Wide Bandgap Compound Research Articles

Performance enhancements of compound semiconductor radiation detectors using digital pulse processing techniques

Abstract The potential benefits of using compound semiconductors for X-ray and gamma ray spectroscopy are already well known. Radiation detectors based on high atomic number and wide band gap compound semiconductors show high detection efficiency and good spectroscopic performance even at room temperature. Despite these appealing properties, incomplete charge collection is a critical issue. Generally, incomplete charge collection, mainly due to the poor transport properties of the holes, produces energy resolution worsening and the well known hole tailing in the measured spectra. In this work, we present a digital pulse processing (DPP) system for high resolution spectroscopy with compound semiconductor radiation detectors. The DPP method, implemented on a PC platform, performs a height and shape analysis of the detector pulses (preamplifier output pulses), digitized by a 14-bit, 100 MHz ADC. Fast and slow shaping, automatic pole-zero adjustment, baseline restoration and pile-up rejection allow precise pulse height measurements both at low and high counting rate environments. Pulse shape analysis techniques (pulse shape discrimination, linear and nonlinear pulse shape corrections) to compensate for incomplete charge collection were also implemented. The results of spectroscopic measurements on a planar CdTe detector show the high potentialities of the system, obtaining low tailing in the measured spectra and energy resolution quite close to the theoretical limit. High-rate measurements (up to 820 kcps) exhibit the excellent performance of the pulse height analysis and the benefits of pulse shape techniques for peak pile-up reduction in the measured spectra. This work was carried out in the framework of the development of portable X-ray spectrometers for both laboratory research and medical applications.

Low-temperature anomalies in the dynamic elastic moduli of cubic AIIBVIcrystals with 3d-transition metal impurities

A negative paramagnetic contribution to the dynamic elastic moduli is identified in AIIBVI:3d wide band-gap compounds for the first time. It appears as a paramagnetic elastic, or, briefly, paraelastic, susceptibility. These compounds are found to have a linear temperature dependence for the inverse paraelastic susceptibility. This is explained by a contribution from the diagonal matrix elements of the orbit-lattice interaction operators in the energy of the spin-orbital states of the 3d-ion as a function of applied stress (by analogy with the Curie contribution to the magnetic susceptibility). The inverse paraelastic susceptibility of AIIBVI crystals containing non-Kramers 3d-ions is found to deviate from linearity with decreasing temperature and reaches saturation. This effect is explained by a contribution from nondiagonal matrix elements (analogous to the well known van Vleck contribution to the magnetic susceptibility of paramagnets).

Czochralski Growth of Indium Iodide and other Wide Bandgap Semiconductor Compounds

ABSTRACTThe Czochralski pulling process is the most valuable and cost efficient method for producing large oriented single crystals of the group IV and III-V semiconductors. However, there have been only a small number of reported attempts to use the Czochralski process for growing the wide bandgap compound semiconductors, needed for the room temperature operated gamma-ray detectors. The main difficulty is in the low chemical stability and high vapor pressure of the group II, V and VI elements, leading to off-stoichiometric composition, and various related defects. Among the heavy metal halides, indium iodide and indium bromide present an interesting exception. InI has a high molecular disassociation energy and a low vapor pressure, allowing for Czochralski pulling. We will describe the procedures used and the results obtained by Czochralski growth and characterization of indium iodide and the related ternary compounds that appear to be quite encouraging.

Electrical field induced direct-to-indirect bandgap transition in ZnO nanowires

As a wide bandgap (3.37 eV) compound semiconductor with the exciton binding energy of 60 meV, ZnO nanowires have attracted considerable interest because of great potential applications in micro-optoelectronics and nano-optoelectronics. However, ZnO nanowires as basic units of microelectronic and optoelectronic nanodevices usually work in the circumstance of the electrical field. Therefore, the influence of electrical field on the electronic structure of ZnO nanowires partly determines the performance of nanodevices. For this issue, we inhere have studied the effects of the applied electrical field on the electronic structure of ZnO nanowires using the density functional theory. It was found that the applied electrical field cannot only effectively modulate the bandgap of ZnO nanowires but also leads to the bandgap transition from direct to indirect. Meanwhile, the above effects of electrical field depend on the size and shape of ZnO nanowires. The physical understanding of the electrical field effects of the energy band structure of ZnO nanowires is attributed to the bond length changes in lattice and the charge transfer between Zn and O atoms. Thus, these valuable investigations are important for designing the microelectronic and optoelectronic nanodevices based on ZnO nanowires.

