impurity scattering, acoustic and optical phonon scattering, and electron–hole scattering. It is pointed out that low-temperature electron mobilities for a number of thin-well HgTe–CdTe superlattices follow the d6W dependence expected for the interface roughness mechanism.