Ultraviolet electroluminescence from ZnO-based light-emitting diode with p-ZnO:N/n-GaN:Si heterojunction structure

A p-ZnO:N/n-GaN:Si structure heterojunction light-emitting diode (LED) is fabricated on c-plane sapphire by full metal organic chemical vapor deposition (MOCVD) technique. The p-type layer with hole concentration of 8.94×10¹⁶ cm⁻³ is composed of nitrogen-doped ZnO using NH₃ as the doping source with subsequent annealing in N₂O plasma ambient. Silicon-doped GaN film with electron concentration of 1.15×10¹⁸ cm⁻³ is used as the n-type layer. Desirable rectifying behavior is observed from the current-voltage (I-V) curve of the device. The forward turn on voltage is about 4 V and the reverse breakdown voltage is more than 7 V. A distinct ultraviolet (UV) electroluminescence (EL) with a dominant emission peak centered at 390 nm is detected at room temperature from the heterojunction structure under forward bias conditions. The origins of the EL emissions are discussed in comparison with the photoluminescence (PL) spectra.     

MOCVD法制备的p-ZnO/n-SiC 异质结器件及其电致发光性能

采用光辅助金属有机化学汽相沉积(PA-MOCVD)法在n-SiC(6H)衬底上制备出As掺杂的p型ZnO薄膜,并制备出相应的p-ZnO:As/n-SiC异质结器件。X射线衍射(XRD)和光致发光(PL)测试表明,ZnO薄膜具有较好的结构和光学特性。电流-电压(I-V)测试结果表明,该型异质结器件具有良好的整流特性,开启电压为5.0 V,反向击穿电压约为-13 V。正向偏压下,器件的电致发光(EL)谱表现出两个分别位于紫外和可见光区域的发光峰,通过和ZnO、SiC的PL谱对照,证实异质结器件的发光峰来源于ZnO侧的辐射复合。     

Current transport in ZnO/Si heterostructure grown by laser molecular beam epitaxy

The n-ZnO/p-Si heterojunction was fabricated by depositing high quality single crystalline aluminium-doped n-type ZnO film on p-type Si using the laser molecular beam epitaxy technique. The heterojunction exhibited a good rectifying behavior. The electrical properties of the heterojunction were investigated by means of temperature dependence current density-voltage measurements. The mechanism of the current transport was proposed based on the band structure of the heterojunction. When the applied bias V is lower than 0.15 V, the current follows the Ohmic behavior. When 0.15V 0.6 V), the space charge limited effect becomes the main transport mechanism. The current-voltage characteristic under illumination was also investigated. The photovoltage and the short circuit current density of the heterojunction aproached 270 mV and 2.10 mA/cm 2 , respectively.     

Electrical characterization of p-ZnO/p-Si heterojunction

Nitrogen doped p-ZnO film, with urea as nitrogen source, is fabricated by pulsed laser deposition on well-cleaned p-type (1 0 0) Si substrates. The structural and electrical properties of the p-p heterojunction are investigated by current-voltage (I-V) and capacitance-voltage (C-V) measurements. It shows a diode-like behavior with turn-on voltage of 0.5 V. The ideality factor η determined by applying positive potential in p-ZnO and negative potential along p-Si is found to be 6. Such a high value of η is attributed to lattice mismatch between ZnO and Si. and other factors responsible are thermoionic emission, minority carrier injection and recombination. C-V results indicate an abrupt interface and a band bending of 0.9 V in the silicon. Heterojunction band diagram for p-ZnO/p-Si is proposed.     

Electrical and magnetic properties of Ni-Cu-Si heterojunction prepared by the liquid phase epitaxy technique

Ni-Cu-Si heterojunction was prepared by the liquid phase epitaxy (LPE) technique. Two growth solutions containing Indium (In) with Cu pieces and In with Ni pieces were employed during the fabrication process. The as-formed junction was directly characterized by different techniques including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and thin film X-ray diffraction (XRD) measurements. The current-voltage (I-V) characteristics of the Au/Ni/Cu/Si diode were found to be nonlinear, asymmetric, having a good rectification behavior with a very small leakage current of 0.003 μA at a reverse bias voltage of 2.0 V. The value of turn on voltage was located at 0.2 V. The magnetic properties were also evaluated at room temperature with a vibrating sample magnetometer. Systematic study of junction fabrication and characterization of such a heterosystem, comparison of the behavior of flat silicon and nanoporous silicon as substrates are presented and thoroughly discussed.     

