Mg-doped Films Research Articles

Large third-order optical nonlinearity of Mg-doped PbS/PVA freestanding nanocomposite films

Mg-doped PbS/PVA freestanding nanocomposite films were successfully prepared by a simple chemical route. X-ray diffraction patterns confirm the formation of all the films with cubic phase of PbS. Optical absorption spectra showed the blue shifted phenomena and the band gaps of Mg : PbS/PVA films increase with an increase in Mg concentration. The increase in resistivity with reducing particle size was examined for these films. The Z-scan results showed that the freestanding nanocomposite films exhibit the large nonlinear refractive index and absorption coefficients with negative sign and they are increased with Mg doping. Third-order nonlinear optical susceptibility and figure of merit were calculated and the maximum value is found to be about 3.405 × 10−5 esu and 2.067 × 10−9 esu m, respectively, for higher Mg-doped film which is three orders of magnitude larger than that observed in the substrate-supported films as reported earlier in the literature. The results show that the Mg : PbS/PVA films have great potential applications in future optical devices.

Effects of growth conditions on the acceptor activation of Mg-doped p-GaN

Mg-doped p-GaN films with various Mg flow rates were grown on a sapphire substrate by metal–organic chemical-vapor deposition (MOCVD) at a very low reactor pressure of 50mbar. The surface roughness of Rrms decreased from 23.4 to 0.72nm with a decrease in the Mg flow rate from 118nmolmin−1 to 30nmolmin−1. In addition, a high Mg activation efficiency of ∼5% was achieved by an optimized Mg flow rate. The photoluminescence (PL) and (002) X-ray diffraction (XRD) measurements indicated that the density of MgGa–VN complexes and screw-type dislocations in the p-GaN films were reduced under a low Mg flow rate. Fitting the variable-temperature Hall data indicated that the acceptor activation energy and the compensation ratio were ∼151meV and ∼10%, respectively. It is therefore believed that the decrease in the compensation effect, by decreasing the density of the compensating center such as MgGa–VN complex and/or screw dislocation, plays an important role in improving the Mg acceptor activation efficiency in the low pressure growth of p-GaN films.

Carrier transport properties of Mg-doped InAlN films

The carrier transport properties of 100 nm-thick Mg-doped InAlN films were investigated using a transmission line model. The electrical resistivity of InAlN:Mg films was 7400 Ωcm, indicating a semi-insulator. Temperature-dependent current-voltage measurements revealed that the predominant carrier transport is due to hopping conduction through deep-level defect states. The carrier transport at the metal contact/InAlN:Mg interface could be explained in terms of hopping conduction through deep-level states located 1.28 eV above the valence band.

Carrier recombination processes in Mg-doped N-polar InN films

We investigate the nonradiative carrier recombination (NR) process in Mg-doped p-InN films having lower photoluminescence (PL) intensity than n-InN films. The NR activation energy in the p-type films is found to be in a range of 9–15 meV, which is smaller than that in n-InN films (40–65 meV). We also investigate the effect of the greater mean free path of minority carriers in p-InN. At room temperature the collision rate of minority carriers with NR centers within the radiative lifetime in p-InN is found to be three orders of magnitude greater than that in n-InN.

Effects of hydrogen treatment on ohmic contacts to p-type GaN films

This study investigated the effects of hydrogen (H 2) treatment on metal contacts to Mg-doped p-GaN films by Hall-effect measurement, current–voltage ( I– V) analyzer and X-ray photoemission spectra (XPS). The interfacial oxide layer on the p-GaN surface was found to be the main reason for causing the nonlinear I– V behavior of the untreated p-GaN films. The increased nitrogen vacancy (V N) density due to increased GaN decomposition rate at high-temperature hydrogen treatment is believed to form high density surface states on the surface of p-GaN films. Compared to untreated p-GaN films, the surface Fermi level determined by the Ga 2p core-level peak on 1000 °C H 2-treated p-GaN films lies about ∼2.1 eV closer to the conduction band edge (i.e., the surface inverted to n-type behavior). The reduction in barrier height due to the high surface state density pinned the surface Fermi level close to the conduction band edge, and allowed the electrons to easily flow over the barrier from the metal into the p-GaN films. Thus, a good ohmic contact was achieved on the p-GaN films by the surface inversion method.

