Ultra-violet photo-response characteristics of p-Si/i-SiO2/n-ZnO heterojunctions based on hydrothermal ZnO nanorods

Hydrothermal zinc oxide (ZnO) nanorod (NR)-based p-Si/n-ZnO and p-Si/i-SiO₂/n-ZnO heterojunctions were fabricated, and the effects of interfacial native SiO₂ (~4 nm) on the I-V characteristics of heterojunctions under dark and ultra-violet illumination conditions were investigated. First, the structural and optical properties of ZnO seed crystals grown by sol-gel method and hydrothermal ZnO NRs on two different substrates of p-Si and p-Si/i-SiO₂ were examined, and more improved optical and crystalline quality was obtained as revealed by photoluminescence and X-ray diffraction. The p-i-n heterojunctions showed ~3 times greater forward-bias currents and enhanced rectifying property than those of p-n junctions, which is attributed to the role of native SiO₂ in carrier confinement by promoting the electron-hole recombination current through the deep level states of ZnO crystal. The measured ratios of photocurrent to dark current of the p-i-n structure were also greater under reverse bias (92–260) and forward bias (2.3–7.1) conditions than those (28–225 for reverse bias, 1.6–6.8 for forward bias) of p-n structure, and the improved photosensitivity of the p-i-n structure under reverse bias is due to lower density of recombination centers in the ZnO NR crystals. Fabricated ZnO NR heterojunction showed repeatable and fast photo-response transients under forward bias condition of which response and recovery times were 7.2 and 3.5 s for p-i-n and 4.3 and 1.7 s for p-n structures, respectively.     

Reconstruction of dependences of the tunneling current on the oxide voltage using the dynamic current-voltage characteristics of the n +-Si-SiO2- n -Si heterostructures

Precision measurements of dynamic current-voltage characteristics of an Al-n ⁺-Si-SiO₂-n-Si structure with a thin (<50 ?) oxide make it possible to separate the active (I _a ) and capacitive (I _c ) components from the total current. An algorithm for the analysis of the capacitive component is developed; this algorithm makes it possible to determine in a single experiment the doping level of n-Si, the oxide capacitance C _i , and also the density and sign of the charge fixed in the oxide. Dependences of the surface potential in n-Si and the voltage drop across the oxide on the gate potential V _g in the transverse electric fields |F| ≤ 10 MV/cm were calculated based on the above data without using any adjustable parameters. At maximum values of |F|, the sheet density of electrons (holes) in n-Si does not exceed 10¹³ cm⁻², which is indicative of the degeneracy and size quantization of electron gas. The dependences I _t (V _g ) and V _i (V _g ) were used to recover the current-voltage characteristics of the tunneling current I _t (V _i ) ≡ I _a (V _i ); these characteristics were measured within more than ten orders of magnitude of their range of variation in the conditions of both the enhancement of the n-Si surface and the inversion. The observed I _t (V _i ) characteristics are not quantitatively described in the context of existing concepts of the tunnel effect. Original Russian Text ? A.G. Zhdan, N.F. Kukharskaya, V.G. Naryshkina, G.V. Chucheva, 2007, published in Fizika i Tekhnika Poluprovodnikov, 2007, Vol. 41, No. 9, pp. 1135–1142.     