Preface: Phys. Status Solidi C 6/12

AbstractThis issue contains a selection of papers presented at the 15th International Semiconducting and Insulating Materials Conference (SIMC‐XV), held at the Vilnius University, Vilnius, Lithuania, from 15 to 19 June 2009. It was jointly organized by the Semiconductor Physics Institute and the Vilnius University, both in Vilnius, Lithuania. SIMC‐XV is the latest in a biennial conference series, initiated in 1980 in Nottingham, UK. This conference has a long history of providing a forum for exchanging ideas for researchers active in the fields of semiconducting and insulating materials. Over its 29 year long history, during which SIMC was held in 11 countries on 4 continents, one feature has not changed: it is a premier conference series that builds bridges between experts investigating material technology, their properties, and device applications.SIMC‐XV was attended by about 100 people from 22 countries. The technical sessions encompassed many new and developing topics, which included wide bandgap compound semiconductors, diluted nitride and bismide compounds, low dimensional semiconductor nanostructures, and complex oxide materials. The conference was arranged into 16 oral sessions that included 22 invited talks presented by distinguished experts in their fields and 39 contributed talks. 35 more contributions were presented as posters. The selection of the papers presented in this volume reflects the multidisciplinary and multinational nature of the SIMC series.We offer our thanks to all members of the conference committees staff and students of the Semiconductor Physics Institute and all the SIMC‐XV participants for their contributions to the success of the conference. We are especially indebted to Dr. Renata Butkutė, Dr. Vaidas Pačebutas, and Prof. Kęstutis Jarašiūnas for their invaluable help in organizing the conference. Our acknowledgements also go to our sponsors – Lithuanian Physical Society, Vilnius University, Lithuanian State Science and Studies Foundation, Office of Naval Research Global, TECO René Koch, Labochema, SVT Associates, Inc., Veeco, TeraVil, and EKSPLA for their financial support that has made SIMC‐XV possible.We wish Prof. Sebastian Lourdudoss, the chair of SIMC‐XVI, success in organizing the next conference in Stockholm, Sweden, in 2011 (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanoparticles doped with TM and RE ions for applications in optoelectronics

Nanoparticles of wide band gap compounds, when doped with rare earth or transition metal ions, are perspective candidates for efficient phosphors in a new generation of light sources for an overhead illumination, i.e., in compact fluorescence lamps and in semiconductor-based white light emitting diodes. Mechanisms of emission enhancement in doped nanoparticles are discussed based on the relevant experimental results. Mechanisms observed are due to carrier confinement, n-type co-doping, due to surface plasmons generation and super radiance.

The recent advances of research on p-type ZnO thin film

ZnO is a direct wide-band gap (3.37 eV) compound semiconductor with large exciton binding energy (60 meV) at room temperature. Therefore it has a strong potential for various short-wavelength optoelectronic device, and now attracts tremendous renew interests for developing highly efficient ZnO-based optoelectronic devices. While high quality ZnO p–n junction materials obtained is the key step of its optoelectronic application. Whereas ZnO thin film is naturally only n-type conductivity due to a large number of native defects, such as oxygen vacancies and zinc interstitials, which lead to difficulty in achieving p-type ZnO thin film. Therefore the fabrication of p-type ZnO thin film has been a key and hotspot of the research on ZnO. This article summarizes the recent advances of the studies on p-type ZnO thin film and the correlative several important breakthroughs in ZnO homo-junction devices based on succeeding on fabrication of p-type ZnO film. Although the achievement obtained as summarized, there is also a long way from the real application of ZnO-based optoelectronic device. We here also discuss the problem and relevant possible solution for the fabrication of p-type ZnO film and its optoelectronic application. And forecast the preparation trends of p-type ZnO thin film.

