Effect of Si doping on optical and thermal properties of MOVPE GaN thin layers

Photothermal deflection spectroscopy is used to investigate thermal and optical properties of MOVPE grown GaN thin layers deposited on sapphire substrate. The effects of Si doping on absorption spectrum and gap energy are revealed. Also, doping-induced free carrier absorption is extracted from absorption in the sub-gap region. Moreover, the variations of photothermal signal versus modulation frequency are used to determine thermal properties of these films. The measured thermal conductivity is clearly decreased by Si doping, the main reason should be the phonon scattering on point defects.     

Comparison of electrical characteristic between AIN/GaN and AIGaN/GaN heterostructure Schottky diodes

Ni/Au Schottky contacts on A1N/GaN and A1GaN/GaN heterostructures are fabricated. Based on the measured current-voltage and capacitance-voltage curves, the electrical characteristics of AlN/GaN Schottky diode, such as Schottky barrier height, turn-on voltage, reverse breakdown voltage, ideal factor, and the current-transport mechanism, are analyzed and then compared with those of an A1GaN/GaN diode by self-consistently solving Schrodinger＇s and Poisson＇s equations. It is found that the dislocation-governed tunneling is dominant for both AlN/GaN and AlGaN/GaN Schottky diodes. However, more dislocation defects and a thinner barrier layer for AlN/GaN heterostrncture results in a larger tunneling probability, and causes a larger leakage current and lower reverse breakdown voltage, even though the Schottky barrier height of AlN/GaN Schottky diode is calculated to be higher that of an A1GaN/GaN diode.     

Comparison of electrical characteristic between AlN/GaN and AlGaN/GaN heterostructure Schottky diodes

Ni/Au Schottky contacts on AlN/GaN and AlGaN/GaN heterostructures are fabricated.Based on the measured current–voltage and capacitance–voltage curves,the electrical characteristics of AlN/GaN Schottky diode,such as Schottky barrier height,turn-on voltage,reverse breakdown voltage,ideal factor,and the current-transport mechanism,are analyzed and then compared with those of an AlGaN/GaN diode by self-consistently solving Schrdinger’s and Poisson’s equations.It is found that the dislocation-governed tunneling is dominant for both AlN/GaN and AlGaN/GaN Schottky diodes.However,more dislocation defects and a thinner barrier layer for AlN/GaN heterostructure results in a larger tunneling probability,and causes a larger leakage current and lower reverse breakdown voltage,even though the Schottky barrier height of AlN/GaN Schottky diode is calculated to be higher that of an AlGaN/GaN diode.     

Influence of defects on the optical and structural properties of Ge dots embedded in an Si/Ge superlattice

In this work we studied the influence of high-energy proton irradiation on the optical and structural properties of an Si/Ge superlattice (SL) with embedded Ge quantum dots (QDs). The presence of QDs in the as-grown samples was established by transmission electron microscopy and photoluminescence (PL). The samples were irradiated with 2.0 MeV protons to fluences in the range 2×10¹²-2×10¹⁴ cm^-2. The structural characterization made by X-ray reciprocal space mapping, X-ray reflection and Rutherford backscattering/channelling has shown no changes in the as-grown heterostructure due to the irradiation. In spite of the expected high concentration of nonradiative recombination centres caused by the proton-induced damage, the PL emission from the Ge dots has been observed even for the highest irradiation fluence. The studied QD-in-SL structure has shown an extraordinarily high radiation hardness when compared with previously studied QD heterostructures.     