Comparative studies of Pt and Ir schottky contacts on undoped Al0.36Ga0.64N

Schottky contacts of Pt and Ir on undoped Al_0.36Ga_0.64N have been fabricated and the ideality factor, the built-in voltage and the reverse bias current were determined using current–voltage measurements to make a comparison.The smallest ideality factors, the lowest reverse bias current and the highest built-in voltages have been obtained for Ir Schottky contacts.We have studied the effect of an annealing for Pt and Ir Schottky contacts, on the ideality factor, the built-in voltage and the reverse bias current. A decrease of the ideality factor and the reverse bias current associated to an increase of the built-in voltage have been obtained except for high annealing temperature (T > 400 °C).Reductions of 37% and 43% of the ideality factor and improvements of 24% and 41% of the built-in voltage have been obtained for Pt and Ir Schottky contacts, respectively, after an annealing performed at 350 °C during 30 min.Two different electrical stresses have also been applied on the ohmic and Schottky contacts during 164 h to study the reliability of the employed technology. In a first time, the devices have been stressed with a drain-to-source voltage V_DS of 20 V and a gate-to-source voltage V_GS of −5 V to submit the devices to an electrical field only and not to a thermal effect induced by the electrical current. In a second time, the aging stress has been applied for a V_DS of 20 V and for a V_GS of 0 V in order to study the impact of the electrical field and the thermal effect induced by the drain current on the electrical behaviours of Al_0.36Ga_0.64N/GaN transistors. This study has also shown the existence of electrical traps in the device structure and proved the good reliability of the involved technology.These comparative studies demonstrate that Ir is a better candidate than Pt for the realisation of Schottky contacts on undoped Al_0.36Ga_0.64N.     

Technological Features of Irradiation of Sip+-n-n+ Diodes with Electrons at Elevated Temperatures

The behavior of the lifetime of nonequilibrium charge carriers τ_p, the reverse current I _R, and the forward-voltage drop U _F in electron-irradiated (E _irr = 6 MeV) commercial p ⁺-n-n ⁺ diodes at irradiation temperatures in the range of T _irr = 20–400°C is studied. Studies have been performed for samples fabricated on a single-crystal Si substrate either doped with phosphorus in the course of growth by the Czochralski method (Cz-n-Si:P) or doped with phosphorus by nuclear transmutations, neutron-transmutation doped Si (NTD n-Si:P). It is shown that, by choosing the temperature conditions of technological irradiation, one can solve the problem of attaining small values of τ_p at a minimal increase in U _F and I _R in fast-response diodes. It is established that, in the case of comparable variations in τ_p in the base region of diodes, the best relation between U _F and I _R is observed at T _irr = 300°C in n-Si:P samples doped by the Czochralski method and at T _irr = 350°C in samples doped by reactions induced by thermal neutrons.     

From Long Infrared to Very Long Infrared Wavelength Focal Plane Arrays Made with HgCdTe n + n ?/ p Ion Implantation Technology

This paper aims at studying the feasibility of very long infrared wavelength (VLWIR) (12–18 μm) focal plane arrays using n-on-p planar ion-implanted technology. To explore and analyze the feasibility of such VLWIR detectors, a set of four Cd _x Hg_1−x Te LPE layers with an 18 μ cutoff at 50 K has been processed at Defir (LETI/LIR–Sofradir joint laboratory), using both our “standard” n-on-p process and our improved low dark current process. Several 320 × 256 arrays, 30-μm pitch, have been hybridized on standard Sofradir readout circuits and tested. Small dimension test arrays characterization is also presented. Measured photonic currents with a 20°C black body suggest an internal quantum efficiency above 50%. Typical I(V) curves and thermal evolution of the saturation current are discussed, showing that standard photodiodes remain diffusion limited at low biases for temperatures down to 30 K. Moreover, the dark current gain brought by the improved process is clearly visible for temperatures higher than 40 K. Noise measurements are also discussed showing that a very large majority of detectors appeared background limited under usual illumination and biases. In our opinion, such results demonstrate the feasibility of high-performance complex focal plane arrays in the VLWIR range at medium term.     

