Low-temperature photoluminescence in holmium-doped silicon

In this paper the photoluminescence (PL) of holmium-doped silicon is discussed. The silicon was first implanted with holmium ions at energies of 1–2 MeV and doses of 1×10¹³–3×10¹⁴ cm⁻², and then annealed at temperatures of 620–900 °C for 0.5–1 h. In order to increase the concentration of electrically and optically active centers, the silicon was implanted a second time with oxygen ions at energies of 0.14–0.29 MeV and doses of 1×10¹⁴–3×10¹⁵ cm⁻². Several photoluminescence lines, which are attributable to the transitions of electrons from the first excited state of the Ho³⁺ ion (⁵ I ₇) to the ground state (⁵ I ₈), were observed. The amplitudes of the most intense lines, which correspond to transitions at frequencies 5119 and 5103 cm⁻¹, decreased by more than an order of magnitude in the temperature range 4.2−78 K. The PL intensity of the holmium ions increased with increasing concentrations of the implanted rare-earth ions and oxygen. Fiz. Tekh. Poluprovodn. 33, 420–422 (April 1999)     

Specific Features and Nature of the 890 nm Photoluminescence Band Detected in SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Films after Low-Temperature Annealing

A band with a peak at 890 nm is detected in the photoluminescence spectra of SiO _x (x ≈ 1.3) films deposited by thermal evaporation of SiO and annealed in air at 650–1150°C. The 890-nm band appears after low-temperature (∼650°C) annealing and exhibits a number of features: (i) as the annealing temperature is elevated to 1150°C, the position of the band peak remains unchanged, whereas the intensity increases by two orders of magnitude; (ii) the effects of the annealing atmosphere (air, vacuum) and the excitation wavelength and power density on the intensity of the 890-nm band differ from the corresponding effects on the well-known bands observable in the ranges 600–650 and 700–800 nm; and (iii) the photoluminescence decay is first fast and then much slower, with corresponding lifetimes of ∼9 and ∼70 μs. The observed features are inconsistent with the interpretation of photoluminescence observed in SiO _x so far. Specifically, the earlier observed photoluminescence was attributed to transitions between the band and defect states in the matrix and between the states of band tails, transitions inside Si nanoclusters, and intraion transitions in rare-earth impurity ions. Therefore, we consider here the possibility of attributing the 890-nm band to transitions in local centers formed by silicon ions twofold- and/or threefold-coordinated with oxygen; i.e., we attempt to interpret the 890-nm band in the same manner as was done for luminescence in SiO₂ glasses and films slightly deficient in oxygen.     

Structure and Thermoelectric Properties of Te- and Ge-Doped Skutterudites CoSb<Subscript>2.875−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>0.125</Subscript>Te<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

n-Type CoSb_2.875−x Ge_0.125Te _x (x = 0.125 to 0.275) compounds with different Te contents have been synthesized by a melt–quench–anneal–spark plasma sintering method, and the effects of Te content on the structure and thermoelectric properties have been investigated. The results show that all specimens exhibited n-type conduction characteristics. The solubility limit of Te in CoSb_2.875−x Ge_0.125Te _x is found to be x = 0.25. The solubility of Te in CoSb₃ is increased through charge compensation of the element Ge. The room-temperature carrier concentration N _p of CoSb_2.875−x Ge_0.125Te _x skutterudites increases with increasing Te content, and the compounds possess high power factors. The maximum power factor of 3.89 × 10⁻³ W m⁻¹ K⁻² was obtained at 720 K for the CoSb_2.625Ge_0.125Te_0.25 compound. The thermal conductivity decreases dramatically with increasing Te content due to strong point defect scattering. The maximum value of the thermoelectric figure of merit ZT = 1.03 was obtained at 800 K for CoSb_2.625Ge_0.125Te_0.25, benefiting from a lower thermal conductivity and a higher power factor. The figure of merit is competitive with values reported for single-filled skutterudites.     

