Deep levels in low‐energy electron‐irradiated 4H‐SiC

Deep levels introduced by low‐energy (200 keV) electron irradiation in n‐type 4H‐SiC epitaxial layers grown by chemical vapour deposition were studied by deep level transient spectroscopy (DLTS) and photoexcitation electron paramagnetic resonance (photo‐EPR). After irradiation, several DLTS levels, EH1, EH3, Z_1/2, EH5 and EH6/7, often reported in irradiated 4H‐SiC, were observed. In irradiated freestanding films from the same wafer, the EPR signals of the carbon vacancy in the positive and negative charge states, V_C⁺ and V_C^‐, respectively, can be observed simultaneously under illumination with light of certain photon energies. Comparing the ionization energies obtained from DLTS and photo‐EPR, we suggest that the EH6/7 (at ～E_C – 1.6 eV) and EH5 (at ～E_C – 1.0 eV) electron traps may be related to the single donor (+ | 0) and the double acceptor (1– | 2–) level of V_C, respectively. Judging from the relative intensity of the DLTS signals, the EH6/7 level may also be contributed to by other unidentified defects. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sulfur doping of AlN and AlGaN for improved n‐type conductivity

Achieving high levels of n‐type conductivity in AlN and high Al‐content nitride alloys is a long standing problem; significant decreases in conductivity are observed as the Al content is increased, a phenomenon that has been attributed to donors such as oxygen or silicon forming DX centers. We address this problem through a comprehensive first‐principles hybrid density functional study of potential n‐type dopants, identifying S_N and Se_N as two elements which are potential shallow donors because they do not undergo a DX transition. In particular, S_N is highly promising as an n‐type dopant because it also has a low formation energy and hence a high solubility. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Surface passivation schemes for high‐efficiency n‐type Si solar cells

An effective passivation on the front side boron emitter is essential to utilize the full potential of solar cells fabricated on n‐type silicon. However, recent investigations have shown that it is more difficult to achieve a low surface recombination velocity on highly doped p‐type silicon than on n‐type silicon. Thus, the approach presented in this paper is to overcompensate the surface of the deep boron emitter locally by a shallow phosphorus diffusion. This inversion from p‐type to n‐type surface allows the use of standard technologies which are used for passivation of highly doped n‐type surfaces. Emitter saturation current densities (J_0e) of 49 fA/cm² have been reached with this approach on SiO₂ passivated lifetime samples. On solar cells a certified conversion efficiency of 21.7% with an open‐circuit voltage (V_oc) of 676 mV was achieved. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The study of optical properties of amorphous thin films of mixed oxides In<Subscript>2</Subscript>O<Subscript>3</Subscript>—SnO<Subscript>2</Subscript> system,deposited by co-evaporation

A discussion of optical properties of mixed oxides In₂O₃—SnO₂ system is presented. Film thickness, substrate temperature, composition (in molar %) and annealing have a profound effect on the structure and optical properties of these films. Initially the increase in band gap with the increase of SnO₂ content in In₂O₃ is due to the increase in carrier density as a result of donor electrons from tin. The decrease in band gap above the critical Sn content is caused by the defects formed by Sn atoms, which act as carrier traps rather than electron donors. The increase in band gap with film thickness is caused by the increase in free carrier density which is generated by (i) Sn atom substitution of In atom, giving out one extra electron and (ii) oxygen vacancy acting as two electrons donor. The decrease in band gap with substrate temperature and annealing is due either to the severe deficiency of oxygen, which deteriorate the film properties and reduce the mobility of the carriers, or to the formation of indium species of lower oxidation state (In²⁺).     

p‐type conducting ZnO:P microwires prepared by direct carbothermal growth

Phosphorous‐doped (ZnO:P) and undoped ZnO wires with diameter of several micrometers and length of several hundred micrometers were thermally grown directly on phosphorus pentoxide doped and undoped ZnO:graphite targets, respectively. The cathodoluminescence spectra of single ZnO:P microwires show three typical acceptor‐related emissions which are attributed to (A⁰, X), (e, A⁰), and DAP. Three‐terminal, gate voltage dependent electrical measurements of back‐gate field effect transistors with the microwires as channels indicate reproducibly that undoped ZnO and ZnO:P microwires are n‐type and p‐type conductive, respectively. The p‐type conductivity was found to be stable over more than six months. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

n‐type silicon solar cells with surface‐passivated screen‐printed aluminium‐alloyed rear emitter

