Conductivity of layers of a chalcogenide glassy semiconductor Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> in high electric fields

Effect of high electric fields on the conductivity of 0.5-1-μm-thick layers of a chalcogenide glassy semiconductor with a composition Ge₂Sb₂Te₅, used in phase memory cells, has been studied. It was found that two dependences are observed in high fields: dependence of the current I on the voltage U, of the type I ∝ U ⁿ , with the exponent (n ≈ 2) related to space-charge-limited currents, and a dependence of the conductivity σ on the field strength F of the type σ = σ₀exp(F/F ₀) (where F ₀ = 6 × 10⁴ V cm⁻¹), caused by ionization of localized states. A mobility of 10⁻³–10⁻² cm² V⁻¹ s⁻¹ was determined from the space-charge-limited currents.     

Charge carrier transport in Ge<Subscript>20</Subscript>As<Subscript>20</Subscript>S<Subscript>60</Subscript> chalcogenide semiconductor films

Charge-carrier transport in Ge₂₀As₂₀S₆₀ films has been studied using the transit time method under low-injection conditions at room temperature. It was found that drift mobilities of electrons and holes in Ge₂₀As₂₀S₆₀ films are close to each other, i.e., μ _e ≈ μ _h ≈ 2 × 10⁻³ cm² V⁻¹ s⁻¹ at T = 295 K and F = 5 × 10⁴ V/cm. It was shown that the time dependence of the photocurrent during carrier drift and the voltage dependence of the drift mobility allowed the use of the concept of anomalous dispersive transport. Experimental data were explained using the model of transport controlled by carrier trapping by localized states with energy distribution near conduction and valence band edges described by the exponential law with a characteristic energy of ∼0.05 eV.     

Thermoelectric Performance of Zn and Nd Co-doped In<Subscript>2</Subscript>O<Subscript>3</Subscript> Ceramics

Polycrystalline In₂O₃ ceramics co-doped with Zn and Nd were prepared by the spark plasma sintering (SPS) process, and microstructure and thermoelectric (TE) transport properties of the ceramics were investigated. Our results indicate that co-doping with Zn²⁺ and Nd³⁺ shows a remarkable effect on the transport properties of In₂O₃-based ceramics. Large electrical conductivity (~130 S cm⁻¹) and thermopower (~220 μV K⁻¹) can be observed in these In₂O₃-based ceramic samples. The maximum power factor (PF) reaches 5.3 × 10⁻⁴ W m⁻¹ K⁻² at 973 K in the In_1.92Nd_0.04Zn_0.04O₃ sample, with a highest ZT of ~0.25.     

Characterization of Hg0.7Cd0.3Te n- on p-type structures obtained by reactive ion etching induced p- to n conversion

This paper presents transport measurements on both vacancy doped and gold doped Hg_0.7Cd_0.3Te p-type epilayers grown by liquid phase epitaxy (LPE), with N_A=2×10¹⁶ cm⁻³, in which a thin 2 μm surface layer has been converted to n-type by a short reactive ion etching (RIE) process. Hall and resistivity measurements were performed on the n-on-p structures in van der Pauw configuration for the temperature range from 30 K to 400 K and magnetic field range up to 12 T. The experimental Hall coefficient and resistivity data has been analyzed using the quantitative mobility spectrum analysis procedure to extract the transport properties of each individual carrier contributing to the total conduction process. In both samples three distinct carrier species have been identified. For 77 K, the individual carrier species exhibited the following properties for the vacancy and Au-doped samples, respectively, holes associated with the unconverted p-type epilayer with p ≈ 2 × 10¹⁶ cm⁻³, μ ≈ 350 cm²V⁻¹s⁻¹, and p ≈ 6 × 10¹⁵ cm⁻³, μ ≈ 400 cm²V⁻¹s⁻¹; bulk electrons associated with the RIE converted region with n ≈ 3 × 10¹⁵cm⁻³, μ ≈ 4 × 10⁴ cm²V⁻¹s⁻¹, and n ≈ 1.5 × 10¹⁵ cm⁻³, μ ≈ 6 × 10⁴ cm²V⁻¹s⁻¹; and surface electrons (2D concentration) n ≈ 9 × 10¹² cm⁻² and n ≈ 1 × 10¹³ cm⁻², with mobility in the range 1.5 × 10³ cm²V⁻¹s⁻¹ to 1.5 × 10⁴ cm²V⁻¹s⁻¹ in both samples. The high mobility of bulk electrons in the RIE converted n-layer indicates that a diffusion process rather than damage induced conversion is responsible for the p-to-n conversion deep in the bulk. On the other hand, these results indicate that the surface electron mobility is affected by RIE induced damage in a very thin layer at the HgCdTe surface.     