Deep level states and negative photoconductivity in n-ZnO/p-Si hetero-junction diodes

Negative photoconductivity (NPC) was observed in n-ZnO/p-Si heterojunction diode grown by ultra-high vacuum sputtering method under nitrogen ambient. Under the illumination of ultra-violet light, positive photoconductivity was observed at low bias voltages, whereas NPC was observed at high bias voltages. The defect states in the ZnO layers grown on Si were analyzed by photoluminescence and deep level transient spectroscopy measurements. Two deep levels were measured at E_c-0.51 eV and E_c-0.54 eV, which might be originated from oxygen vacancy and nitrogen atom related defects, respectively. Based on the simulation of band diagram, the defect states were located below Fermi level at zero bias voltage. However, as increasing the bias voltages, NPC was observed due to the increase of empty defect states. This analysis allowed us to consider the possibility that the NPC phenomenon in n-ZnO/p-Si heterojunction diode is originated dominantly from the defect states as a carrier recombination center in ZnO layer.     

The fabrication and photoresponse of ZnO/diamond film heterojunction diode

Boron-doped p-type freestanding diamond (FSD) films were prepared by hot filament chemical vapor deposition (HFCVD) method. The effect of B/C ratio on the electrical properties of FSD films was investigated by Hall effect measurement system. A ZnO/diamond heterojunction diode was fabricated successfully by depositing n-type ZnO films on the p-type FSD substrate by radio-frequency (RF) magnetron sputtering method. The wavelength dependent photoresponse properties of the heterojunction diode were investigated by studying the effect of light illumination on current-voltage (I-V) characteristics and photocurrent spectra at room temperature. The diode showed a significant discrimination between ultraviolet (UV) and the visible light under reverse bias conditions and photoresponse of the device was approximately linear related to the increasing reverse bias voltages.     

Electroluminescence from ZnO nanowire-based p-GaN/n-ZnO heterojunction light-emitting diodes

Vertically aligned ZnO nanowires were successfully grown on the sapphire substrate by nanoparticle-assisted pulsed laser deposition (NAPLD), which were employed in fabricating the ZnO nanowire-based heterojunction structures. p-GaN/n-ZnO heterojunction light-emitting diodes (LEDs) with embedded ZnO nanowires were obtained by fabricating p-GaN:Mg film/ZnO nanowire/n-ZnO film structures. The current–voltage measurements showed a typical diode characteristic with a threshold voltage of about 2.5 V. Electroluminescence (EL) emission having the wavelength of about 380 nm was observed under forward bias in the heterojunction diodes and was intensified by increasing the applied voltage up to 30 V.     

Metal-semiconductor-metal ultraviolet photodetector based on GaN

A metal-semiconductor-metal (MSM) ultraviolet photodetector has been fabricated using unintentionally doped n-GaN films grown on sapphire substrates. Its dark current, photocurrent under the illumination with λ = 360 nm light, responsivity, and the dependence of responsivity on bias voltage were measured at room temperature. The dark current of the photodetector is 1.03 Na under 5 V bias, and is 15.3 Na under 10 V bias. A maximum responsivity of 0.166 A/W has been achieved under the illumination with λ= 366 nm light and 15 V bias. It exhibits a typical sharp band-edge cutoff at the wavelength of 366 nm, and a high responsivity at the wavelength from 320 nm to 366 nm. Its responsivity under the illumination with λ= 360 nm light increases when the bias voltage increases.     

CVD法制备p-ZnO薄膜/n-Si异质结发光二极管及其性能研究

利用简单的化学气相沉积方法, 首先在n-Si衬底上生长Sb掺杂p-ZnO薄膜, 并在此基础上制作了p-ZnO/n-Si异质结发光二极管．对制备的Sb掺杂ZnO薄膜 在800 ℃下进行了热退火处理, 发现退火后样品的晶体质量和表面形貌都得到明显提高, 并且薄膜呈现的电导类型为p型, 载流子浓度为9.56× 10¹⁷ cm^-3. 此外, 该器件还表现出良好的整流特性, 正向开启电压为4.0 V, 反向击穿电压为9.5 V. 在正向45 mA的注入电流条件下, 器件实现了室温下的电致发光. 这说明较高质量的ZnO薄膜也可以通过简单的化学气相沉积方法来实现, 这为ZnO基光电器件的材料制备提供了一种简单可行的方法. 关键词： CVD p-ZnO 异质结 电致发光     

Dielectrophoretically aligned GaN nanowire rectifiers

We demonstrate GaN nanowire (NW) current rectifiers which were formed by assembling n-GaN nanowires on a patterned p-Si substrate by means of alternating current (ac) dielectrophoresis. The dielectrophoresis was accomplished at a frequency of 10 kHz with three different ac bias voltages (5, 10, and 15 V_p–p), indicating that the number of aligned GaN nanowires increased with increasing ac bias voltage. The n-GaN NW/p-Si diodes showed well-defined current rectifying behavior with a forward voltage drop of 1.2–1.5 V at a current density of 200 A/cm². We observed that the GaN NW diode functioned well as a half-wave rectifier. PACS 71.20.Nr; 73.40.Cg; 73.40.Ei; 73.40.Kp     