Aluminium‐doped p‐type (Al‐p⁺) silicon emitters fabricated by means of a simple screen‐printing process are effectively passivated by plasma‐enhanced chemical‐vapour deposited amorphous silicon (a‐Si). We measure an emitter saturation current density of only 246 fA/cm², which is the lowest value achieved so far for a simple screen‐printed Al‐p⁺ emitter on silicon. In order to demonstrate the applicability of this easy‐to‐fabricate p⁺ emitter to high‐efficiency silicon solar cells, we implement our passivated p⁺ emitter into an n⁺np⁺ solar cell structure. An independently confirmed conversion efficiency of 19.7% is achieved using n‐type phosphorus‐doped Czochralski‐grown silicon as bulk material, clearly demonstrating the high‐efficiency potential of the newly developed a‐Si passivated Al‐p⁺ emitter. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Impurity band in SnBi<Subscript>4</Subscript>Se<Subscript>7</Subscript>: thermoelectric power and electrical resistivity measurements

Thermoelectric power and electrical resistivity measurements on polycrystalline samples of Bi₂Se₃ and stoichiometric ternary compound in the quasi-binary system SnSe–Bi₂Se₃ in the temperature range of 90–420 K are presented and explained assuming the existence of an impurity band. The variation of the electron concentration with temperature above 300 K is explained in terms of the thermal activation of a shallow donor, by using a single conduction band model. The density of states effective mass m ^*=0.15m ₀ of the electrons, the activation energy of the donors, their concentration, and the compensation ratio are estimated. The temperature dependence of the electron mobility in conduction band is analyzed by taking into account the scattering of the charge carriers by acoustic phonon, optical phonon, and polar optical phonon as well as by alloy and ionized impurity modes. On the other hand, by considering the two-band model with electrons in both the conduction and impurity bands, the change in the electrical resistivity with temperature between 420 and 90 K is explained.     

Analysis of the current linearity at low illumination of high‐efficiency back‐junction back‐contact silicon solar cells

The relation between current and illumination intensity of three structures of high‐efficiency back‐junction back‐contact silicon solar cells was analyzed. Both, n‐type cells with non‐diffused front surface and p‐type cell with floating n‐emitter show a pronounced non‐linearity due to strong illumination dependence of the passivation quality of the non‐diffused surface and the floating junction respectively. Quantum efficiency (QE) of this cell type drops significantly for the illumination lower than 0.5 suns. In contrast the QE of n‐type cells with n⁺‐front surface field (FSF) is linear. Low illumination current characteristics of all three of the analyzed structures could be well described by physical models. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Wide band gap and p‐type conductive Cu–Nb–O films

Cu–Nb–O films with a thickness of ca. 150 nm were prepared on borosilicate glass substrates using CuNbO₃ ceramic target at substrate temperature of 500 °C by pulsed laser deposition. The X‐ray diffraction patterns showed that the Cu–Nb–O films were amorphous or an aggregation of fine crystals. The post‐annealed film at 300 °C in N₂ gas showed 80% transmission in visible light (band gap = 2.6 eV) and high p‐type conductivity of 21 S cm^–1. The Cu–Nb–O film with a thickness of 100 nm, fabricated from the target with a composition of Cu/Nb = 0.9, showed the highest p‐type conductivity of 116 S cm^–1. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hydrogen bonds of extremely high polarisability between molecules adsorbed on silica gel — an IR investigation

The adsorption on silica gel of a series of electron pair donors with a variation of the pKa value from 12 up to the range of negative values was subjected to IR spectroscopic investigation. Adsorption is effected via hydrogen bonds with the OH groups of the surface, indicated by the disappearance of the band of the free OH groups (3750 cm⁻¹) and by the occurrence of a broad band at smaller wave numbers. The shift is roughly proportional to the pKa value. Furthermore, a continuous absorption is observed as the pKa value increases. This shows that protons become detached from the SiOH groups and hydrogen bonds of type NH⁺…N form between the adsorbed molecules. A double minimum potential well occurs in these hydrogen bonds, which are extremely easily polarisable. The anomalous proton conductivity to be expected sometimes in the presence of such groupings is discussed.     