A novel in situ method for simultaneous growth of smart material single crystals and thinfilms

Development of a novel in situ method for simultaneous growth of single crystalsand thin films of a smart material spinel is achieved. Material to be grown asmetal-incorporated single crystal and thin film was taken as a precursor andput into a bath containing acid as a reaction speed-up reagent (catalyst) as wellas a solvent with a metal foil as cation scavenger. By this novel method, zincaluminate crystals having hexagonal facets and thin films having single crystallineorientation were prepared from a single optimized bath. Properties of bothcrystals and thin films were studied using an x-ray diffractometer and EDAX.ZnAl2O4 is a well-known wide bandgap compound semiconductor(Eg = 3.8 eV), ceramic, opto-mechanical and anti-thermal coating in aerospace vehicles. Thus a space_gmr technique was found to be a new low cost and advantageous method for in situand simultaneous growth of single crystals and thin films of a smart material.

Semiconductor materials and radiation detection

While Si and Ge have become detection standards for X-ray and gamma-ray spectroscopy in the laboratory, their use for an increasing range of applications is becoming marginalized by one or more of their physical limitations; namely the need for ancillary cooling systems or bulky cryogenics, their modest stopping powers and radiation intolerance. Wide band-gap compounds offer the ability to operate in a range of chemical, thermal and radiation environments while still maintaining sub-keV spectral resolution at X-ray wavelengths. In addition, these materials encompass such a wide range of physical properties that it is technically feasible to engineer materials to specific applications. However, while compound materials are used routinely in the optical and infrared wave bands, their development at hard X- and gamma-ray wavelengths has been plagued by material and fabrication problems. In this paper an overview of suitable materials is presented and the current progress in producing X- and gamma-ray radiation detectors is reviewed.

A novel growth method for ZnAl2O4 single crystals

ZnAl2O4 is a well-known wide band gap compound semiconductor (Eg=3.8eV), ceramic, opto-mechanical, anti-thermal coating in aero-space vehicles and UV optoelectronic devices. A novel method for the growth of single crystals of a ternary oxide material was developed as a fruit of a long term work. Material to be grown as metal incorporated single crystal was taken as precursor and put into a bath containing acid as reaction speed up reagent (catalyst) as well as solvent with a metal foil as cation scavenger. Using this method, ZnAl2O4 crystals having hexagonal facets are prepared from a single optimized bath. Structural and compositional properties of crystals were studied using Philips, Xpert - MPD: X-ray diffractometer and Philips, ESEM-TMP + EDAX. Thus technique was found to be a new low cost and advantageous method for growth of single crystals of ternary oxide a material. We hope that these data be helpful either as a scientific or technical basis in material processing. Dedicated to Prof. P. Ramasamy © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A novel in situ synthesis and growth of ZnAl 2O 4 thin films

ZnAl 2O 4 is a spinel oxide, widely used as high-temperature material, catalyst, catalyst support, optical coating for spacecrafts and emerging as one of the best wide band gap compound semiconductor ( E g = 3.8 eV ) for UV optoelectronic applications. In this work, spinel oxide is synthesized by the simple, in situ, acid-based chemical reaction and the thin film was deposited as a product of reaction by liquid-phase deposition (LPD). Material to be grown as metal incorporated thin film was taken as precursor and put into a bath containing acid, catalyst. The acid also serves as solvent with a metal foil as cation scavenger. Uniform and highly adherent ZnAl 2O 4 thin films were prepared. Structural, compositional and surface morphological properties of thin films were studied using Philips, Xpert—MPD: X-ray diffractometer and Philips, ESEM—TMP+EDAX, Hitachi S-450: SEM, Nanoscope—III: AFM.

