IV-VI Compound Semiconductors Research Articles

Laser sintered PbSe semiconductor thin films for Mid-IR applications using nanocrystals

Interests in group IV-VI compound semiconductors for MWIR applications have undergone multiple resurgences over the last 70 years due to the high IR responsivity. Many research groups have shown they can achieve high IR response by making modification of the sensitization process; however, nearly all are using traditional deposition methods (Chemical Bath Deposition or Chemical/Physical Vapor Deposition) involving complex chemical processing or vacuum-based equipment. This work introduces a flexible alternative method for producing ultrapure polycrystalline thin films with exceptional morphological control. The process utilizes a planetary ball-mill to wet-mill ultra-pure crystalline ingot into colloidal nanocrystals (NC), followed by rapid deposition in a centrifuge to form a compact, thin film across a silicon substrate. Laser sintering is then used to rapidly form a continuous denser film with excellent substrate adhesion and unique control of the morphological properties. This work demonstrates a new method using laser sintering that can form a 1.5 μm thin film of PbSe with equivalent physical and electrical (p-type with a carrier concentration of 4.5 × 1017/cm3 and resistivity value of 0.17 Ωcm) properties to CBD/PVD. The results are presented for the process development, film morphology, crystal structure, and electrical properties. Lastly, unlike traditional deposition methods, this alternative process is environmentally responsible by producing zero hazardous waste by allowing recovery of all unused PbSe for reuse. The reported method is applicable for achieving thin films from a variety of high purity bulk materials.

Paradox associated with cohesive energy of IV-VI semiconductors

Cohesive energy of elemental systems is simply the heat of sublimation. However, the cohesive energy for compounds depends on the final products when vaporized. Different descriptions exist based on the final products and a wide range of value has been reported. Here, we compare the cohesive energy reported in the literature for IV-VI (compound) semiconductors and discuss the paradox associated with it. By analysing different descriptions and the value obtained, we argue that the heat of sublimation of IV-VI compounds is the cohesive energy per molecule (as they evaporate in a molecular form) and is much lower that the cohesive energy per atom and the cohesive energy per atom depends on the final products when the solid is vaporized into atoms.

Recent progress on Ⅳ-Ⅵ compound semiconductor heterojunction two-dimensional electron gas

Semiconductor heterojunctions play a crucial role in exploring novel physics and developing advanced devices. Due to the characteristic electronic band structure, such as the narrow bandgap and the large spin-orbital interaction, the Ⅳ-Ⅵ compound semiconductor heterojunctions are not only of great importance to infrared detectors, but also arouse extensively concern in the frontier fields of physics, like topological insulators (TIs) and spintronics. Most excitingly, the two-dimensional electron gas (2DGE) with high electron density and high mobility is revealed at the interface of the typical Ⅳ-Ⅵ compound semiconductor CdTe/PbTe heterojunction, the formation of which is attributed to the unique twisted interface of the Ⅳ-Ⅵ compound semiconductor heterojunctions. Further researches demonstrate that the 2DEG system boasts prominent infrared photoresponse and is of Dirac fermion nature. This review presents the major progress in Ⅳ-Ⅵ compound semiconductor heterojunction 2DEG in the past decades. First, the formation mechanism of the twisted heterojunction 2DEG is discussed based on both theoretical and experimental results. By molecular beam epitaxy the novel lattice-mismatch heterostructure CdTe/PbTe with sharp interface was obtained and first-principle calculations revealed that the alternately changed atomic layer spacing played a crucial role in the formation of 2DEG. High resolution transmission electron microscope image of the interface clearly demonstrated the twisted interfacial structure and showed that the interfacial Te-sharing bonding configuration provided the excessive electrons. Second, we show the transport properties of the 2DEG under the condition of low temperature and high magnetic field, and the unambiguous π Berry phase of quantum oscillations indicate that the 2DEG is of Dirac fermion nature and demonstrate its potential for realizing two-dimensional TI and spintronic device. Moreover, the 2DEG exhibits quite high mobility, making it candidate for high electron mobility transistor. At last, the high-performance mid-infrared photodetector is displayed, which is built based on the typical Ⅳ-Ⅵ compound semiconductor CdTe/PbTe heterojunction. The most exciting feature of the detector is that it is able to achieve high-speed response with satisfying detectivity while working at room temperature, which could be a complementation to state-of-art mid-infrared photodetectors. In summary, the Ⅳ-Ⅵ compound semiconductor heterojunctions are of great significance not only in fundamental physics but also in device applications, and this review could provide the researchers with the main results in the field.

