Hydrogen Sensors Based on AlGaN/AlN/GaN Schottky Diodes

Pt/AlGaN/AlN/GaN Schottky diodes are fabricated and characterized for hydrogen sensing. The Pt Schottky contact and the Ti/Al/Ni/Au ohmic contact are formed by evaporation. Both the forward and reverse currents of the device increase greatly when exposed to hydrogen gas. A shift of 0.3 V at 300 K is obtained at a fixed forward current after switching from N2 to 10%H2＋N2. The sensor responses under different concentrations from 50ppm H2 to 10%H2＋N2 at 373K are investigated. Time dependences of the device forward current at 0.5 V forward bias in N2 and air atmosphere at 300 and 373K are compared. Oxygen in air azcelerates the desorption of the hydrogen and the recovery of the sensor. Finally, the decrease of the Schottky barrier height and sensitivity Of the sensor are calculated.     

Effects of Different Plasma Energy Treatments on n-Type Al0.4Ga0.6N Material

El3ctronic properties, surface chemistry and surface morphology of plasma-treated n-Al0.4Ga0.6N material are studied by electrical contact measurements, atomic force microscopy and x-ray photoemission spectroscopy. Although excessive etching can cause the surface roughness to significantly increase, the nitrogen vacancies VN produced by the excessive etching can be compensated for by the negative effects of the rougher surface. Thus, VN produced by excessive etching plays a key role in Ohmic contact of high-A1 content AIGaN and it can reduce Ohmic contact resistance. The effect of rapid thermal annealing on the performance of n-Al0.4Ga0.6N can significantly reduce the etching damage caused by excessive etching.     

Electrical Characteristics of InGaN/AlGaN and InGaN/GaN MQW Near UV-LEDs

Electrical characteristics of In0.05 Ga0.95N/Al0.07Ga0.9aN and In0.05 Ga0.95N/GaN multiple quantum well （MQW） ultraviolet light-emltting diodes （UV-LEDs） at 400hm wavelength are measured. It is found that for InGaN/AlGaN MQW LEDs, both ideality factor and parallel resistance are similar to those of InGaN/GaN MQW LEDs, while series resistance is two times larger. It is suggested that the Al0.07Ga0.93N barrier layer did not change crystal quality and electrical characteristic of p-n junction either, but brought larger series resistance. As a result, InGaN/AlGaN MQW LEDs suffer more serious thermal dissipation problem although they show higher light output efficiency.     

Photoreflectance investigations of AlGaN/GaN heterostructures with a two dimensional electron gas

Undoped AlGaN/GaN heterostructures with different content and thickness of AlGaN layer are investigated by photoreflectance (PR) spectroscopy. We have observed PR resonances related to an absorption in both GaN and AlGaN layers. The character of these resonances has been analyzed, and PR lines associated with excitonic and band-to-band absorption in the GaN layer and band-to-band absorption in the AlGaN layer have been identified. The transition related to band-to-band absorption possesses characteristic Franz–Keldysh oscillations (FKOs) associated with a built-in electric field. The electric field in the AlGaN layer obtained on the basis of the analysis of FKOs has been found to be in the range of 244–341 kV/cm. The value of the field has been found to decrease with the increase in AlGaN thickness and to increase with the increase in Al concentration. The surface potential for AlGaN layers has been found to increase with the increase in Al mole fraction and has been estimated to be in the range of 1.0–1.7 eV.     

High-Temperature Characteristics of Ti/Al/Ni/Au Ohmic Contacts to n-GaN

We present the high-temperature characteristics of Ti/Al/Ni/Au（15 nm/220 nm/40 nm/50 nm） multiplayer contacts to n-type GaN （Nd = 3.7 × 10^17 cm^-3, Nd = 3.0 × 10^18 cm^-3）. The contact resistivity increases with the measurement temperature. Furthermore, the increasing tendency is related to doping concentration. The higher the doped, the slower the contact resistivity with decreasing measurement temperature. Ti/Al/Ni/Au ohmic contact to heavy doping n-GaN takes on better high temperature reliability. According to the analyses of XRD and AES for the n-GaN/Ti/Al/Ni/Au, the Au atoms permeate through the Ni layer which is not thick enough into the AI layer even the Ti layer.     