p‐type conduction in stacking‐fault‐free m ‐plane GaN

Transport measurements of p‐type m ‐plane GaN films grown on low extended‐defect density, free‐standing m ‐plane (10 0) GaN substrates are presented. No significant anisotropy in in‐plane mobility was found for hole concentrations between 2.45 × 10¹⁷ and 8.7 × 10¹⁸ cm^–3. Since faulted, heteroepitaxial m ‐plane films showed significant anisotropy in electron and hole mobility a microstructural feature with anisotropic distribution (basal plane stacking faults) is discussed as a possible source of anisotropic scattering in non‐polar and semi‐polar films. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low‐voltage,high‐gain,and high‐mobility organic complementary inverters based on N,N′‐ditridecyl‐3,4,9,10‐perylenetetracarboxylic diimide and pentacene

The authors describe an organic complementary inverter with N,N′‐ditridecyl‐3,4,9,10‐perylenetetracarboxylic diimide as an n‐type semiconductor and pentacene as a p‐type semiconductor. Each transistor of the inverter exhibited high carrier mobility: 1.62 cm²/Vs for an n‐type drive transistor and 0.57 cm²/Vs for a p‐type switch transistor. The gain of the inverter reached 125. Another inverter using Ta₂O₅ as a high κ gate dielectric performed well with a gain of 500 and an operation voltage of only 5 V.

     

Selective fabrication of n‐ and p‐type SnO films without doping

Undoped n‐ and p‐type tin monoxide (SnO) films have been selectively fabricated by pulsed laser deposition with a Sn target and careful control of oxygen partial pressure. The films are epitaxially grown in optimal growth conditions on yttria‐stabilized zirconia (YSZ) substrates with out‐of‐plane and in‐plane orientation relationships of (001)_SnO//(001)_YSZ and [110]_SnO//[100]_YSZ, respectively. Both Seebeck and Hall measurements show consistent results on the carrier types of the films. The electron Hall mobility is approximately 11 cm²/Vs at room temperature and the carrier activation energy is 0.14 eV for the n‐type film. The growth at increased oxygen partial pressure yields p‐type films, demonstrating the selective fabrication of both n‐ and p‐type SnO films without doping. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Electron traps in wide-gap n-Hg0.3Cd0.7Te characterized by time-resolved photoconductivity

Time-resolved photoconductivity measurements have been used to characterize electron traps in wide-gap n-HgO_0.3Cd_0.7Te for the first time. The characterization was made possible by combining the time-resolved photoconductive data with the analytical method conventionally used in DLTS spectroscopy. Two electron traps were found in the band gap with 61 meV and 79 meV below the conduction band edge, their concentrations are 1.1×10¹³ cm^–3 and 5.8×10¹¹ cm^–3, respectively. Compared with DLTS spectroscopy, this characterization method markedly simplifies sample preparation and experimental procedure.     

Control of the Interface Between Electron‐Hole and Electron‐Ion Plasmas: Hybrid Semiconductor‐Gas Phase Devices as a Gateway for Plasma Science