Temperature-Dependent Studies of Si-Implanted Al<Subscript>0.33</Subscript>Ga<Subscript>0.67</Subscript>N with Different Annealing Temperatures and Times

Electrical activation studies were carried out on Si-implanted Al_0.33Ga_0.67N as a function of ion dose, annealing temperature, and annealing time. The samples were implanted at room temperature with Si ions at 200 keV in doses ranging from 1 × 10¹⁴ cm⁻² to 1 × 10¹⁵ cm⁻², and subsequently proximity-cap annealed from 1150°C to 1350°C for 20 min to 60 min in a nitrogen environment. One hundred percent electrical activation efficiency was obtained for Al_0.33Ga_0.67N samples implanted with a dose of 1 × 10¹⁵ cm⁻² after annealing at either 1200°C for 40 min or at 1300°C for 20 min. The samples implanted with doses of 1 × 10¹⁴ cm⁻² and 5 × 10¹⁴ cm⁻² exhibited significant activations of 74% and 90% after annealing for 20 min at 1300°C and 1350°C, respectively. The mobility increased as the annealing temperature increased from 1150°C to 1350°C, showing peak mobilities of 80 cm²/V s, 64 cm²/V s, and 61 cm²/V s for doses of 1 × 10¹⁴ cm⁻², 5 × 10¹⁴ cm⁻², and 1 × 10¹⁵ cm⁻², respectively. Temperature-dependent Hall-effect measurements showed that most of the implanted layers were degenerately doped. Cathodoluminescence measurements for all samples exhibited a sharp neutral donor-bound exciton peak at 4.08 eV, indicating excellent recovery of damage caused by ion implantation.     

Ultraviolet Photodetection Properties of a Pt Contact on a Mg<Subscript>0.1</Subscript>Zn<Subscript>0.9</Subscript>O/ZnO Composite Film

We report on the ultraviolet (UV) photodetection properties of a Pt contact on a sol-gel Mg_0.1Zn_0.9O/ZnO composite structure on a glass substrate. In the dark, the current–voltage (I–V) characteristics between the Pt and Ag contacts on the top of the ZnO film were linear while that on the Mg_0.1Zn_0.9O/ZnO composite film were rectifying, suggesting the formation of a Schottky diode on the latter. The ideality factor, n, and the reverse leakage current density, J _R , of the Schottky diode were greater than 2 and 2.36 × 10⁻² A cm⁻² at −5 V, respectively. Under ultraviolet light, the I–V characteristics become linear. The maximum photo-to-dark current ratio observed was about 63. The composite film showed good sensitivity to UV light with wavelengths of less than 400 nm, though the photoresponse process was found to be slow.     