Influence of oxygen on the photoluminescence intensity of erbium (at 1.54 µm) in erbium-doped a-Si:H films

The influence of oxygen on the photoluminescence (PL) of erbium (at 1.54 μm) in erbium-doped hydrogenated amorphous silicon (c-Si〈Er〉) is investigated. The a-Si:H〈Er〉 films studied are fabricated by cosputtering Si and Er targets using the technology of dc silane decomposition in a magnetic field. The oxygen concentration is varied from 10¹⁹ to 10²¹ cm⁻³ by increasing the partial pressure of oxygen in the chamber. It is shown that, as in the case of erbium-doped crystalline silicon (c-Si〈Er〉), oxygen has an effect on the intensity of the 1.54 μm photoluminescence in a-Si:H〈Er〉 films. The values of the erbium and oxygen concentrations at which the maximum Er PL intensity is observed are two orders of magnitude higher than in crystalline silicon. The increase in the Er PL intensity at room temperature and the weaker temperature dependence of the Er PL in comparison to c-Si〈Er,O〉 attest to the prospect of using a-Si:H〈Er〉 films in optoelectronic applications. Fiz. Tekh. Poluprovodn. 32, 1384–1389 (November 1998)     

Effects of High-Energy Electron Irradiation on ZnO/Si MSM Photodetectors

ZnO/Si metal–semiconductor–metal photodetectors (MSM-PDs) were subjected to high-energy electron irradiation (HEEI) to total fluence of 2 × 10¹³ cm⁻². ZnO/Si MSM-PDs demonstrated at least 43% greater radiation resistance than similar Si devices. Room-temperature annealing of radiation damage was observed as 63% recovery of photocurrent over 47 days. The current transport mechanism for ZnO/Si MSM-PDs was dominated by space-charge-limited conduction (SCLC) with minimal effect on conduction regime due to HEEI. Analysis of photoluminescence (PL) data indicates that the radiation-induced defects are likely oxygen and zinc vacancies, i.e., (V₀⁺) and (V_Zn ^- - H ⁺ )⁰ (\hbox{V}_{\rm{Zn}}^{ - } - \hbox{H}^{ + } )^{0} , respectively.     

Effects of Sm<Superscript>3+</Superscript> Doping on Dielectric Properties of Anatase TiO<Subscript>2</Subscript> Nanoparticles Synthesized by a Low-Temperature Hydrothermal Method

Samarium-doped thermally stable TiO₂ nanoparticles in the anatase phase have been synthesized by a low-temperature hydrothermal method. The formation of the anatase phase has been investigated by x-ray diffraction. Thermogravimetry and differential thermal analysis have been used for thermal studies. The morphology and composition of synthesized powders have been studied using scanning electron microscopy, transmission electron microscopy, and energy-dispersive spectroscopy. Surface areas were studied by the Brunauer–Emmett–Teller method. Dielectric properties were studied for dopant levels of 0.2 mol% and 0.5 mol% at 300 K in the frequency range of 42 Hz to 5 MHz. At low frequency, charge carriers at the grain boundary produce interfacial polarization giving rise to a high dielectric constant (ε′), which is significantly reduced by doping with samarium ions (Sm³⁺). Strong frequency dependence of the dielectric loss was also observed for each concentration. Conductivity studies showed that the reduction in conductivity is due to the decrease in particle size with the increase in Sm³⁺ dopant level.     

Oxygen and Erbium related donor centers in Czochralski grown silicon implanted with erbium

The Hall effect measurements were conducted on Czochralski-grown silicon after implantation of erbium and two-step annealing at 700 °C and 900 °C. After the first step the formation of oxygen-related shallow donors was observed at E _c in the range 20–40 meV and erbium-related donor centers at ≈E _c -70 meV and ≈E _c -120 meV. Along with the same oxygen-related shallow thermal donors and donor centers at ≈E _c -70 meV, other donor centers at ≈E _c -150 meV are formed following the 900°C anneal, instead of those at ≈E _c -120 meV. The new donor states are of particular interest because of their possible involvement in the photoluminescence process. The obtained results for erbium-implanted silicon are compared with some fragmentary DLTS data found in the current literature on the donors with ionization energies less than 0.2 eV. Fiz. Tekh. Poluprovodn. 33, 1192–1195 (October 1999)     

Influence of the intensity of gg irradiation on the photoluminescence of GaAs:Te

The influence of gg irradiation (⁶⁰Co) of various intensities (P _γ≈1.7−7.5kGR/h) on the photoluminescence of GaAs:Te single crystals [n ₀=(1.2–2.3×10¹⁸ cm⁻³] is investigated. Together with the known photoluminescence impurity bands (hν _max≈1.2 eV and/or hν _max≈1.35 eV) and edge band (hν _max≈1.51 eV), new bands are also observed in the spectra at hν _max≈1.3 eV and hν _max≈1.48 eV. The observed effects are attributed to radiation-stimulated ordering of the donor impurity and deep impurity centers. Fiz. Tekh. Poluprovodn. 32, 38–39 (January 1998)     