Bulk crystal growth of zinc aluminate spinel at room temperature

Zinc aluminate ZnAl2O4 is a well known wide band gap compound semiconductor (Eg=3·8 eV) used in UV photoelectronic devices, antithermal coatings for spacecraft, and ceramic and catalyst materials. In the present work, the development of a single step process to synthesise and grow ZnAl2O4 spinel at low temperature is described. In this context, novel, economical and spontaneous growth of a bulk crystal of a ternary oxide material is reported. ZnAl2O4 crystals were grown at room temperature (30°C). Material to be grown as metal incorporated single crystals was taken as precursor and put into a neutral bath containing acid as reaction acceleration reagent (catalyst) as well as solvent with a metal foil (cation scavenger). Zinc aluminate crystals have been grown in a single step process using a chemical bath under optimised conditions in an average period of 15 days. Crystals were grown with hexagonal facets and XRD measurement confirmed the growth of a single crystal. Elemental (Zn, Al, O) and chemical compositional characterisation was done by EDAX. Thus the technique was found to be an advantageous low cost method for the growth of bulk single crystals of ternary oxide materials.

Molecular beam epitaxial growth and characterization of Cd-based II–VI wide-bandgap compounds on Si substrates

We have carried out a detailed study on the growth of Cd-based II–VI compounds on Si substrates using molecular beam epitaxy (MBE). CdTe, CdSe, CdSeTe, and CdZnSeTe layers were nucleated and grown on Si(211) substrates in order to study a broad range of semiconductor properties, such as crystal structure, fundamental bandgap, surface morphology, and defect and dislocation density as a function of the constituent elements. For structural characterization, we used transmission electron microscopy (TEM) and x-ray diffraction, which indicated that cubic Cd1-xSexTe material had been grown on Si substrates throughout the entire composition range. Likewise, photoreflectance (PR) and photoluminescence (PL) were used to measure the optical response in the near bandgap (Eg) region. Results indicated nonrandom ordering of CdSeTe/Si material. Studies on quaternary CdZnSeTe material indicated that Zn and Se concentrations directly impact surface morphology with extremely smooth surfaces obtained as Zn content is decreased.

Compound semiconductor radiation detectors

We discuss the potential benefits of using compound semiconductors for the detection of X- and γ-ray radiation. While Si and Ge have become detection standards for energy dispersive spectroscopy in the laboratory, their use for an increasing range of applications is becoming marginalized by one or more of their physical limitations; namely the need for ancillary cooling systems or bulky cryogenics, their modest stopping powers and radiation intolerance. Compound semiconductors encompass such a wide range of physical properties that it is technically feasible to engineer a material to any application. Wide band-gap compounds offer the ability to operate in a wide range of thermal and radiation environments, whilst still maintaining sub-keV spectral resolution at hard X-ray wavelengths. Narrow band-gap materials, on the other hand, offer the potential of exceeding the spectral resolution of both Si and Ge, by as much as a factor of 3. Assuming that the total system noise can be reduced to a level commensurate with Fano noise, spectroscopic detectors could work in the XUV, effectively bridging the gap between the ultraviolet and soft X-ray wavebands. Thus, in principle, compound semiconductor detectors can provide continuous spectroscopic coverage from the far infrared through to γ-ray wavelengths. However, while they are routinely used at infrared and optical wavelengths, in other bands, their development has been plagued by material and fabrication problems. This is particularly true at hard X- and γ-ray wavelengths, where only a few compounds (e.g., GaAs, CdZnTe and HgI2) have evolved sufficiently to produce working detection systems. In this paper, we examine the current status of research in compound semiconductors and by a careful examination of material properties and future requirements, recommend a number of compounds for further development. In the longer term, when material problems are sufficiently under control, we believe the future lies in the development of heterostructures and inserted interface layers to overcome contacting problems and quantum heterostructures and superlattices to facilitate low-noise readout.