Dielectric properties of nano Si/C/N composite powder and nano SiC powder at high frequencies

The dielectric properties of nano Si/C/N composite powder and nano SiC powder at high frequencies have been studied. The nano Si/C/N composite powder and nano SiC powder were synthesized from hexamethyldisilazane ((Me₃Si)₂NH) (Me:CH₃) and SiH₄–C₂H₂, respectively, by a laser-induced gas-phase reaction. The complex permittivities of the nano Si/C/N composite powder and nano SiC powder were measured at a frequency range of 8.2–12.4 GHz. The real part (′) and imaginary part (″) of the complex permittivity, and dissipation factor (tg δ=″/′) of nano Si/C/N composite powder are much higher than those of nano SiC powder and bulk SiC, Si₃N₄, SiO₂, and Si, especially the tg δ. The promising features of nano Si/C/N composite powder would be due to more complicated Si, C, and N atomic chemical environment than in a mixture of pure SiC and Si₃N₄ phase. The charged defects and quasi-free electrons moved in response to the electric field, diffusion or polarization current resulted from the field propagation. Because there exists graphite in the nano Si/C/N composite powder, some charge carries are related to the sp³ dangling bonds (of silicon and carbon) and unsaturated sp² carbons. The high ″ and tg δ of nano Si/C/N composite powder were due to the dielectric relaxation. The nano Si/C/N composite powder would be a good candidate for electromagnetic interface shielding material.     

Near-field optical characterization of GaN and InxGa1−xN/GaN heterostructures grown on freestanding GaN substrates

Using near-field scanning optical microscopy (NSOM), we report the spatial distribution of photoluminescence (PL) intensity in III-nitride-based semiconductor layers grown on GaN substrates. Undoped GaN, In_0.11Ga_0.89N, and In_0.13Ga_0.87N/GaN multi-quantum wells (MQWs) were grown by metal organic chemical vapor deposition (MOCVD) on freestanding GaN substrates. Micro-Raman spectroscopy has been used to evaluate the crystalline properties of the GaN homoepitaxial layers. The variation of the PL intensity from the NSOM imaging indicates that the external PL efficiency fluctuates from 20% to 40% in the 200 nm InGaN single layer on freestanding GaN, whereas it fluctuates from 20% to 60% in InGaN/GaN MQWs. In the NSOM-PL images, bright island-like features are observed. After deconvolution with the spatial resolution of the NSOM, the size of these features is estimated to be in the range of 150–250 nm.     

Structure and optoelectronic properties of Si/O superlattice

We have carried out structural study of the Si/O semiconductor atomic superlattices (SAS) with up to 18 Si/O layers fabricated by molecular beam epitaxy and in situ oxygen exposure on both Sb-doped and undoped Si buffer layers, and correlated the results with our photoluminescence, electroluminescence (EL) and I–V data. The Si/O SAS is a new type of superlattice, where monolayers of oxygen are sandwiched between the Si layers. High-resolution cross-sectional transmission electron microscopy (TEM) study has confirmed the presence of the superlattice and shown epitaxy in the Si/O superlattices. The high structural quality of the layers grown on the undoped Si buffer layers with low density of stacking faults—less than 10⁷/cm²—was established by TEM. Although structure perfection is very important allowing this new class of superlattices to be extended to other systems, it is important to point out that a 9-period SAS-based EL device with emission of light in green has been life-tested with stable output for over 1 year of continuous operation. The Si/O superlattice also serves as an epitaxially grown insulating layer as possible replacement of silicon-on-insulator. Together with the tailor-made effective band gap, this epitaxially grown superlattice may serve as future silicon-based three-dimensional integrated circuits.     

Micro-structural and temperature dependent electrical characterization of Ni/GaN Schottky barrier diodes

Micro-structural investigation of Ni/GaN Schottky barrier diodes has been carried out using high-resolution transmission electron microscopy and electron diffraction spectrum in order to emphasize the role of Ni/GaN interface in controlling the Schottky diode behavior. Variable temperature Hall effect measurement of GaN samples along with the current–voltage (I–V) characteristics of Ni/n-GaN Schottky barrier diodes have been measured in 100–380 K temperature range. Results are analyzed in terms of thermionic emission theory by incorporating the concept of barrier inhomogeneity at the metal/semiconductor interface. The observed anomaly of temperature dependence of Schottky barrier height and ideality factor are explained by invoking two sets of Gaussian distribution of SBH in the temperature ranges of 100–180 K and 220–380 K, respectively. The value of A** (effective Richardson constant) as determined from the modified Richardson plot is 29.2 A/(cm² K²), which shows an excellent agreement with the theoretical value (26.4 A/(cm² K²)) in the temperature range of 220–380 K.     