Electrical and Optical Characterization of AlGaN/GaN HEMTs with <Emphasis Type="Italic">In Situ</Emphasis> and <Emphasis Type="Italic">Ex Situ</Emphasis> Deposited SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Layers

A comparative study of AlGaN/GaN high-electron-mobility transistor (HEMT) surface passivation using ex situ and in situ deposited SiN _x is presented. Performing ex situ SiN _x passivation increased the reverse gate leakage and off-state channel leakage by about three orders of magnitude. The in situ SiN _x layer was characterized using transmission electron microscopy (TEM) and capacitance–voltage (CV) measurements. Photoluminescence (PL) spectra indicated a reduction of nonradiative recombination centers in in situ SiN _x -passivated samples, indicating improved crystal quality. CV measurements indicated a reduction of surface state density as well, and thus better overall passivation using in situ SiN _x . Electroluminescence (EL) images of the channel regions in AlGaN/GaN HEMT devices operating in forward blocking mode with up to 400 V drain bias demonstrated reduced channel emission profiles of in situ-passivated devices. Compared with a nonpassivated reference sample, the reduced EL emission profiles correlated with a reduced channel temperature on ex situ SiN _x -passivated devices.     

New dielectric material system of Nd(Mg1/2Ti1/2)O3–CaTiO3 at microwave frequency

The microwave dielectric properties of (1 − x)CaTiO₃–xNd(Mg_1/2Ti_1/2)O₃ (0.1  x  1.0) ceramics prepared by the conventional solid state method have been investigated. The system forms a solid solution throughout the entire compositional range. The dielectric constant decreases from 152 to 27 as x varies from 0.1 to 1.0. In the (1 − x)CaTiO₃–xNd(Mg_1/2Ti_1/2)O₃ system, the microwave dielectric properties can be effectively controlled by varying the x value. At 1400 °C, 0.1CaTiO₃–0.9Nd(Mg_1/2Ti_1/2)O₃ has a dielectric constant (ε_r) of 42, a Q × f value of 35 000 GHz and a temperature coefficient of resonant frequency (τ_f) of −10 ppm/°C. As the content of Nd(Mg_1/2Ti_1/2)O₃ increases, the highest Q × f value of 43 000 GHz for x = 0.9 is achieved at the sintering temperature 1500 °C.     

Temperature dependent current-voltage characteristics of n-MgxZn1−xO/p-GaN junction diodes

This study investigates the temperature dependence of the current-voltage (I-V) characteristics of n-Mg_xZn_1−xO/p-GaN junction diodes. The n-Mg_xZn_1−xO films were deposited on p-GaN using a radio-frequency (rf) magnetron sputtering system followed by annealing at 500, 600, 700, and 800 °C in nitrogen ambient for 60 s, respectively. The n-Mg_xZn_1−xO/p-GaN diode at a substrate temperature of 25 °C had the lowest leakage current in reverse bias. However, the leakage current of the diodes increased with an increase in annealing temperatures. The temperature sensitivity coefficients of the I-V characterizations were obtained at different substrate temperatures (25, 50, 75 100, and 125 °C) providing extracted values of 26.4, 27.2, 17.9, and 0.0 mV/°C in forward bias and 168.8, 143.4, 84.6, and 6.4 mV/°C in reverse bias, respectively. The n-Mg_xZn_1−xO/p-GaN junction diode fabricated with Mg_xZn_1−xO annealed at 800 °C demonstrated the lowest temperature dependence. Based on these findings, the n-Mg_xZn_1−xO/p-GaN junction diode is feasible for GaN-based heterojunction bipolar transistors (HBTs).     

Electric and photoelectric properties of n-GaxIn1−x N/p-Si anisotypic heterojunctions

We have developed a technology for producing n-type Ga_xIn_1−x N/p-Si heterostructures by combined pyrolysis of indium and gallium monoammoniate chlorides, making it possible to obtain heterolayers with composition varying over wide limits (from GaN up to InN). The composition and basic electric and optical characteristics of nitride films were determined. The electric and photoelectric properties of the heterostructures with Ga_xIn_1−x N films of different composition were investigated. It was shown that the anisotypic heterojunction n-Ga_xIn_1−x N/p-Si is a promising photosensitive element for detecting visible-range radiation. The maximum values of the specific detectivity were D*=1.2×10¹¹ Hz^1/2·W⁻¹ at 290 K. A band diagram of the heterojunction was constructed. Fiz. Tekh. Poluprovodn. 32, 461–465 (April 1998)     