Model of the temperature field in tube furnaces and its application to a system of “contactless” crystal growth from the vapour

The temperature field created in a tube furnace by thermal radiation is described here with an easy to use model suitable for the determination of the axial and radial temperature profiles in the furnace chamber. Particular attention is paid to the radial temperature differences and their role in the evaporation-condensation processes—these differences ensure “contactless” crystal growth from the vapour at around 1100K by preventing condensation on the ampoule wall. It is shown that the creation of radial temperature differences of a few degrees is achievable by interaction of the solid material with more distant zones of the furnace. The model has proved to be a tool for the description and control in self-selecting vapour growth as initially applied to IV–VI compounds and extended to II–VI wide bandgap compounds and, especially, their solid solutions. Broader use of the model is expected.

Structure and electronic properties of new model dinitride systems: a density-functional study of CN 2, SiN 2, and GeN 2

The dinitrides CN 2, SiN 2, and GeN 2 in assumed pyrite-type structures are studied by means of density functional theory using both ultrasoft pseudopotentials and the augmented spherical wave (ASW) method. The former two materials constitute the large- x limit of the broader class of CN x and SiN x compounds, which are well known for their interesting mechanical and electronic properties. For CN 2 a large bulk modulus B 0 of 405 GPa was determined. While SiN 2 is found to be a wide band gap compound, the calculated gaps of CN 2 and GeN 2 are considerably smaller. The trends in structural and electronic properties, as e.g., bond lengths, band gaps and covalency are well understood in terms of the interplay of different types of bonding.

Synthesis and characterization of the wide band-gap compound Pr 2Te 4O 11

Single crystals of Pr 2Te 4O 11 have been grown through the reaction of Pr 6O 11 and TeO 2 in a CsCl flux at 1123 K. This compound, which is isostructural with Nd 2Te 4O 11 and Ho 2Te 4O 11, crystallizes in space group C2/ c of the monoclinic system with four formula units in a cell of dimensions at 153 K of a=12.6880(6) Å, b=5.2361(3) Å, c=16.2920(8) Å, β=106.052(1)°, and V=1040.17(9) Å 3. The three dimensional structure is made from the interconnection between ∞ 2[Pr 2O 10 14−] and ∞ 2[Te 4O 11 6−] networks. Pr 2Te 4O 11 is a Curie–Weiss paramagnetic with an effective magnetic moment of 3.59(2) μ B. Optical diffuse reflectance spectroscopy shows typical 4f–4f optical transitions for Pr(III), in addition to a wide band gap of 3.65 eV.

Blue-Violet Avalanche-Photodiode (APD) and its Ionization Coefficients in II–VI Wide Bandgap Compound Grown by Molecular Beam Epitaxy

We present II–VI wide bandgap semiconductor ZnSe and ZnSSe based blue-violet avalanche photo-diode (APD) operation grown on GaAs substrate by molecular beam epitaxy (MBE). Because of highly improved crystal quality of the active layer, the ZnSSe APD reveals stable and high avalanche gain (G ≥ 60) in the blue-violet optical region under high field operation condition (E = 1.15 × 106 V/cm) at room temperature (RT). High electric field induced ionization coefficients α (for electrons) and β (for holes) are determined as a function of electric field strength E(V/cm); α(E) = 1.7 × 107 exp (—4.9 × 106/E) and β(E) = 3.7 × 106 exp (—4.0 × 106/E).

Mechanism of Slow-Mode Degradation in II–VI Wide Bandgap Compound Based Blue-Green Laser Diodes

Mechanism of Slow-Mode Degradation in II–VI Wide Bandgap Compound Based Blue-Green Laser Diodes

Mechanism of Slow-Mode Degradation in II-VI Wide Bandgap Compound Based Blue-Green Laser Diodes

Dislocation-free blue-green laser diodes of (ZnCd)Se/(ZnMg)(SSe) show a “slow-mode” degradation during device operation. This degradation is caused not by generation and propagation of macroscopic defects, but by microscopic-point defect reaction (marked enhancement in its concentration and resulting migration process) during high-density carrier-injection. It is also evidenced experimentally that the direct driving force on the marked defect reaction is derived from minority carrier injection induced e–h non-radiative recombination process at localized point defect centers in the p-type cladding layer of the LD devices.