Photoluminescence of silicon after deposition of polycrystalline diamond films

Low-temperature (5K) photoluminescence of silicon substrates in the range 0.8–1.2 eV is studied before and after deposition of polycrystalline diamond films. The diamond films were deposited in the microwave plasma onto high-purity dislocation-free silicon (with the resitivity ρ ≈ 3 kΩ cm) subjected to mechanical polishing or more delicate chemical and mechanical polishing. The deposition temperature was 750–850°C. In the photoluminescence spectra of the samples with the substrates polished chemically and mechanically, two lines, D ₁ and D ₂, corresponding to the dislocation-related emission are recorded. Generation of dislocations in the substrates is caused by efficient adhesion of the diamond film and, as a result, by internal stresses that relax with the formation of dislocations. The experimental spectra are practically identical to the photoluminescence spectra observed in silicon (ρ ≈ 100 Ω cm) with the density of dislocations ∼10⁴ cm⁻².     

The E3 Defect in MgxZn1?xO

The authors used highly rectifying PdO _y /Mg _x Zn_1−x O Schottky barriers to determine the apparent capture cross-section and the thermal activation energy of the E3 defect in Mg _x Zn_1−x O thin films by thermal admittance spectroscopy and deep-level transient spectroscopy for x < 0.08. The samples were grown by pulsed-laser deposition. It is observed that both the apparent capture cross-section and the thermal activation energy increase with increasing Mg mole fraction.     

Hopping conduction in heavily doped bulk n-type SiC

The electronic properties of heavily doped n-type 4H, 6H, and 15R SiC have been studied with temperature dependent Hall effect, resistivity measurements, and thermal admittance spectroscopy experiments. Hopping conduction was observed in the resistivity experiments for samples with electron concentrations of 10¹⁷ cm⁻³ or higher. Both band and hopping conduction were observed in all three polytypes in resistivity and Hall effect experiments. The hopping conduction activation energy ε₃ obtained from the resistivity measurements varied from 0.003 to 0.013 eV. The ε₃ value obtained from thermal admittance spectroscopy measurements were slightly lower. The nitrogen ionization levels were observed by thermal admittance spectroscopy only in those samples where hopping conduction was not detected by this experiment. Free carrier activation energy E_a for nitrogen was difficult to determine from temperature dependent Hall effect measurements because of the effects of hopping conduction. A new feature in the apparent carrier concentration vs inverse temperature data in the hopping regime was observed.     

Shallow and Deep Centers in As-Grown and Annealed MgZnO/ZnO Structures with Quantum Wells

Shallow and deep centers in ZnO(P)/MgZnO/ZnO/MgZnO/ZnO(Ga) structures grown by pulsed laser deposition on sapphire were studied before and after annealing in oxygen atmosphere at high temperatures of 850°C to 950°C. In both as-grown and annealed structures, microcathodoluminescence spectra in the near-bandgap region demonstrate a blue-shift by 0.13 eV compared with bulk ZnO films, indicating carrier confinement in the MgZnO/ZnO/MgZnO quantum well (QW). Annealing strongly decreases the concentration of shallow uncompensated donors from ~10¹⁷ cm⁻³ to ~10¹⁶ cm⁻³ and makes it possible to probe the region of the QW by capacitance–voltage (C–V) profiling. This profiling confirms charge accumulation in the QW. The dominant electron traps in the as-grown films are the well-known traps with activation energies of 0.3 eV and 0.8 eV. After annealing, the electron traps observed in the structure have activation energies of 0.14 eV, 0.33 eV, and 0.57 eV, with the Fermi level in the n-ZnO(P) pinned by the 0.14-eV traps. The annealing also introduces deep compensating defects that decrease the intensity of band-edge luminescence and produce a deep luminescence defect band at 2.2 eV. In addition, a defect vibrational band becomes visible in Raman spectra near 650 cm⁻¹. No conversion to p-type conductivity was detected. The results are compared with the data for the structures successfully converted to p-type, and possible reasons for the observed differences are discussed.     

Stoichiometry-dependent deep levels in undoped p-type Al0.38Ga0.62As grown by liquid phase epitaxy

Photocapacitance (PHCAP) and photoluminescence (PL) measurements were applied to unintentionally doped p-type Al_0.38Ga_0.62As grown by liquid phase epitaxy using the temperature difference method under controlled vapor pressure. PHCAP spectra revealed three dominant deep levels at E_v+0.9, E_v + 1.45, and E_v+1.96 eV, and a deep level at E_v+0.9−1.5 eV which was not neutralized by forward bias injection. These level densities increase with increasing arsenic vapor pressure and net shallow acceptor density. Furthermore, PL spectra reveal a deep level at 1.6–1.7 eV. The PL intensity of this deep level increases with increasing arsenic vapor pressure. These deep levels are thought to be associated with excess As.     