Effect of heating on electrical transport in AlGaN/GaN Schottky barrier diodes on Si substrate

Thermal properties of the AlGaN/GaN Schottky barrier diodes were investigated, using a pulsed-IV measurement technique. The thermally degraded mobility in the DC-bias configuration was restored, when the pulse-bias voltages were applied. It was observed that heat generation was minimized, using a pulse width of 500 ns and pulse period of 10 ms. For the SBDs consisting of 5 μm of anode–cathode distance, on-resistance measured by the pulse-IV and DC-IV were 1.6 and 6.2 Ω-mm, respectively. We also demonstrated the device-width dependence of the thermal properties of the SBDs. We found that the performance of the power devices can be greatly influenced by the heat generation.     

Influence of p-GaN annealing on the optical and electrical properties of InGaN/GaN MQW LEDs

Optical and electrical properties of InGaN/GaN multiple quantum wells (MQWs) light emitting diodes (LEDs) annealed in pure O₂ ambient (500 °C) and pure N₂ ambient (800 °C) were systematically investigated. The temperature-dependent photoluminescence measurements showed that high-temperature thermal annealing in N₂ ambient can induce indium clusters in InGaN MQWs. Although the deep traps induced by indium clusters can act as localized centers for carriers, there are many more dislocations out of the trap centers due to high-temperature annealing. As a result, the radiative efficiency of the sample annealed in N₂ ambient was lower than that annealed in O₂ ambient at room temperature. Electrical measurements demonstrated that the LEDs annealed in O₂ ambient were featured by a lower forward voltage and there was an increase of ~41% in wall-plug efficiency at 20 mA in comparison with the LEDs annealed in N₂ ambient. It is thus concluded that activation of the Mg-doped p-GaN layer should be carried out at a low-temperature O₂ ambient so as to obtain LEDs with better performance.     

Comparative study of electrical properties of Au/n-Si (MS) and Au/Si3N4/n-Si (MIS) Schottky diodes

In this paper, the electrical parameters of Au/n-Si (MS) and Au/Si₃N₄/n-Si (MIS) Schottky diodes are obtained from the forward bias current-voltage (I-V) and capacitance-voltage (C-V) measurements at room temperature. Experimental results show that the rectifying ratios of MS and MIS diode at ± 5 V are found to be 1.25×10³ and 1.27×10⁴, respectively. The main electrical parameters of MS and MIS diode, such as the zero-bias barrier height (Φ _Bo) and ideality factor (n) are calculated to be 0.51 eV (I-V), 0.53 eV (C-V), and 4.43, and 0.65 eV (I-V), 0.70 eV (C-V), and 3.44, respectively. Also, the energy density distribution profile of the interface states (N_ss) is obtained from the forward bias I-V. In addition, the values of series resistance (R_s) for the two diodes are calculated from Cheung's method and Ohm's law.     

Investigation of optical properties of modulation doped GaN/AlGaN MQWS nanostructures

Due to many important applications, the group III-nitride semiconductors have recently attracted remarkable attention among the semiconductor researchers and engineers. In this paper, we report on the impact of the extrinsic and temporal carriers on the screening of the polarization internal fields. The optical efficiency of GaN/AlGaN multiple quantum well (MQW) nanostructures were studied by means of photoluminescence (PL) and time-resolved PL measurements. Extrinsic carriers come from Si doping in the barriers while temporal carriers originate when the samples are excited by the laser beam. The emission peaks of MQWs in PL spectra of the undoped and low-doped samples show a shift towards higher energy levels as excitation intensity increases, while the other samples do not exhibit such a phenomenon due to the dominance of the extrinsic carriers. The transient data confirm the results of the PL measurements.     