Improvement of Thermoelectric Power Factor of Hydrothermally Prepared Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Compared with its Solvothermally Prepared Counterpart

We report on the successful hydrothermal synthesis of Bi_0.5Sb_1.5Te₃, using water as the solvent. The products of the hydrothermally prepared Bi_0.5 Sb_1.5Te₃ were hexagonal platelets with edges of 200–1500 nm and thicknesses of 30–50 nm. Both the Seebeck coefficient and electrical conductivity of the hydrothermally prepared Bi_0.5Sb_1.5Te₃ were larger than those of the solvothermally prepared counterpart. Hall measurements of Bi_0.5Sb_1.5Te₃ at room temperature indicated that the charge carrier was p-type, with a carrier concentration of 9.47 × 10¹⁸ cm⁻³ and 1.42 × 10¹⁹ cm⁻³ for the hydrothermally prepared Bi_0.5Sb_1.5Te₃ and solvothermally prepared sample, respectively. The thermoelectric power factor at 290 K was 10.4 μW/cm K² and 2.9 μW/cm K² for the hydrothermally prepared Bi_0.5Sb_1.5Te₃ and solvothermally prepared sample, respectively.     

Vanadium-based ohmic contacts to n-type Al<Subscript>0.6</Subscript>Ga<Subscript>0.4</Subscript>N

Four vanadium-based contacts to n-type Al_0.6Ga_0.4N were compared in this work. Both V/Al/Pd/Au and V/Al/V/Au contacts with optimized layer thicknesses provided lower specific-contact resistances than did the previously reported V/Al/Pt/Au ohmic contact. Specific contact resistances of the V/Al/Pd/Au (15 nm/85 nm/20 nm/95 nm) and V/Al/V/Au (15 nm/85 nm/20 nm/95 nm) contacts were 3×10⁻⁶ Ω·cm² and 4×10⁻⁶ Ω·cm², respectively. On the other hand, an analogous V/Al/Mo/Au contact never became ohmic, even after it was annealed at 900°C for 30 sec. Compared to the V/Al/Pd/Au contact, the V/Al/V/Au contact required a less severe annealing condition (30 sec at 700°C instead of 850°C). The V/Al/V/Au contact also provided a smoother surface, with a root-mean-square (RMS) roughness of 39 nm.     

Current-voltage characteristics of ZnGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> compound polycrystals

Current-voltage characteristics of the In-ZnGa₂Se₄-In structure have been studied in the temperature range of 90–335 K. Based on the data calculated for the concentration of three trap types in ZnGa₂Se₄, the values N _t = 1.4 × 10¹³, 8.2 × 10¹², and 2.6 × 10¹² cm⁻³ are obtained. The contact region transparency D _k ^*= 10⁻⁵, surface recombination velocity S _k = 0.65 m/s, and carrier lifetime τ = 1.5 × 10⁻⁴ s were determined. It was found that the current transmission mechanism in electric fields weaker than 10³ V/cm is caused by monopolar carrier injection.     

Effect of Proton Irradiation on Interface State Density in Sc<Subscript>2</Subscript>O<Subscript>3</Subscript>/GaN and Sc<Subscript>2</Subscript>O<Subscript>3</Subscript>/MgO/GaN Diodes

Proton irradiation of Sc₂O₃/GaN and Sc₂O₃/MgO/GaN metal-oxide semiconductor diodes was performed at two energies, 10 MeV and 40 MeV, and total fluences of 5 × 10⁹ cm⁻², corresponding to 10 years in low-earth orbit. The proton damage causes a decrease in forward breakdown voltage and a flat-band voltage shift in the capacitance-voltage characteristics, indicating a change in fixed oxide charge and damage to the dielectric. The interface state densities after irradiation increased from 5.9 × 10¹¹ cm⁻² to 1.03 × 10¹² cm⁻² in Sc₂O₃/GaN diodes and from 2.33 × 10¹¹ to 5.3 × 10¹¹ cm⁻² in Sc₂O₃/MgO/GaN diodes. Postannealing at 400°C in forming gas recovered most of the original characteristics but did increase the interfacial roughness.     