Synthesis and Electrical Properties of γ-Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript>2</Subscript>O<Subscript>4</Subscript> via a Citrate Sol–Gel Method with Polyethylene Glycol 400

In this work, a citrate sol–gel method (Sol–Gel) with polyethylene glycol 400 (Sol-Gel-PEG400) was developed to prepare γ-Na _x Co₂O₄ by using sodium and cobalt nitrates as the raw materials, citric acid as a complexing agent, and PEG400 as a dispersant. At 800°C, single-phase γ-Na _x Co₂O₄ crystals were obtained using Sol-Gel-PEG400. With the addition of 1 vol.% PEG400, smaller, flaky particles exhibited a well-tiled structure along the plane direction of the flaky particles. Moreover, polycrystalline sintered bulk γ-Na _x Co₂O₄ with more highly oriented crystals and greater compact density was fabricated using the Sol-Gel-PEG400 synthesized powders compared with the powders synthesized by citrate Sol–Gel. The electrical conductivity (σ) values of Sol-Gel-PEG400 samples were higher than those of Sol–Gel samples between 400 K and 900 K. The σ value of Sol-Gel-PEG400 increased to 3.13 × 10⁴ Sm⁻¹ at 400 K and to 1.84 × 10⁴ Sm⁻¹ at 900 K. Between 400 K and 850 K, the Seebeck coefficient (α) values of Sol-Gel-PEG400 samples were slightly lower than those of Sol–Gel samples. Near 900 K, the α values of these two methods were nearly equal, at 164 μV K⁻¹. Between 400 K and 900 K, the power factor (P) of Sol-Gel-PEG400 was evidently larger than that of Sol–Gel.     

Characteristic features of the temperature dependence of the photoluminescence polarization of {Ga vacancy}-SnGa(SiGa) complexes in GaAs produced as a result of resonant polarized excitation

The excitation and induced polarization spectra of the photoluminescence band with the maximum near a photon energy of 1.18 eV in Sn-or Si-doped n-GaAs with an electron density ∼ 10¹⁸ cm⁻³ are measured at various temperatures. It is shown that the temperature dependence of the induced polarization of this photoluminescence due to V _GaSn_Ga or V _GaSi_Ga complexes in the temperature range 77–230K is close to the corresponding dependence for V _GaTe_As complexes. In addition, a slight decrease in the induced polarization as the temperature is increased, not observed for V _GaTe_As complexes, is observed in the range 77–125 K for the investigated complexes. It is hypothesized that the difference is attributable to the existence of excited configurations in the absorbing and emitting states of the V _GaSn_Ga and V _GaSi_Ga complexes, where the populations of these configurations in the absorbing state increase with the temperature. The difference between the total energies of the excited and ground configurations of the absorbing state is 10–20 meV for V _GaSn_Ga complexes and 15–30 meV for V _GaSi_Ga complexes. Fiz. Tekh. Poluprovodn. 33, 42–46 (January 1999)     

Investigation of Trap States in AlInN/AlN/GaN Heterostructures by Frequency-Dependent Admittance Analysis

We present a systematic study on the admittance characterization of surface trap states in unpassivated and SiN _x -passivated Al_0.83In_0.17N/AlN/GaN heterostructures. C–V and G/ω–V measurements were carried out in the frequency range of 1 kHz to 1 MHz, and an equivalent circuit model was used to analyze the experimental data. A detailed analysis of the frequency-dependent capacitance and conductance data was performed, assuming models in which traps are located at the metal–AlInN surface. The density (D _t) and time constant (τ _t) of the surface trap states have been determined as a function of energy separation from the conduction-band edge (E _c − E _t). The D _st and τ _st values of the surface trap states for the unpassivated samples were found to be D_st @ (4 - 13) ×10¹² D_{\rm{st}} \cong (4 - 13) \times 10^{12}  eV ^{- 1} cm ^{- 2} {\hbox{eV}}^{ - 1} {\hbox{cm}}^{ - 2} and τ _st ≈ 3 μs to 7 μs, respectively. For the passivated sample, D _st decreased to 1.5 ×10¹² 1.5 \times 10^{12}  eV ^{- 1} cm ^{- 2} {\hbox{eV}}^{ - 1} {\hbox{cm}}^{ - 2} and τ _st to 1.8 μs to 2 μs. The density of surface trap states in Al_0.83In_0.17N/AlN/GaN heterostructures decreased by approximately one order of magnitude with SiN _x passivation, indicating that the SiN _x insulator layer between the metal contact and the surface of the Al_0.83In_0.17N layer can passivate surface states.     