The effects of annealing on Au/pyronine-B/MD n-InP Schottky structure

Thin film of non-polymeric organic compound pyronine-B has been fabricated on moderately doped (MD) n-InP substrate as an interfacial layer using spin coating technique for the electronic modification of Au/MD n-InP Schottky contact. The electrical characteristics have been determined at room temperature. The barrier height and the ideality factor values for Au/pyronine-B/MD n-InP Schottky diode have been obtained from the forward bias I-V characteristics at room temperature as 0.60 eV and 1.041; 0.571 and 1.253 eV after annealing at 100 and 250 °C, respectively. An increase in annealing temperature at the Au/n-InP Schottky junction is shown to increase the reverse bias leakage current by about one order of magnitude and decrease the Schottky barrier height by 0.027 eV. Furthermore, the barrier height values for the Au/pyronine-B/MD n-InP Schottky diode have also been obtained from the C-V characteristics at room temperature as 1.001 and 0.709 eV after annealing at 100 and 250 °C, respectively. Finally, it was seen that the diode parameters changed with increase in the annealing temperature.     

Growth of AlGaN Epitaxial Film with Chemical Vapour High A1 Content by Metalorganic Deposition

A high-Al-content AlGaN epilayer is grown on a low-temperature-deposited AlN buffer on （0001） sapphire by low pressure metalorganic chemical vapour deposition. The dependence of surface roughness, tilted mosaicity, and twisted mosaicity on the conditions of the AlGaN epilayer deposition is evaluated. An AlGaN epilayer with favourable surface morphology and crystal quality is deposited on a 2Onto low-temperature-deposited AlN buffer at a low V/Ⅲ flow ratio of 783 and at a low reactor pressure of 100 Torr, and the adduct reaction between trimethylaluminium and NH3 is considered.     

Recombination in tensile-strained In0.3Ga0.7As quantum well on InP

Photoluminescence (PL) measurements under different excitation powers were carried out at low temperature on tensile-strained In_0.3Ga_0.7As single wells of 6 nm with InGaAs barriers lattice matched to InP substrate. PL measurements taken at 2 K show a main emission band at 0.762 eV probably originating from a type-II transition. The insertion of an ultrathin InAs layer at In_0.3Ga_0.7As on In_0.53Ga_0.47As interface reveals an additional feature at 0.711 eV as well as an excited-state luminescence emission at high pump powers. The InAs insertion improves heterointerface quality, which was confirmed by an increase in PL intensity.     

Evolution of Ge and SiGe Quantum Dots under Excimer Laser Annealing

We present different relaxation mechanisms of Ge and SiGe quantum dots under excimer laser annealing. Investigation of the coarsening and relaxation of the dots shows that the strain in Ge dots on Ge films is relaxed by dislocation since there is no interface between the Ge dots and the Ge layer, while the SiGe dots on Si0.77Ge0.23 film relax by lattice distortion to coherent dots, which results from the obvious interface between the SiGe dots and the Si0.77Ge0.23 film. The results are suggested and sustained by Vanderbilt and Wickham＇s theory, and also demonstrate that no bulk diffusion occurs during the excimer laser annealing.     

Surface and interface electronic properties of AlGaN(0001) epitaxial layers

AlGaN layers with Al content varying over the whole range of compositions were grown by molecular beam epitaxy (MBE) on n-6H-SiC substrates. The band gap energy is obtained from the vanishing of Fabry–Pérot oscillations in a fit to optical reflection spectra near the band gap absorption edge. The surface potential was determined by in-situ X-ray photoemission spectroscopy (XPS) and is found to increase as a function of the Al content from (0.5±0.1) eV to (1.3±0.1) eV, from GaN to AlN. A Si₃N₄ thin passivation layer was formed in-situ onto a 2DEG AlGaN/GaN structure. The mechanism underlying the passivation of high electron mobility transistor (HEMT) structures is suggested to be based on the formation of interface states, which keep the Fermi level fixed at a position close to that of the free AlGaN surface. PACS 73.20.-r; 73.40.-c; 73.40.Kp     

Oxygen Recovery in Hf Oxide Films Fabricated by Sputtering

The chemical structure of ultrathin Hf oxide films （〈 10 nm） fabricated by a standard sputtering method is investigated using x-ray spectroscopy and Rutherford backseattering spectroscopy. According to the experiments, oxygen species are impacted to the HfO2/Si interface during the initial sputtering, and then released back to the upper HfO2 region driven by the oxygen concentration grads. A vacuum annealing can greatly enhance this recovery process. Additionally, significant SiO2 reduction in the interface is observed after the vacuum annealing for the thick HfO2 films in our experiment. It might be an effective method to confine the interracial layer thickness by sputtering thick HfO2 in no-oxygen ambient.     