Tunable p-Type Conductivity and Transport Properties of AlN Nanowires via Mg Doping

Arrays of well-aligned AlN nanowires (NWs) with tunable p-type conductivity were synthesized on Si(111) substrates using bis(cyclopentadienyl)magnesium (Cp(2)Mg) vapor as a doping source by chemical vapor deposition. The Mg-doped AlN NWs are single-crystalline and grow along the [001] direction. Gate-voltage-dependent transport measurements on field-effect transistors constructed from individual NWs revealed the transition from n-type conductivity in the undoped AlN NWs to p-type conductivity in the Mg-doped NWs. By adjusting the doping gas flow rate (0-10 sccm), the conductivity of AlN NWs can be tuned over 7 orders of magnitude from (3.8-8.5) × 10(-6) Ω(-1) cm(-1) for the undoped sample to 15.6-24.4 Ω(-1) cm(-1) for the Mg-doped AlN NWs. Hole concentration as high as 4.7 × 10(19) cm(-3) was achieved for the heaviest doping. In addition, the maximum hole mobility (∼6.4 cm(2)/V s) in p-type AlN NWs is much higher than that of Mg-doped AlN films (∼1.0 cm(2)/V s). (2) The realization of p-type AlN NWs with tunable electrical transport properties may open great potential in developing practical nanodevices such as deep-UV light-emitting diodes and photodetectors.

Anomalous Hall mobility kink observed in Mg-doped InN: Demonstration of p-type conduction

The p-type conduction in Mg-doped InN film is identified by an anomalous Hall mobility kink observed at ∼600 K in temperature-dependent Hall-effect measurements. The good agreement between experimental results and ensemble Monte Carlo simulation confirms the p-type bulk conduction under the surface electron accumulation layer. Furthermore, it is found that there is an exponential relationship between the hole concentration in the p-type bulk layer and the reciprocal kink temperature, which provides an effective way to evaluate the hole concentration in Mg-doped InN bulk layer through Hall-effect measurements.

Low temperature dielectric characterization of Mg-doped SrTiO3 thin films prepared by sol-gel

Voltage dependence of dielectric constant ε′ of ferroelectrics and low dielectric loss tan δ of incipient ferroelectrics make them attractive for tuning elements in microwave circuits. In this letter, field dependence of the low-temperature dielectric permittivity and polarization of Mg-doped SrTiO3 films is studied. Incorporation of Mg on both Sr and Ti sites decreases the ε′ and relative tunability nr of sol-gel derived SrTiO3 films, whereas polarization is reduced by Ti site substitution only. tan δ of the studied films is ≤0.012, decreasing at low temperatures down to 0.001 when Ti is substituted by 5% of Mg.

Electrical and Structural Characterization of Epitaxial-Grown Mg-Doped C60 Thin Films

The epitaxial growth of Mg-doped C60 films has been investigated. It is found that the epitaxial growth of Mg-doped C60 film is enabled by using mica (001) substrate in the low Mg concentration region (Mg/C60 molar ratio < 1). The crystal quality of the epitaxial Mg-doped C60 film is improved drastically in compared with micro-crystalline film on glass substrate. Such drastic improvement of crystal quality in the epitaxial films results significant increase in conductivity. This result may indicate the significant increase of carrier mobility.

Structural characterization of Mg-doped C60 thin films

The epitaxial growth of Mg-doped C60 films has been investigated. It is found that the epitaxial growth of Mg-doped C60 film is enabled by using mica (001) substrate in the low Mg concentration region (Mg/C60 molar ratio < 1). The crystal quality of the epitaxial Mg-doped C60 film is improved drastically in compared with micro-crystalline film on glass substrate. Such drastic improvement of crystal quality in the epitaxial films results significant increase in conductivity. This result may indicate the significant increase of carrier mobility. In addition, the crystal structure change is observed in high Mg concentration region (Mg/C60 molar ratio > 2). The C60 polymer sheet formation is suggested.