CdTe thin film solar cells with a SnTe buffer layer in back contact

In recent years CdTe thin film solar cells (SCs) have demonstrated promising market application foreground. However, a relatively large gap between the achieved SCs efficiencies and the theoretically predicted maximum efficiency still exists. In particular, the open circuit voltage (Voc) of the CdTe thin film SCs could be further increased, which is limited by high-work-function of CdTe. In this work, p-type narrow-gap SnTe thin films are proposed as a buffer layer in the back contact of CdTe SCs. The band alignment at SnTe/CdTe interface renders a small valance band offset, which expedites hole transport from the CdTe absorber to the hole electrode and improves the Ohmic contact for CdTe. With the insertion of a SnTe single buffer layer at the back contact, a Cu-free CdTe SC has been developed, which displays the same cell efficiency as the traditional cells using CuxTe as the back contact. The Voc of cells with ZnTe: Cu/SnTe double-layer at back contact is notably higher than the CdTe SCs fabricated with CuxTe or ZnTe: Cu single-layer at the back contact. Furthermore, accretion of hole concentration is also demonstrated by the diffusion of Sn in the CdTe absorber layer. This work shows the potential to improve CdTe SCs by using a narrow-gap IV-VI compound semiconductor.

Electron-conducting quantum dot solids: novel materials based on colloidal semiconductor nanocrystals

We review the optical and electrical properties of solids that are composed of semiconductor nanocrystals. Crystals, with dimensions in the nanometre range, of II-VI, IV-VI and III-V compound semiconductors, can be prepared by wet-chemical methods with a remarkable control of their size and shape, and surface chemistry. In the uncharged ground state, such nanocrystals are insulators. Electrons can be added, one by one, to the conduction orbitals, forming artificial atoms strongly confined in the nanocrystal. Semiconductor nanocrystals form the building blocks for larger architectures, which self-assemble due to van der Waals interactions. The electronic structure of the quantum dot solids prepared in such a way is determined by the orbital set of the nanocrystal building blocks and the electronic coupling between them. The opto-electronic properties are dramatically altered by electron injection into the orbitals. We discuss the optical and electrical properties of quantum dot solids in which the electron occupation of the orbitals is controlled by the electrochemical potential.

Atomic Ordering in Self-assembled Epitaxial II-VI and IV-VI Compound Semiconductor Quantum Dot Systems

ABSTRACTTransmission electron microcopy (TEM) in both the parallel illumination and scanning probe mode revealed atomically ordered entities within a 5 to 250 nm range in a IV-VI and a II-VI compound semiconductor quantum dot (QD) system. While the II-VI system was a nominal [001] CdSe/(Mn0.1Zn0.9)Se multilayer structure with the QDs embedded, the IV-VI system nominally consisted of [111] PbSe islands. The comparison of photoluminescence (PL) spectra from the CdSe/(Mn0.1Zn0.9)Se structure with those of a reference structure, that was grown to the same nominal specification under otherwise identical conditions except that no Mn was incorporated into the cladding layers, revealed for the former sample two peaks at approximately 2 and 2.1 eV. We tentatively attribute these two PL peaks to two groups of atomically ordered entities.

IV-VI Compound Semiconductor Mid-Infrared Vertical Cavity Surface Emitting Lasers Grown by MBE

AbstractMid-infrared vertical cavity surface emitting lasers (VCSELs) using PbSe as the active material and broadband high reflectivity Pb1−xSrxSe/BaF 2 distributed Bragg reflectors (DBR) as bottom and top mirrors were grown by molecular beam epitaxy. By pulsed optical pumping, this first IV-VI semiconductor VCSEL operated up to 290K at a wavelength of 4.5 µm. Further optimization of such VCSELs could lead to room temperature continuos wave operation.

High pressure phase transitions and elastic properties of IV-VI compound semiconductors

Abstract We have investigated the high pressure phase transition and elastic behaviour of IV-VI compound semiconductors (PbS, PbSe, PbTe and SnTe) by means of a three-body potential (TBP) model, which includes the effects of three-body interactions (TBI) and van der Waals attraction in the framework of the Hafemeister and Flygare potential. This TBP model has been found to be successful in predicting the experimental data on pressure induced phase-transitions and associated volume collapses and elastic properties of these semiconductors. The significance of the TBI has been illustrated by computing the results with and without the inclusion of TBI effects. The results obtained from the TBP model are in better agreement with experimental data than those achieved from other models and from our model II which excludes the effects of TBI.