Improved Electrical Performance and Reliability of Poly-Si TFTs Fabricated by Drive-In Nickel-Induced Crystallization with Chemical Oxide Layer

Ni-metal-induced crystallization (MIC) of amorphous Si (α-Si) has been employed to fabricate low-temperature polycrystalline silicon thin-film transistors (TFTs). However, the Ni residues degrade the device performance. In this study, a new method for manufacturing MIC–TFTs using drive-in Ni-induced crystallization with a chemical oxide layer (DICC) is proposed. Compared with that of MIC–TFTs, the on/off current ratio (I _on/I _off) of DICC–TFTs was increased by a factor of 9.7 from 9.21 × 10⁴ to 8.94 × 10⁵. The leakage current (I _off) of DICC–TFTs was 4.06 pA/μm, which was much lower than that of the MIC–TFTs (19.20 pA/μm). DICC–TFTs also possess high immunity against hot-carrier stress and thereby exhibit good reliability.     

A 1.8-V 0.7 ppm/°C high order temperature-compensated CMOS current reference

A high order curvature compensation technique for current reference generator which exploits the I–V characteristic of MOS to achieve I _SC (T ^m ) (m ≥ 2) is described. I _SC (T ^m ) is a self-compensated current which corrects its negative three-order TC (Temperature Coefficient) and linear TC by itself. Then, I (T ²) is achieved also by exploiting the I–V characteristic of MOS, for correcting the other negative high order parts of I _SC (T ^m ). This circuit operates on a 1.8 V power supply and is compatible with a standard n-well 0.5-μm digital CMOS process. The circuit realizes a temperature coefficient of 0.7 ppm/°C, a deviation of the simulated output current of 0.011% from −20°C to + 150°C and 97.5 dB PSRR through HSPICE simulation.     

Controlled oxygen incorporation in indium gallium arsenide and indium phosphide grown by metalorganic vapor phase epitaxy

The defect engineering in metalorganic vapor phase epitaxy In_xGa_1-xAs and InP by controlled oxygen doping using diethyl aluminum ethoxide (DEALO) was developed in this study. DEALO doping has led to the incorporation of Al and O, and the compensation of shallow Si donors in In_xGa_1−xAs: Si with 0 ≤ x ≤ 0.25. With the same DEALO mole fraction during growth, the incorporation of Al and O was found to be independent of x, but the compensation of Si donors decreases with increasing In content. Deep level transient spectroscopy analysis on a series of In_xGa_1-xAs: Si. samples with 0 ≤ x ≤ 0.18 revealed that oxygen incorporation led to a set of deep levels, similar to those found in DEALO doped GaAs. As the In composition was increased, one or more of these deep levels became resonant with the conduction band and led to a high electron concentration in oxygen doped In_0.53Ga_0.47As. Low temperature photoluminescence emission measurements at 12K on the same set of samples revealed the quenching of the near-band edge peak, and the appearance of new oxygen-induced emission features. DEALO doping in InP has also led to the incorporation of Al and O, and the compensation of Si donors due to oxygen-induced multiple deep levels.     

Spatial Localization of Carrier Traps in 4H-SiC MOSFET Devices Using Thermally Stimulated Current

Carrier traps in 4H-SiC metal–oxide–semiconductor (MOS) capacitor and transistor devices were studied using the thermally stimulated current (TSC) method. TSC spectra from p-type MOS capacitors and n-channel MOS field-effect transistors (MOSFETs) indicated the presence of oxide traps with peak emission around 55 K. An additional peak near 80 K was observed due to acceptor activation and hole traps near the interface. The physical location of the traps in the devices was deduced using a localized electric field approach. The density of hole traps contributing to the 80-K peak was separated from the acceptor trap density using a gamma-ray irradiation method. As a result, hole trap density of N _t,hole = 2.08 × 10¹⁵ cm⁻³ at 2 MV/cm gate field and N _t,hole = 2.5 × 10¹⁶ cm⁻³ at 4.5 MV/cm gate field was extracted from the 80-K TSC spectra. Measurements of the source-body n ⁺–p junction suggested the presence of implantation damage in the space-charge region, as well as defect states near the n ⁺ SiC substrate.