Development of Molecular Beam Epitaxially Grown Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te for High-Density Vertically-Integrated Photodiode-Based Focal Plane Arrays

Hg_1−x Cd _x Te samples of x ~ 0.3 (in the midwave infrared, or MWIR, spectral band) were prepared by molecular beam epitaxy (MBE) for fabrication into 30-μm-pitch, 256 × 256, front-side-illuminated, high-density vertically-integrated photodiode (HDVIP) focal plane arrays (FPAs). These MBE Hg_1−x Cd _x Te samples were grown on CdZnTe(211) substrates prepared in this laboratory; they were ~10-μm thick and were doped with indium to ~5 × 10¹⁴ cm⁻³. Standard HDVIP process flow was employed for array fabrication. Excellent array performance data were obtained from these MWIR arrays with MBE HgCdTe material. The noise-equivalent differential flux (NEΔΦ) operability of the best array is 99.76%, comparable to the best array obtained from liquid-phase epitaxy (LPE) material prepared in this laboratory.     

Solid-State Synthesis of Te-Doped Mg<Subscript>2</Subscript>Si

Te-doped Mg₂Si (Mg₂Si:Te _m , m = 0, 0.01, 0.02, 0.03, 0.05) alloys were synthesized by a solid-state reaction and mechanical alloying. The electronic transport properties (Hall coefficient, carrier concentration, and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, thermal conductivity, and figure of merit) were examined. Mg₂Si was synthesized successfully by a solid-state reaction at 673 K for 6 h, and Te-doped Mg₂Si powders were obtained by mechanical alloying for 24 h. The alloys were fully consolidated by hot-pressing at 1073 K for 1 h. All the Mg₂Si:Te _m samples showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased and the absolute value of the Seebeck coefficient decreased with increasing Te content, because Te doping increased the electron concentration considerably from 10¹⁶ cm⁻³ to 10¹⁸ cm⁻³. The thermal conductivity did not change significantly on Te doping, due to the much larger contribution of lattice thermal conductivity over the electronic thermal conductivity. Thermal conduction in Te-doped Mg₂Si was due primarily to lattice vibrations (phonons). The thermoelectric figure of merit of intrinsic Mg₂Si was improved by Te doping.     

Electrodeposition of <Emphasis Type="Italic">p</Emphasis>-Type Sb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Thermoelectric Films

Thermoelectric Sb _x Te _y films were potentiostatically electrodeposited in aqueous nitric acid electrolyte solutions containing different concentrations of TeO₂. Stoichiometric Sb _x Te _y films were obtained by applying a voltage of −0.15 V versus saturated calomel electrode (SCE) using a solution consisting of 2.4 mM TeO₂, 0.8 mM Sb₂O₃, 33 mM tartaric acid, and 1 M HNO₃. The nearly stoichiometric Sb₂Te₃ films had a rhombohedral structure, R[`3]m R\bar{3}m , with a preferred orientation along the (015) direction. The films had hole concentration of 5.8 × 10¹⁸/cm³ and exhibited mobility of 54.8 cm²/Vs. A more negative potential resulted in higher Sb content in the deposited Sb _x Te _y films. Furthermore, it was observed that the hole concentration and mobility decreased with increasingly negative deposition potential, and eventually showed insulating properties, possibly due to increased defect formation. The absolute value of the Seebeck coefficient of the as-deposited Sb₂Te₃ thin film at room temperature was 118 μV/K.     

Thermoelectric Properties of Sb-Doped Mg<Subscript>2</Subscript>Si<Subscript>0.3</Subscript>Sn<Subscript>0.7</Subscript>

Mg₂(Si_0.3Sn_0.7)_1−y Sb _y (0 ≤ y ≤ 0.04) solid solutions were prepared by a two-step solid-state reaction method combined with the spark plasma sintering technique. Investigations indicate that the Sb doping amount has a significant impact on the thermoelectric properties of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y compounds. As the Sb fraction y increases, the electron concentration and electrical conductivity of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y first increase and then decrease, and both reach their highest value at y = 0.025. The sample with y = 0.025, possessing the highest electrical conductivity and one of the higher Seebeck coefficient values among all the samples, has the highest power factor, being 3.45 mW m⁻¹ K⁻² to 3.69 mW m⁻¹ K⁻² in the temperature range of 300 K to 660 K. Meanwhile, Sb doping can significantly reduce the lattice thermal conductivity (κ _ph) of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y due to increased point defect scattering, and κ _ph for Sb-doped samples is 10% to 20% lower than that of the nondoped sample for 300 K < T < 400 K. Mg₂(Si_0.3Sn_0.7)_0.975Sb_0.025 possesses the highest power factor and one of the lower κ _ph values among all the samples, and reaches the highest ZT value: 1.0 at 640 K.     