High-Operating-Temperature HgCdTe Avalanche Photodiodes

In this communication we report the first results of electro-optical characterization of planar heterostructure HgCdTe avalanche photodiodes (APDs), which enables the operation of APDs at high gain, at low bias, and with low dark current and/or at high operating temperature (HOT). The APD is based on a heterostructure in which the photons are detected in a wide-band-gap layer, and the photoelectrons are amplified in a vertical junction in a confined narrow-gap layer. The dark diffusion current and thermal background sensitivity of the device are limited by using a thin narrow-band-gap amplification layer. In addition, the defect-limited dark current is also expected to be reduced due to the reduced volume of the narrow-band-gap depletion layer. The electro-optical performance was characterized at T = 80 K and T = 200 K for two devices with a nominal thickness of the amplification layer of w = 100 nm and 500 nm, realized in x _Cd = 0.3 Hg-vacancy-doped layers grown by molecular-beam epitaxy (MBE). The measurements show an average gain of 〈M〈 = 10 at a reverse bias of 5 V, which is slightly reduced compared with a conventional APD with x _Cd = 0.3. The thermal diffusion current measured at low reverse bias, V _b = 0.1 V, and at T = 200 K is about 0.1 mA/cm² to 0.3 mA/cm², which is a factor of 50 lower than standard x _Cd = 0.3 n-on-p APDs. The quantum efficiency due to absorption in the gain layer is high (QE_peak > 30%), although no antireflecting coating was used, indicating that the device can also be used for high-operating-temperature thermal detection.     

Comparative study of AlGaN/GaN HEMTs robustness versus buffer design variations by applying Electroluminescence and electrical measurements

By means of step stressing tests on AlGaN/GaN HEMTs the robustness properties of devices fabricated on wafers with different buffer designs have been compared to each other (standard UID GaN buffer and UID Al_0.05Ga_0.95N back-barrier in combination with GaN channel layer). The devices with GaN buffer showed an abrupt increase of gate leakage current after reaching drain bias values in the range of 30 V while devices with Al_0.05Ga_0.95N back-barrier did not show any degradation up to 120 V drain bias. All DC-Step-Stress tests have been accompanied by Electroluminescence (EL) analysis and electrical characterization techniques before, during and after stress. It has been shown that EL at forward and reverse bias conditions can be used as an indicator of potential device degradation. Devices comprising an AlGaN back-barrier design demonstrated superior robustness.     

Electrical Properties of Organic–Inorganic Semiconductor Device Based on Rhodamine-101

Rhodamine-101 (Rh101) thin films on n-type Si substrates have been formed by means of evaporation, thus Sn/Rh101/n-Si heterojunctions have been fabricated. The Sn/Rh101/n-Si devices are rectifying. The optical energy gaps have been determined from the absorption spectra in the wavelength range of 400 nm to 700 nm. Rh101 has been characterized by direct optical absorption with an optical edge at 2.05 ± 0.05 eV and by indirect optical absorption with␣an optical edge at 1.80 ± 0.05 eV. It was demonstrated that trap-charge-limited current is the dominant transport mechanism at large forward bias. A␣mobility value of μ = 7.31 × 10⁻⁶ cm² V⁻¹ s⁻¹ for Rh101 has been obtained from the forward-bias current–voltage characteristics.     

Degradation of MOS tunnel structures at high current density

The stability of tunneling-thin (2–3 nm) SiO₂ films during prolonged flow of high-density currents (10²–10³ A/cm²) was investigated. A sharp increase in the charge which a tunneling MOS structure is capable of transmitting without degradation on switching from Fowler-Nordheim injection to direct tunneling (10³ C/cm² and 10⁷ C/cm², respectively) was observed. The degradation of SiO₂ films was investigated using Al/SiO₂/n-Si/p ⁺-Si thyristor structures with a positive bias on the semiconductor, i.e., with reverse bias of the MOS structure. The use of these devices accounted for the uniformity of the current distribution over the area and made it possible to monitor the state of the insulator layer by measuring the device gain in the phototransistor mode. Fiz. Tekh. Poluprovodn. 32, 743–747 (June 1998)     