InGaN/GaN p-i-n Photodiodes Fabricated with Mg-Doped p-InGaN Layer

Mg-doped p-InGaN layers with In composition of about 10% are grown by metalorganic chemical vapor deposition (MOCVD). The effect of the annealing temperature on the p-type behavior of Mg-doped InGaN is studied. It is found that the hole concentration in p-InGaN increases with a rising annealing temperature in the range of 600-850°C, while the hole mobility remains nearly unchanged until the annealing temperature increases up to 750°C, after which it decreases. On the basis of conductive p-type InGaN growth, the p-In_0.1Ga_0.9N/i-In_0.1Ga_0.9N/n-GaN junction structure is grown and fabricated into photodiodes. The spectral responsivity of the InGaN/GaN p-i-n photodiodes shows that the peak responsivity at zero bias is in the wavelength range 350-400nm.     

Band Gap Energies and Refractive Indices of Epitaxial Pb1-xSrxTe Thin Films

Pb1-x Srx Te thin films with different strontium （St） compositions axe grown on BaF2 （111） substrates by molecular beam epitaxy （MBE）. Using high resolution x-ray diffraction （HPLXRD）, we obtain Pb1-xSrxTe lattice constants, which vary in the range 6.462-6.492 A. According to the Vegard law and HRXRD data, Sr compositions in Pb1-xSrxTe thin films range from 0.0-8.0%. The Pb1-xSrxTe refractive index dispersions are attained from infrared transmission spectrum characterized by Fourier transform infrared （FTIR） transmission spectroscopy. It is found that refractive index decreases while Sr content increases in Pb1-xSrx Te. We also simulate the Pb1-xSrxTe transmission spectra theoretically to obtain the optical band gap energies which range between 0.320 e V and 0.449 e V. The simulated results are in good agreement with the FTIR data. Finally, we determine the relation between Pb1-xSrx Te band gap energies and Sr compositions （Eg = 0.320＋0.510x-0.930x^2 ＋184x^3 （eV））.     

Ferroelectric Properties of Bi3.25La0.75Ti3O12 Thin Films Crystallized inDifferent N2/O2 Ambients

Bi3.25La0.75 Ti3O12 （BLT） ferroelectric thin films are deposited by sol-gel method and annealed for crystallizaion in total l eccm N2/02 mixed gas with various ratio at 750℃ for 30rain. The effect of crystallization ambient on the structural and ferroelectric properties of the BLT films is studied. The growth direction and grain size of BLT film are revealed to affect ferroeleetric properties. Alter the BLT film is annealed in 20%O2, the largest P~ value is obtained, which is ascribed to an increase of random orientation and large grain size. The fatigue property is improved with the concentration of oxygen in the ambient increasing, which is ascribed to annealing in the ambient with high concentrated oxygen adequately decreasing the defects related to lack of oxygen.     

Measurement and Analysis of Composition and Depth Profile of H in Amorphous Si1-xCx:H Films

Composition in amorphous Si1-xCx：H heteroepitaxial thin films on Si （100） by plasma enhanced chemical vapour deposition （PECVD） is analysed. The unknown x （0.45-3.57） and the depth profile of hydrogen in the thin films are characterized by Rutherford backscattering spectrum （RBS）, resonance-nuclear reaction analysis （R-NRA） and elastic recoil detection （ERD）, respectively. In addition, the depth profile of hydrogen in the unannealed thin films is compared to that of the annealed thin films with rapid thermal annealing （RTA） or laser spike annealing （LSA） in nitrogen atmosphere. The results indicate that the stoichiometric amorphous SiC can be produced by PECVD when the ratio of CH4/SiH4 is approximately equal to 25. The content of hydrogen decreases suddenly from 35% to 1% after 1150℃ annealing. RTA can reduce hydrogen in SiC films effectively than LSA.     