Crystallization behavior and ferroelectric properties of PbTiO<Subscript>3</Subscript>/Ba<Subscript>0.85</Subscript>Sr<Subscript>0.15</Subscript>TiO<Subscript>3</Subscript>/PbTiO<Subscript>3</Subscript> sandwich thin film on Pt/Ti/SiO<Subscript>2</Subscript>/Si substrates

A PbTiO₃/Ba_0.85Sr_0.15TiO₃/PbTiO₃ (PT/BST15/PT) sandwich thin film has been prepared on Pt/Ti/SiO₂/Si substrates by an improved sol-gel technique. It is found that such films under rapid thermal annealing at 700°C crystallize more favorably with the addition of a PbTiO₃ layer. They possess a pure, perovskite-phase structure with a random orientation. The polarization-electric field (P-E) hysteresis loop and current-voltage (I-V) characteristic curves reveal that a PT/BST15/PT film exhibits good ferroelectricity at room temperature. However, no sharp peak, only a weak maximum, is observed in the curves of the dielectric constant versus temperature. The dielectric constant, loss tangent, leakage current density at 20 kV/cm, remnant polarization, and coercive field of the PT/BST15/PT film are 438, 0.025, 1.3 × 10⁻⁶ Acm⁻², 2.46 μCcm⁻², and 41 kVcm⁻¹, respectively, at 25°C and 10 kHz. The PT/BST15/PT film is a candidate material for high sensitivity elements for uncooled, infrared, focal plane arrays (UFPAs) to be used at near ambient temperature.     

Thermoelectric Properties of Zr<Subscript>3</Subscript>Mn<Subscript>4</Subscript>Si<Subscript>6</Subscript> and TiMnSi<Subscript>2</Subscript>

The Seebeck coefficient, electrical resistivity, and thermal conductivity of Zr₃Mn₄Si₆ and TiMnSi₂ were studied. The crystal lattices of these compounds contain relatively large open spaces, and, therefore, they have fairly low thermal conductivities (8.26 Wm⁻¹ K⁻¹ and 6.63 Wm⁻¹ K⁻¹, respectively) at room temperature. Their dimensionless figures of merit ZT were found to be 1.92 × 10⁻³ (at 1200 K) and 2.76 × 10⁻³ (at 900 K), respectively. The good electrical conductivities and low Seebeck coefficients might possibly be due to the fact that the distance between silicon atoms in these compounds is shorter than that in pure semiconductive silicon.     

Thermoelectric Properties of NdGd<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript> Prepared by CS<Subscript>2</Subscript> Sulfurization

Ternary rare-earth sulfides NdGd_1+x S₃, where 0 ≤ x ≤ 0.08, were prepared by sulfurizing Ln₂O₃ (Ln = Nd, Gd) with CS₂ gas, followed by reaction sintering. The sintered samples have full density and homogeneous compositions. The Seebeck coefficient, electrical resistivity, and thermal conductivity were measured over the temperature range of 300 K to 950 K. All the sintered samples exhibit a negative Seebeck coefficient. The magnitude of the Seebeck coefficient and the electrical resistivity decrease systematically with increasing Gd content. The thermal conductivity of all the sintered samples is less than 1.9 W K⁻¹ m⁻¹. The highest figure of merit ZT of 0.51 was found in NdGd_1.02S₃ at 950 K.     