Modification of Drude model on the free carrier property of IV-VI compound semiconductor in magnetic field

A general analytical expression is reported for the Drude model's dielectric function of free carrier in multi-valley IV-VI compound semiconductor in the magnetic field. The modifications of the energy band non-parabolicity and free carrier re-population are introduced to the dielectric function in combination with the\(\vec k \cdot \vec P\) model. The difference of the dielectric function between the modified Drude model and the classical Drude model is demonstrated by the calculation for the typical IV-VI compound semiconductor material PbTe.

Double Heterostructure Pb1-xSrxS/Pb1-ySryS Lasers Prepared Using Hot Wall Epitaxy

Pb1-xSrxS/Pb1-ySryS double heterostructure (DH) lasers were prepared for the first time using a hot wall epitaxy (HWE) system and their operating characteristics were determined. Under pulsed measurement conditions, the operational wavelength for the lasers became as low as 2.07 µm at 140 K. This wavelength is the shortest wavelength ever reported for a IV-VI compound semiconductor laser.

Vacancies in II-VI- and IV-VI Compound Semiconductors Studied by Positron Lifetime Spectroscopy

Vacancies in II-VI- and IV-VI Compound Semiconductors Studied by Positron Lifetime Spectroscopy

Physics and applications of IV-VI compound semiconductor lasers

Lasers made of lead salt compounds have proven to be best suited for coverage of the mid-infrared region from 3 to 30 mu m. Nowadays double-heterostructure (DH) lasers are the standard, using as active layers PbEuSSe for the short-wavelength region below 4 mu m, PbEuTeSe or PbEuSe for the 4-8 mu m range and PbSnTe or PbSnSe for wavelengths beyond 8 mu m. Originally liquid phase epitaxy but now molecular beam and hot wall epitaxy techniques are most commonly used to produce a wide variety of laser structures. While simple DH lasers with contact or mesa stripes for lateral confinement prevail in commercial devices, highly sophisticated prototypes with buried layer, corrugated distributed Bragg reflector (DBR) and distributed feedback (DFB) and single- and multi-quantum-well structures have been successfully manufactured. Gas spectroscopy has remained the main field of application where mode quality and tuning properties of the emitted radiation are most important. The dominant development goal has been the increase of the operating temperature to make the use of simple cooling equipment possible. Theoretical models for threshold current calculations have been established; however, the reliability of the results has been impaired by the fact that basic material properties of the ternary and quaternary compounds are not well know. In the course of this paper the state of the art and the development trends of lead salt lasers are described. The various fields of applications are summarised. Two examples-one dealing with car exhaust monitoring, the other with clinical isotopic ratio measurements of the exhaled breath-are discussed in some detail.

Investigation of the bulk band structure of IV-VI compound semiconductors: PbSe and PbTe.

The bulk band structure of the IV-VI semiconductors PbSe and PbTe was determined experimentally through angle-resolved photoemission from cleaved PbSe(100) and PbTe(100) surfaces, using synchrotron radiation for photoexcitation. By analyzing spectra recorded in electron emission normal to the surface over a wide range of photon energies, it was found that a large number of photoemission peaks showed dispersion with photon energy, proving that the spectra can be interpreted through direct (i.e., k-conserving) transitions. We analyze the dispersion in terms of transitions from occupied to unoccupied bands, on the basis of the free-electron final-state model. In order to relate the experimental data to calculated band structures, we have carried out linearized muffin-tin-orbital calculations for both solids. The experimental occupied band structure shows excellent agreement with these calculations. We discuss our results in the light of previous experimental and theoretical studies of these materials.

Reflection high-energy electron diffraction intensity oscillations in IV-VI compound semiconductors

Reflection high-energy electron diffraction intensity oscillations were observed for the first time during layer-by-layer growth of IV-VI compound semiconductors. Epitaxial PbSe and PbTe layers were grown by molecular beam epitaxy on cleaved BaF2 (111) at a substrate temperature range of 210–350 °C. The growth rate for both compounds was measured using in situ intensity oscillations, and was verified by stylus thickness measurements. Very little variation in growth rate was observed while varying the substrate temperature in this range.