Subgap anomaly and above-energy-gap structure in chains of diffusive SNS junctions

We present the results of low-temperature transport measurements on chains of superconductor-normal constriction-superconductor (SNS) junctions fabricated on the basis of superconducting PtSi film. A comparative study of the properties of the chains, consisting of 3 and 20 SNS junctions in series, and single SNS junctions reveals essential distinctions in the behavior of the current-voltage characteristics of the systems: (i) a gradual decrease of the effective suppression voltage for the excess conductivity observed at zero bias as the quantity of the SNS junctions increases; (ii) a rich fine structure on the dependences dV/dI-V at dc bias voltages higher than the superconducting gap and corresponding to some multiples of 2Δ/e. A model explaining this above-energy-gap structure based on the energy relaxation of electrons via Cooper-pair-breaking in the superconducting island connecting normal metal electrodes is proposed.     

Modulating microstructure and optical properties of hydrogenated nanocrystalline silicon photovoltaic materials prepared under hydrogen diluted silane PECVD by various DC bias

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were fabricated by plasma enhanced chemical vapor deposition under the various negative substrate bias voltages with hydrogen as a diluent of silane. The microstructure and optical properties of nc-Si:H thin films were studied by Raman scattering spectroscopy, X-ray diffraction (XRD), transmission electron microscopy, and optical transmission spectroscopy. Raman spectra and XRD pattern reveal that applying negative bias voltages at the moderate level favors the enhancement of crystalline volume fraction, increase of crystallite sizes and decrease of residual stress. We also demonstrated that the negative direct current bias can be used to modulate the volume fraction of voids, refractive index, absorption coefficient, compactness and ordered degree of nc-Si:H films. It is found that the film deposited at −80 V shows not only high crystallinity, size of crystallite, and static index n₀ but also low residual stress and volume fraction of voids. Furthermore, the microstructural evolution mechanism of nc-Si:H thin films prepared at different bias voltages is tentatively explored.     

Abnormal current-voltage characteristics and metal-insulator transition of amorphous carbon film/silicon heterojunction

The amorphous carbon film/n-Si (a-C/n-Si) heterojunctions have been fabricated by direct current magnetron sputtering at room temperature, and their current-voltage characteristics have been investigated. The results show that these junctions have good rectifying properties in the temperature range 80-300 K. The interesting result is that the current-voltage curve changes dramatically with increasing applied voltage and temperature. For the forward bias voltages, the junction shows Ohmic mechanism characteristic in the temperature range 240-300 K. However, the conduction mechanism changes from Ohmic for the low bias voltages to space charge limited current for the high bias voltages in the temperature range 80-240 K. While for the reverse bias voltages, it changes from Schottky emission to breakdown with increasing voltage. Another important phenomenon is that the temperature dependence of the junction resistance shows a metal-insulator transition, whose transition temperature can be controlled by the bias voltage.     

Growth of Ni-Mn-Ga high-temperature shape memory alloy thin films by magnetron sputtering technique

Ni-Mn-Ga thin films have been fabricated by using magnetron sputtering technique under various substrate negative bias voltages. The effect of substrate negative bias voltage on the compositions and surface morphology of Ni-Mn-Ga thin films was systematically investigated by energy dispersive X-ray spectrum and atomic force microscopy, respectively. The results show that the Ni contents of the thin films increase with the increase of the substrate negative bias voltages, whereas the Mn contents and Ga contents decrease with the increase of substrate negative bias voltages. It was also found that the surface roughness and average particle size of the thin films remarkably decrease with the increase of substrate negative bias voltages. Based on the influence of bias voltages on film compositions, a Ni₅₆Mn₂₇Ga₁₇ thin film was obtained at the substrate negative bias voltage of 30 V. Further investigations indicate that the martensitic transformation start temperature of this film is up to 584 K, much higher than room temperature, and the film has a non-modulated tetragonal martensitic structure at room temperature. Transmission electron microscopy observations reveal that microstructure of the thin film exhibits an internally (1 1 1) type twinned substructure. The fabrication of Ni₅₆Mn₂₇Ga₁₇ high-temperature shape memory alloy thin film will contribute to the successful development of microactuators.     