Coupling electron‐hole (e^–‐ h⁺) and electron‐ion plasmas across a narrow potential barrier with a strong electric field provides an interface between the two plasma genres and a pathway to electronic and photonic device functionality. The magnitude of the electric field present in the sheath of a low temperature, nonequilibrium microplasma is sufficient to influence the band structure of a semiconductor region in immediate proximity to the solid‐gas phase interface. Optoelectronic devices demonstrated by leveraging this interaction are described here. A hybrid microplasma/semiconductor photodetector, having a Si cathode in the form of an inverted square pyramid encompassing a neon microplasma, exhibits a photosensitivity in the ～420–1100 nm region as high as 3.5 A/W. Direct tunneling of electrons into the collector and the Auger neutralization of ions arriving at the Si surface appear to be facilitated by an n ‐type inversion layer at the cathode surface resulting from bandbending by the microplasma sheath electric field. Recently, an npn plasma bipolar junction transistor (PBJT), in which a low temperature plasma serves as the collector in an otherwise Si device, has also been demonstrated. Having a measured small signal current gain h_fe as large as 10, this phototransistor is capable of modulat‐ing and extinguishing the collector plasma with emitter‐base bias voltages <1 V. Electrons injected into the base when the emitter‐base junction is forward‐biased serve primarily to replace conduction band electrons lost to the collector plasma by secondary emission and ion‐enhanced field emission in which ions arriving at the base‐collector junction deform the electrostatic potential near the base surface, narrowing the potential barrier and thereby facilitating the tunneling of electrons into the collector. Of greatest significance, therefore, are the implications of active, plasma/solid state interfaces as a new frontier for plasma science. Specifically, the PBJT provides the first opportunity to control the electronic properties of a material at the boundary of, and interacting with, a plasma. By specifying the relative number densities of free (conduction band) and bound (valence band) electrons at the base‐collector interface, the PBJT's emitter‐base junction is able to dictate the rates of secondary electron emission (including Auger neutralization) at the semiconductor‐plasma interface, thereby offering the ability to vary at will the effective secondary electron emission coefficient for the base surface (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Negative FIR-photoconductivity in n-GaAs

Negative photoconductivity in n-GaAs has been observed in the far infrared spectral range between 20 and 29 cm^–1. Negative photoconductivity occurs when a magnetic field is applied to the samples and impact ionization of shallow donors by the electric bias field is the dominant mechanism of electron excitation to the conduction band. A conceivable model qualitatively explaining the experimental results is proposed, which involves optical transitions from the lowest Landau subband to higher bound states of shallow donors.     

Shallow donor in natural MoS2

Using electron paramagnetic resonance and density functional theory calculations, we show that the shallow donor responsible for the n‐type conductivity in natural MoS₂ is rhenium (Re) with a typical concentration in the low 10¹⁷ cm^–3 range and the g ‐values: g_|| = 2.0274 and g_⊥ = 2.2642. In bulk MoS₂, the valley–orbit (VO) splitting and ionization energy of the Re shallow donor are determined to be ～3 meV and ～27 meV, respectively. Calculations show that the VO splitting of Re approaches the value in bulk if the number of MoS₂ layers is larger than four and increases to 97.9 meV in a monolayer. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Homoepitaxy of ZnO by pulsed‐laser deposition

ZnO thin films were grown homoepitaxially on O‐face ZnO single crystals by pulsed‐laser deposition. The ZnO substrates grown by the hydrothermal method were heat‐treated in oxygen ambient at 1000 °C for 2 h prior to deposition. After the thermal treatment the substrates show bilayer steps between 200–400 nm wide terraces and a considerably improved crystalline structure. Thin film surfaces exhibit closed loop spirals and show steps of c /2 or c. The FWHM of the (0002) rocking curve of the best sample is 29″. Similar to the substrates used, Al is contained in the thin films (<10¹⁴ cm^–3) as photoluminescence (PL) and thermal admittance spectroscopy suggest. However, deep levels between 200 and 400 meV below the conduction band are the dominant donors at room temperature. Low temperature PL is dominated by (Al⁰,X) (I6, FWHM: 200 µeV) and extremely homogeneous (σ ≈ 1%).

     

Resonant two‐magnon Raman scattering in high‐Tc cuprates

Resonance enhancement of one‐phonon, two‐phonon, and two‐magnon Raman scattering in a general, exactly solvable, multiband model is explained in a way that is in accordance with the general analytical properties of the total optical conductivity tensor. Using this approach, the charge‐transfer limit of the Emery three‐band model is examined to explain resonance enhancement of the two‐magnon Raman spectra of high‐T_c cuprates, which is found in experiments to be of 3 orders of magnitude. While previous Raman and optical conductivity analyzes of the cuprates, based on the single‐band Hubbard model, are found to be consistent with the picture where one hole per one CuO₂ unit is localized on the Cu ion, the present three‐band approach allows the study of the opposite, strong copper‐oxygen hybridization limit, which is found to be in agreement with the results of nuclear magnetic resonance (NMR) and one‐phonon Raman scattering experiments. Copyright © 2010 John Wiley & Sons, Ltd.