THz Emission from InN

AbstractWe report the terahertz (THz) emission from the wurzite indium nitride (InN) films grown by molecular beam epitaxy (MBE). More than two orders of magnitude of THz power enhancement has been achieved from the InN film grown along the a-axis and magnesium (Mg) doped InN with a critical carrier concentration. The primary radiation mechanism of the a-plane InN film is found to be due to the acceleration of photoexcited carriers under the polarization-induced in-plane electric field perpendicular to the a-axis. Apparent azimuthal angle dependences of THz wave amplitude and the second harmonic generation are observed from a-plane InN. In the Mg-doped films, Mg as the acceptors compensate the native donors in the InN films and large band bending over a wider space-charge region causes the enhancement of THz emission power compared to the undoped InN.

Probing and modulating surface electron accumulation in InN by the electrolyte gated Hall effect

The surface contribution to the electrical transport properties of InN was directly measured and modulated by the electrolyte gated Hall effect. Undoped and Mg-doped films show different behaviors that can be effectively described by a multilayer model, taking into account the conduction contribution from both the surface and interface with the buffer layer. Gated photoluminescence experiments further show the surface accumulation layer enhances radiative electron-hole recombination in undoped InN.

Growth of free-standing highly luminescent undoped and Mg-doped GaN thick films with a columnar structure

Undoped and Mg-doped GaN thick films with a columnar structure have been grown on gold-covered fused silica substrates in a three-zone horizontal quartz-tube reactor. The undoped films were grown by a two-step chemical vapor deposition method using gallium and ammonium chloride as starting reagents and ammonia as carrier gas. The Mg-doped films were deposited by the same method, but with an initial Ga–Mg alloying step. These GaN thick films have a wurtzite structure and exhibit strong room-temperature luminescence with the characteristic GaN band-edge, and the Mg-related emissions for the doped films. Their optoelectronic properties are comparable to GaN films grown epitaxially with other more elaborate techniques. This growth technique can be used for inexpensive, large-area electroluminescent devices.

Dielectric properties of MgO-doped compositionally graded multilayer barium strontium titanate films

We have grown 5mol% MgO-doped multilayered Ba1−xSrxTiO3 (BST) films having a nominal thickness of 220nm with compositions of each layer as BST60/40, BST75/25, and BST90/10 (upgraded). We also fabricated undoped upgraded BST and uniform BST60/40 films for comparison. Results show that Mg-doping improves dielectric loss (tanδ=0.008) and yields better surface roughness (∼3.1nm) compared to undoped upgraded BST. Mg-doped films displayed excellent temperature stability with temperature coefficient of capacitances of −0.94 and 1.14ppt∕°C from 20to90°C and 20to−10°C, respectively. Mg doping resulted in a moderate dielectric tunability (29%) compared to undoped BST (65.5%) at 444kV∕cm.

A study on maskless selective growth of Mg-doping p-type GaAs using low-energy focused ion beam

Maskless selective growth of Mg-doped GaAs films was performed by using a low-energy (30–200 eV) Mg–Ga focused ion beam (FIB) under the ambient of As 4 molecular beam. Under suitable As 4 pressure, all samples showed p-type and the carrier concentrations were around 10 17 cm −3. We achieved very high sticking coefficient of Mg ( k Mg of nearly 10 −1) compared with the result obtained at Molecular beam epitaxy (MBE) growth ( k Mg of nearly 10 −5). A p–n junction was formed on n +-type GaAs substrate by maskless selective growth. The diode showed typical I– V characteristics and the quality factor ( n-value) was approximately 1.75 for the sample grown at 60 eV. These results indicate that this method would be suitable for making maskless selective micro-device fabrication.

ZnO Nanowire/p-GaN Heterojunction LEDs

AbstractThis article reports forward and reverse biased emission in vertical ZnO nanowire/p-GaN heterojunction light emitting diodes (LEDs) grown out of solution on Mg-doped p-GaN films. The electroluminescence spectra under forward and reverse bias are distinctly different. Forward bias showed two peaks centered around 390 nm and 585 nm, while reverse bias showed a single peak at 510 nm. Analysis of the current-voltage characteristics and electroluminescence spectra is presented to determine the transport mechanism and location of electron hole recombination. Reverse bias transport and luminescence are attributed to hot-hole injection from the ZnO nanowires into the GaN film through tunneling breakdown. Forward bias transport and luminescence are attributed to hole injection from the GaN into the ZnO and recombination at defect states inside the ZnO yielding distinct color variations between individual wires. Major resistive losses occurred in the GaN lateral thin film connecting to the vertical ZnO nanowires.