Memory Effect in the Charge Transport in Strongly Disordered Antimony Films

We study conductivity of strongly disordered amorphous antimony films under high bias voltages. We observe non-linear current-voltage characteristic, where the conductivity value at zero bias is one of two distinct values, being determined by the sign of previously applied voltage. Relaxation curves demonstrate high stability of these conductivity values on a large timescale. Investigations of the antimony film structure allows to determine the percolation character of electron transport in strongly disordered films. We connect the memory effect in conductivity with modification of the percolation pattern due to recharging of some film regions at high bias voltages.     

Transparent CdO/n-GaN(0001) heterojunction for optoelectronic applications

Undoped CdO films were prepared by sol–gel method. Transparent heterojunction diodes were fabricated by depositing n-type CdO films on the n-type GaN (0001) substrate. Current–voltage (I–V) measurements of the device were evaluated, and the results indicated a non-ideal rectifying characteristic with I_F/I_R value as high as 1.17×10³ at 2 V, low leakage current of 4.88×10⁻⁶ A and a turn-on voltage of about 0.7 V. From the optical data, the optical band gaps for the CdO film and GaN were calculated to be 2.30 eV and 3.309 eV, respectively. It is evaluated that interband transition in the film is provided by the direct allowed transition. The n-GaN (0001)/CdO heterojunction device has an optical transmission of 50–70% from 500 nm to 800 nm wavelength range.     

Enhancing photoresponse time of low cost Pd/ZnO nanorods prepared by thermal evaporation techniques for UV detection

High quality undoped ZnO nanorods have been synthesized at 850 °C by vapor-solid (VS) technique without a catalyst through a low cost process on silicon substrates. Then, ZnO nanorods have been characterized by using scanning electron microscopy (SEM), X-ray diffractometer (XRD), and photoluminescence (PL) spectroscopy. Metal-semiconductor-metal (MSM) photodetectors with palladium (Pd) as contact electrodes have been successfully constructed for ultraviolet (UV) detection. Under dark and UV illumination, the load resistance of the Pd/ZnO junction was found to be 80.4 kΩ, and 23.5 kΩ referring to the maximum allowed bias voltage; the barrier height was estimated to be about 0.8 eV, and 0.76 eV, at 5 V applied bias voltage, respectively. It was found that the maximum responsivity of the Pd/ZnO MSM photodetector was 0.106 A/W at 300 nm which corresponds to a quantum efficiency of 43.8% at 5 V applied bias voltage. The transient photoresponse of the fabricated device is reported under different applied biases at 1 V, 3 V, and 5 V.     

Ti-Cu-N hard nanocomposite films prepared by pulse biased arc ion plating

In this work, Ti-Cu-N hard nanocomposite films were deposited on high-speed-steel (HSS) substrates using a TiCu (88:12 at.%) single multi-component target by pulse biased arc ion plating. The influence of pulse bias voltages was examined with regard to elemental composition, structure, morphology and mechanical properties of the films. The Cu atomic content of Ti-Cu-N films was determined by Electron Probe Micro-Analyzer (EPMA). The structure and morphology were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Hardness and film/substrate adhesion were determined by nanoindenter and scratch test, respectively. The results showed that the content of Cu appeared to be in the range of 1.75-4.5 at.%, depending on pulse bias voltages. The films exhibit a preferred orientation TiN (1 1 1) texture when the substrate bias voltages were −100 V and −300 V, while the preferred orientation change to be a preferred orientation TiN (2 2 0) one when the substrate bias voltages increase to −600 V and −900 V. And no obvious sign of metal copper phase was observed. The SEM morphologies showed some macroparticles (MPs) on the surface of the films and the relative content of the MPs decreased significantly when the substrate bias voltages increased from −100 to −900 V. The maximum value (74 N) of the film/substrate adhesion of the films was obtained when the substrate bias voltage was −600 V with Cu content of 1.75 at.%. Hardness enhancement was observed, the value of the hardness increased firstly and reached a maximum value of 31.5 GPa, corresponding to Cu content of 1.75 at.%, and then it decreased when the substrate bias voltage changed from −100 to −900 V. The hardness enhancement was discussed related to the concept for the design of hard materials.     

CuO薄膜的制备及其光伏特性

通过反应磁控溅射在n型硅和玻璃衬底上制备了p型CuO薄膜.使用X射线衍射仪和紫外-可见光-近红外光度计研究了p型CuO薄膜的结构和光学特性,得出其平均晶粒尺寸和光学带隙分别为8 nm和1.36 eV.通过研究其电压-电流关系确定了在p型CuO薄膜和n型硅衬底之间形成了p-n结.在AM 1.5光照条件下p-CuO/n-Si电池的开路电压为0.33 V,短路电流密度为6.27 mA/cm², 填充因数和能量转化效率分别为0.2和0.41%.