Effects of Ir Substitution and Processing Conditions on Thermoelectric Performance of p-Type Zr0.5Hf0.5Co1?xIrxSb0.99Sn0.01 Half-Heusler Alloys

A series of samples with the composition Zr_0.5Hf_0.5Co_1−x Ir _x Sb_0.99Sn_0.01 (x = 0.0 to 0.7) were synthesized by high-temperature solid-state reaction at 1173 K. High-density pellets of the powders were obtained using hot press (HP) and spark plasma sintering (SPS) techniques. The thermoelectric properties of the pellets were measured from 300 K to 750 K. Independently of the pressing conditions, all Ir-containing samples (x > 0) showed p-type semiconducting behavior. At 300 K, the electrical conductivity and thermopower of Zr_0.5Hf_0.5Co_1−x Ir _x Sb_0.99Sn_0.01 materials surprisingly increased with increasing Ir concentration. The largest electrical conductivity and thermopower values of 150 S/cm and 140 μV/K, respectively, were observed at 300 K for x = 0.7. The thermal conductivity of the synthesized materials decreased with increasing Ir content, went through a minimum value (x = 0.3), and increased thereafter with further addition of Ir. Pellets fabricated by SPS showed smaller thermal conductivity than pellets of the same composition obtained from uniaxial hot pressing. A thermal conductivity value of ∼2.0 W/m K was observed at 300 K for an SPS pellet with the com- position Zr_0.5Hf_0.5Co_0.5Ir_0.5Sb_0.99Sn_0.01. The thermal conductivity of Zr_0.5Hf_0.5- Co_1−x Ir _x Sb_0.99Sn_0.01 decreased with rising temperature, and the smallest value of ∼1.5 W/m K was observed at 750 K for the SPS specimen with x = 0.5.     

Silicon nanowire array architecture for heterojunction electronics

Photosensitive nanostructured heterojunctions n-TiN/p-Si were fabricated by means of titanium nitride thin films deposition (n-type conductivity) by the DC reactive magnetron sputtering onto nano structured single crystal substrates of p-type Si (100). The temperature dependencies of the height of the potential barrier and series resistance of the n-TiN/p-Si heterojunctions were investigated. The dominant current transport mechanisms through the heterojunctions under investigation were determined at forward and reverse bias. The heterojunctions under investigation generate open-circuit voltage V _oc = 0.8 V, short-circuit current I _sc = 3.72 mA/cm² and fill factor FF = 0.5 under illumination of 100 mW/cm².

     

Capacitance–conductance–current–voltage characteristics of atomic layer deposited Au/Ti/Al2O3/n-GaAs MIS structures

We have studied the admittance and current–voltage characteristics of the Au/Ti/Al₂O₃/n-GaAs structure. The Al₂O₃ layer of about 5 nm was formed on the n-GaAs by atomic layer deposition. The barrier height (BH) and ideality factor values of 1.18 eV and 2.45 were obtained from the forward-bias ln I vs V plot at 300 K. The BH value of 1.18 eV is larger than the values reported for conventional Ti/n-GaAs or Au/Ti/n-GaAs diodes. The barrier modification is very important in metal semiconductor devices. The use of an increased barrier diode as the gate can provide an adequate barrier height for FET operation while the decreased barrier diodes also show promise as small signal zero-bias rectifiers and microwave. The experimental capacitance and conductance characteristics were corrected by taking into account the device series resistance R_s. It has been seen that the non-correction characteristics cause a serious error in the extraction of the interfacial properties. Furthermore, the device behaved more capacitive at the reverse bias voltage range rather than the forward bias voltage range because the phase angle in the reverse bias has remained unchanged as 90° independent of the measurement frequency.     