Effects of doping on the crystalline quality and composition distribution in InGaN/GaN structure grown by MOCVD

The effects of Si and Mg doping on the crystalline quality and In distribution in the InGaN films were studied by atomic force microscope (AFM), triple crystal X-ray diffraction (TCXRD) and Rutherford backscattering spectrometry (RBS). The undoped, Si-doped and Mg-doped InGaN films were grown by metalorganic chemical vapor deposition (MOCVD) on (0 0 0 1) sapphire substrates. The electronic concentration in the Si-doped InGaN is about 2×10 19 cm −3. It is found that the crystalline quality and In distribution in InGaN is slightly affected by the Si doping. In the Mg doped-case, the hole concentration is about 4×10 18 cm −3 after annealing treatment. The surface morphology and crystalline quality of the Mg-doped InGaN are deteriorated significantly compared with the undoped InGaN. The growth rate of Mg-doped InGaN is higher than the undoped InGaN. Mg doping enhances the In incorporation in the InGaN alloy. The increase in In composition in the growth direction is more severe than the undoped InGaN.

Preparation of transparent CuCrO2:Mg/ZnO p–n junctions by pulsed laser deposition

Transparent p–n heterojunctions composed of zinc oxide, copper–chromium oxide, and indium tin oxide (ITO) films were fabricated by pulsed laser deposition (PLD) on glass substrates at temperatures as low as 500 °C. The rectifying characteristics were observed in the current–voltage curves of the prepared junctions. Undoped and Mg-doped CuCrO2 films were examined for their suitability as the p-type semiconductor layer for these junctions. A p–n junction with an n+-ZnO/n-ZnO/p-CuCrO2:Mg/ITO/glass structure exhibited the highest conductivity among all the samples. This 0.4-μm-thick junction exhibited an optical transparency of greater than 80% in the visible range.

Electroluminescence from ZnO nanowires in n-ZnO film/ZnO nanowire array/p-GaN film heterojunction light-emitting diodes

ZnO nanowire-array-embedded n-ZnO∕p-GaN heterojunction light-emitting diodes were fabricated by growing Mg-doped p-GaN films, ZnO nanowire arrays, and polycrystalline n-ZnO films consecutively. Electroluminescence emission having the wavelength of 386nm was observed under forward bias in the heterojunction diodes and the UV-violet light was emerged from the ZnO nanowires. The heterojunction diode was thermal treated in hydrogen ambient to increase the electron injection rate from the n-ZnO films into the ZnO nanowires. High concentration of electrons supplied from the n-ZnO films activated the radiative recombination in the ZnO nanowires, i.e., increased the light-emitting efficiency of the heterojunction diode.

Fabrication of magnesium-doped gallium nitride nanorods and microphotoluminescence characteristics

High density magnesium (Mg)-doped gallium nitride (GaN) nanorods were fabricated by inductively coupled plasma reactive ion etching technique from the epitaxial film. Under the fixed Cl2∕Ar flow rate of 10∕25SCCM (SCCM denotes cubic centimeter per minute at STP) and inductively coupled plasma/bias power of 200∕200W, the nanorods were fabricated with a density of 108–1010cm2 and dimension of 20–100nm by varying the chamber pressure from 10to30mTorr. A large blueshift was observed in the photoluminescence (PL) peak energy of Mg-doped GaN nanorods under HeCd laser (325nm) excitation. The PL spectra of nanorods show a typical donor-acceptor-pair emission around 3.0eV with a large blueshift compared to the Mg-doped GaN film. The blueshift energy increases from 8to67meV as the excitation intensity varies from 12to56kW∕cm2. Possible reasons causing the power dependence of spectral shift in the PL emission energy are discussed.