Conductivity compensation in <Emphasis Type="Italic">p</Emphasis>-6<Emphasis Type="Italic">H</Emphasis>-SiC in irradiation with 8-MeV protons

Carrier removal rate (V _d ) in p-6H-SiC in its irradiation with 8-MeV protons has been studied. The p-6H-SiC samples were produced by sublimation in vacuum. V _d was determined by analysis of capacitance-voltage characteristics and from results of Hall effect measurements. It was found that complete compensation of samples with initial value of N _a − N _d ≈ 1.5 × 10¹⁸ cm⁻³ occurs at an irradiation dose of ∼1.1 × 10¹⁶ cm⁻². In this case, the carrier removal rate was ∼130 cm⁻¹.     

The temperature and pressure dependence of the electron and hole mobilities in GaxIn1-xAsyP1-y alloys

A wide range of samples of both n-type and p-type Ga_xIn_1-xAs_yP_1-y on InP has been grown by LPE with carrier concentrations in the low 10¹⁶cm⁻³range. The electron mobility (μ_e) at room temperature decreased from about 4000 cm²V⁻¹s⁻¹ at y = 0 and passed through a shallow minimum near y = 0.25. At high y values, μ_e rose steeply, reaching 11 000 cm²V⁻¹s⁻¹ at the ternary boundary. In the p-type material the hole mobility (μ_p) varied from 140 cm V⁻¹s⁻¹ in InP, passed through a minimum of about 70 cm²V⁻¹s⁻¹ near y = 0.5 and then increased swiftly towards the ternary boundary. The temperature dependence of both μ_e and μ_p suggested the presence of alloy or space-charge scattering. In order to distinguish between these two mechanisms the pressure coefficient of the direct band-gap dE_o/dP was measured as a function of y by observing the movement with pressure of the photoconductive edge. From dE_o/dP the pressure variation of the effective mass was deduced. By measuring the change in electron and hole mobilities with pressure, it was then possible to establish that alloy scattering rather than space-charge scattering was occurring. From the composition dependence of the alloy scattering potentials for electrons and holes predictions have been made of the variation of μ_e and μ_P with temperature, pressure and dopant Presently a Nuffield Science Fellow concentration. At room temperature a maximum electron mobility of about 11,200 cm²V⁻¹ s⁻¹ is indicated. Presently a Nuffield Science Fellow     

Role of Si-doped Al<Subscript>0.3</Subscript>Ga<Subscript>0.7</Subscript>As layers in the high-frequency conductivity of GaAs/Al<Subscript>0.3</Subscript>Ga<Subscript>0.7</Subscript>As heterostructures under conditions of the quantum hall effect

The complex high-frequency conductivity of GaAs/Al_0.3Ga_0.7As heterostructures that are δ-doped and modulation-doped with silicon was investigated by acoustic methods under conditions of the integer quantum Hall effect. Both the real (σ₁) and imaginary (σ₂) parts of the complex conductivity σ(ω, H)=σ_i?iσ₂ were determined from the dependences of the absorption and velocity of surface acoustic waves on magnetic field. It is shown that, in the heterostructures with electron density n_s=(1.3–7)×10¹¹ cm^?2 and mobility μ=(1–2)×10⁵ cm²/(V s), the high-frequency conductivity near the centers of the Hall plateau is due to electron hopping between localized states. It is established that, with filling numbers 2 and 4, the conductivity of the Al_0.3Ga_0.7As:Si layer efficiently shunts the high-frequency hopping conductivity of the two-dimensional interface layer. A method of separating the contributions of the interface and Al_0.3Ga_0.7As:Si layers to the hopping conductivity σ(ω, H) is developed. The localization length of electrons in the interface layer is determined on the basis of the nearest neighbor hopping model. It is shown that, near the centers of the Hall plateau, both σ(ω, H) and n_s depend on the cooling rate of a GaAs/Al_0.3Ga_0.7As sample. As a result, the sample “remembers” the cooling conditions. Infrared light and static strain also change both σ(ω, H) and n_s. We attribute this behavior to the presence of two-electron defects (so-called DX^? centers) in the Al_0.3Ga_0.7As:Si layer.