Voltage-tunable SiO2-isolated a-SiC:H and/or a-SiN:H p–i–n thin-film LEDs fabricated on c-Si

A voltage-tunable amorphous p–i–n thin-film light emitting diodes (TFLEDs) with SiO₂-isolation on n⁺-type crystalline silicon (c-Si) has been proposed and fabricated successfully. The structure of the device with i-a-SiC:H and i-a-SiN:H luminescent layers is indium–tin–oxide (ITO)/p⁺-a-Si:H/p⁺-a-SiC:H/i-a-SiC:H/i-a-SiN:H/n⁺-a-SiCGe: H/n⁺-a-SiC:H/n⁺-c-Si/Al. This device revealed a brightness of 695 cd/m² at an injection current density of 300 mA/cm². Its EL (electroluminescence) peak wavelength exhibited blue-shift from 655 to 565 nm with applied forward-bias (V) increasing from 15 to 19 V, but the EL peak wavelength was red-shifted from 565 to 670 nm with further increase of V from 19 to 23 V. By comparing with the EL spectra from p–i–n TFLEDs with i-a-SiC:H or i-a-SiN:H luminescent layer only, the EL spectrum of this TFLED could consist of three bands of radiations from the tail-to-tail-state recombinations in (1) i-a-SiC:H layer, (2) i-a-SiN:H layer, and (3) i-a-SiC:H/p⁺-a-SiC:H junction.     

Electroluminescence from Au/Si/SiO2/p-Si structure

Si/SiO₂ films have been grown using the two-target alternation magnetron sputtering technique. The thickness of the SiO₂ layer in all the films was 8 nm and that of the Si layer in five types of the films ranged from 4 to 20 nm in steps of 4 nm. Visible electroluminescence (EL) has been observed from the Au/Si/SiO₂/p-Si structures at a forward bias of 5 V or larger. A broad band with one peak 650–660 nm appears in all the EL spectra of the structures. The effects of the thickness of the Si layer in the Si/SiO₂ films and of input electrical power on the EL spectra are studied systematically.     

Electrical and gas sensing properties of <Emphasis Type="Italic">por</Emphasis>-Si/SnO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanocomposite layers

The electrical characteristics and chemical reactant sensitivity of layers of heterogeneous nanocomposites based on porous silicon and nonstoichiometric tin oxide por-Si/SnO _x , fabricated by the magnetron sputtering of tin with subsequent oxidation, are studied. It is shown that, in the nanocomposite layers, a system of distributed heterojunctions (Si/SnO _x nanocrystals) forms, which determine the electrical characteristics of such structures. The sensitivity of test sensor structures based on por-Si/SnO _x nanocomposites to NO₂ is determined. A mechanism for the effect of the adsorption of NO₂ molecules on the current-voltage characteristics of the por-Si(p)/SnO _x (n) heterojunctions is suggested.     

Thermoelectric Properties of Triple-Filled BaxYbyInzCo4Sb12 Skutterudites

Indium-filled skutterudites are promising power generation thermoelectric materials due to the presence of an InSb nanostructure that lowers the thermal conductivity. In this work, we have investigated thermoelectric properties of triple-filled Ba _x Yb _y In _z Co₄Sb₁₂ (0 ≤ x, y, z ≤ 0.14 actual) compounds by measuring their Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient. All samples were prepared by a melting–annealing–spark plasma sintering method, and their structure was characterized by x-ray diffraction and transmission electron microscopy (TEM). TEM results show the development of an InSb nanostructure with a grain size of 30 nm to 500 nm. The nanostructure is present in all samples containing In and is also detected by specific heat measurements. The Seebeck and Hall coefficients indicate that the compounds are n-type semiconductors. Electrical conductivity increases with increasing Ba content. Thermal conductivity is strongly suppressed upon the presence of In in the skutterudite structure, likely due to enhanced boundary scattering of phonons on the nanometer-scale InSb inclusions. The highest thermoelectric figure of merit is achieved with Ba_0.09Yb_0.07In_0.06Co₄Sb_11.97, reaching ZT = 1.25 at 800 K.