The nature of molecular beam epitaxial growth examined via computer simulations

Abstract Man-made synthetic structures involving epitaxially grown thin films of one or more compound semiconductors have been finding an increasingly important role in semiconductor device technology.1 Remarkable advances have been made over the past decade in the realization of a variety of such structures involving III-V, II-VI. and IV-VI compound semiconductors, as well as combinations of semiconductors with dielectrics and metals.' The major part of this progress has come about primarily due to the advent of, and refinements in, two vapor phase growth techniques — molecular beam epitaxy (MBE)' and metal-organic chemical vapor deposition (MO-CVD).3 The underlying motivation has been the remarkably altered and potentially useful electronic and optical properties of electrons, holes, and light arising from their confinement in a quasi-two-dimensional environment in the ultrathin films (thickness less than the particle deBroglie wavelength).4 However, confinement of the par- ticles in such ultrathin laye...

Universal band structures for group-V elements and IV-VI compound semiconductors

The tight-binding band structures of the group-V elements As, Sb, and Bi and the IV-VI compound semiconductors have been found by the fitting of previous calculations and photoemission data. The two-center interactions are found to scale with bond length $d$. The $\mathrm{pp}\ensuremath{\sigma}$ interaction scales as ${d}^{\ensuremath{-}2.6}$ and the $\mathrm{ss}\ensuremath{\sigma}$ interaction as ${d}^{\ensuremath{-}4.6}$. In the tetravalent diamond semiconductors both $s$ and $p$ interactions scale as ${d}^{\ensuremath{-}2}$. The different scaling law reflects bonding differences; both $s$ and $p$ electrons are involved in bonding in diamond while only $p$ electrons are used in the group-V and IV-VI compound solids. The states of the group-V elements are reasonably well reproduced with only first-neighbor interactions. Second-neighbor interactions or excited ${s}^{*}$ and ${d}^{*}$ states are needed to fit the narrow gaps of the IV-VI compound semiconductors. The electronic structure of the rocksalt phase of InSb is also calculated and used to illustrate the differences in bonding between III-V and IV-VI compounds.

Experimental Study on the Epitaxial Films of PbSnTe Grown by Hot Wall Epitaxy Technique

The narrow gap lead-tin-telluride (Pb1-x Snx Te) compound semiconductor has assumed an increasingly important role in the field of IR detectors. One of the techniques which has contributed significantly to the preparations of epitaxial films of IV-VI compound semiconductors with bulk-like properties is the Hot Wall Epitaxy Technique (HWE). This HWE technique has been employed for the growth of epitaxial films of Pb0.8Sn0.2 Te by us. Films have been grown on (100) KCI substrates at various temperatures ranging from 250°C to 350°C. The crystallinity and the surface qualities have been examined by X-ray techniques and Selected area Channelling patterns using Scanning Electron Microscope (SEM) Electrical characteristics such as resistivity and mobility are measured using Van der Pauw Technique. IR radiation response of these films is also investigated from room temperature to liquid nitrogen temperature.

Ferroelectric lattice instabilities in narrow band gap semiconductors

Abstract The recent investigation of the ferroelectric lattice instability in IV-VI compound semiconductors is presented with emphasis on the interaction of to phonons with electrons.

The Transverse Effective Charge of the IV-VI Compound Semiconductors

Littlewood's assumption in the calculation of the transverse effective charge (TEC) of IV-VI compound semiconductors is studied by the simple calculation at W -point on the Jones zone surfaces based on the Heine-Jones model. It is shown that the main peak of optical absorption spectrum cannot be assigned only to (111) component of the effective charge in contrast with Littlewood's assumption. The TEC is directly calculated by executing the band calculation with the potential obtained by the EPM in the framework of Vogl's expression. The calculated result gives better agreement with experiment than Littlewood's. The acoustic sum rule is also studied by using the similar formulation given by Vogl.

Theory of anomalous resistivity associated with structural phase transitions in IV-VI compounds

A theoretical description of electrical-resistivity anomalies associated with structural phase transitions of IV-VI compound semiconductors is presented, taking account of interband electron---soft-TO-phonon scattering. The sublattice displacement and rhombohedral strain in the low-temperature phase are calculated within the framework of a self-consistent mean-field theory, with emphasis on the effect of electron-phonon couplings. The resistivity due to the scattering of free carriers from acoustic phonons and optic phonons is examined, including the carrier transfer among the four valleys due to valley splitting below the transition temperature (${T}_{c}$). The theoretical calculations of electrical resistivity as well as order parameters are in good accord with data on $p$-type SnTe at the quantitative level.