Ni/Au Schottky gate oxidation and BCB passivation for high-breakdown-voltage AlGaN/GaN HEMT

A new AlGaN/GaN high electron mobility transistor (HEMT) employing Ni/Au Schottky gate oxidation and benzocyclobutene (BCB) passivation is fabricated in order to increase a breakdown voltage and forward drain current. The Ni/Au Schottky gate metal with a thickness of 50/300 nm is oxidized under oxygen ambient at 500 C and the highly resistive NiO is formed at the gate edge. The leakage current of AlGaN/GaN HEMTs is decreased from 4.94 μA to 3.34 nA due to the formation of NiO. The BCB, which has a low dielectric constant, successfully passivates AlGaN/GaN HEMTs by suppressing electron injection into surface states. The BCB passivation layer has a low capacitance, so BCB passivation increases the switching speed of AlGaN/GaN HEMTs compared with silicon nitride passivation, which has a high dielectric constant. The forward drain current of a BCB-passivated device is 199 mA /mm, while that of an unpassivated device is 172 mA /mm due to the increase in two-dimensional electron gas (2DEG) charge.     

Influence of thermal stress on the characteristic parameters of AlGaN/GaN heterostructure Schottky contacts

Ni Schottky contacts on AlGaN/GaN heterostructures have been fabricated. The samples are then thermally treated in a furnace with N₂ ambient at 600 ^circC for different times (0.5, 4.5, 10.5, 18, 33, 48 and 72 h). Current-voltage (I-V) and capacitance-voltage (C-V) relationships are measured, and Schrödinger's and Poisson's equations are self-consistently solved to obtain the characteristic parameters related to AlGaN/GaN heterostructure Schottky contacts: the two-dimensional electron gas (2DEG) sheet density, the polarization sheet charge density, the 2DEG distribution in the triangle quantum well and the Schottky barrier height for each thermal stressing time. Most of the above parameters reduce with the increase of stressing time, only the parameter of the average distance of the 2DEG from the AlGaN/GaN interface increases with the increase of thermal stressing time. The changes of the characteristic parameters can be divided into two stages. In the first stage the strain in the AlGaN barrier layer is present. In this stage the characteristic parameters change rapidly compared with those in the second stage in which the AlGaN barrier layer is relaxed and no strain is present.     

Structural and Magnetic Properties of [Fe/Ni]N Multilayers

[Fe/Ni]N multilayered structure grows epitaxially on the single crystalline MgO substrate. Due to the different directions of magnetic easy axes of Fe and Ni and the strong strain, large anisotropy dispersion is assumed. According to the layer model, the magnetization of Fe and Ni layers cannot follow each easy axis because of exchange coupling, and then the anisotropies are averaged out. The reduction of the effective anisotropy enhances with the decrease of periodic thickness. Thus, the coercivity of [Fe/Ni]N multilayers reduces with decreasing periodic thickness.     

Effect of CO on Characteristics of AlGaN/GaN Schottky Diode

Pt Schottky diode gas sensors for CO are fabricated using A1GaN/GaN high electron mobility transistor（HEMTs）structure. The diodes show a remarkable sensor signal （3 mA, in N2, 2mA in air ambient） biased 2 V after 1% CO is introduced at 50℃. The Schottky barrier heights decrease for 36meV and 27meV in the two cases respectively. The devices exhibit a slow recovery characteristic in air ambient but almost none in the background of pure N2, which reveals that oxygen molecules could accelerate the desorption of CO and offer restrictions to CO detection.     

AlGaN-Based Solar-Blind Schottky Photodetectors Fabricated on AlN/Sapphire Template

We report AlGaN-based back-illuminated solar-blind Schottky-type ultraviolet photodetectors with the cutoff- wavelength from 280nm to 292nm without bias. The devices show low dark current of 2.1× 10^-6A/cm^2 at the reverse bias of 5 V. The specific detectivity D＊ is estimated to be 3.3 × 10^12cmHz^1/2 W^-1 . To guarantee the performance of the photodetectors, the optimization of AlGaN growth and annealing condition for Schottky contacts were performed. The results show that high-temperature annealing method for Ni/Pt Schottky contacts is effective for the reduction of leakage current.