Direct-current and radio-frequency characteristics of passivated AlGaN/GaN/Si high electron mobility transistors

AlGaN/GaN/Si HEMTs grown by molecular beam epitaxy have been investigated using spectroscopy capacitance, direct and pulse current–voltage and small-signal microwave measurements. Passivation of the HEMT devices by SiO₂/SiN with NH₃ and N₂O pretreatments is made in order to reduce the trapping effects. As has been found from DLTS data, some of electron traps are eliminated after passivation. This has led to an improvement in the drain current. To describe the electron transport, we have developed a charge-control model by including the deep traps observed from DLTS experiments. The thermal and trapping effects have been, on the other hand, studied from a comparison between direct-current and pulsed conditions. As a result, a gate-lag and a drain-lag were revealed indicating the presence of deep lying centers in the gate-drain spacing. Finally, small-signal microwave results have shown that the radio-frequency parameters of the AlGaN/GaN/Si transistors are improved by SiO₂/SiN passivation and more increasingly with N₂O pretreatment.     

Effect of doping on structural,optical and electrical properties of nanostructure ZnO films deposited onto a-Si:H/Si heterojunction

We investigated the structural; optical and electrical properties of ZnO thin films as the n-type semiconductor for silicon a-Si:H/Si heterojunction photodiodes. The ZnO film forms the front contact of the super-strata solar cell and has to exhibit good electrical (high conductivity) and optical (high transmittance) properties. In this paper we focused our attention on the influence of doping on device performance. The results show that the X-ray diffraction (XRD) spectra revealed a preferred orientation of the crystallites along c-axis. SEM images show that all films display a granular, polycrystalline morphology and the ZnO:Al exhibits a better grain uniformity. The transmittance of the doped films was found to be higher when compared to undoped ZnO. A low resistivity of the order of 2.8 × 10⁻⁴ Ω cm is obtained for ZnO:Al using 0.4 M concentration of zinc acetate. The photoluminescence (PL) spectra exhibit a blue band with two peaks centered at 442 nm (2.80 eV) and 490 nm (2.53 eV). It is noted that after doping the ZnO films a shift of the band by 22 nm (0.15 eV) is recorded and a high luminescence occurs when using Al as a dopant. Dark I–V curves of ZnO/a-Si:H/Si structure showed large difference, which means there is a kind of barrier to current flow between ZnO and a-Si:H layer. Doping films was applied and the turn-on voltages are around 0.6 V. Under reverse bias, the current of the ZnO/a-Si:H/Si heterojunction is larger than that of ZnO:Al/a-Si:H/Si. The improvement with ZnO:Al is attributed to a higher number of generated carriers in the nanostructure (due to the higher transmittance and a higher luminescence) that increases the probability of collisions.     

Structural and local electrical properties of AlInN/AlN/GaN heterostructures

GaN layers and Al_1−xIn_xN/AlN/GaN heterostructures have been studied by scanning probe microscopy methods. Threading dislocations (TDs), originating from the GaN (0 0 0 1) layer grown on sapphire, have been investigated. Using Current-Atomic Force Microscopy (C-AFM) TDs have been found to be highly conductive in both GaN and AlInN, while using semi-contact AFM (phase-imaging mode) indium segregation has been traced at TDs in AlInN/AlN/GaN heterostructures. It has been assessed that In segregation is responsible for high conductivity at dislocations in the examined heterostructures.     

表面钝化前后Al0.22Ga0.78N/GaN异质结势垒层应变的高温特性

用高分辨X射线衍射仪(HRXRD)研究了表面钝化前后Al_0.22Ga_0.78N/ GaN异质结势垒层应变的高温特性，温度变化范围从室温到813K.结果表明，对未钝化的异质 结，当测试温度高于523K时，Al_0.22Ga_0.78N势垒层开始出现应变 弛豫；钝化后，在Al_0.22Ga_0.78N势垒层中会产生一个附加的平面 拉伸应变，并随着温度的增加，势垒层中的平面拉伸应变会呈现出一个初始的增加，接着应 变将减小，对100nm厚的Al_0.22Ga_0.78N势垒层，应变只是轻微地减 小，但对于50nm厚的Al_0.22Ga_0.78N势垒层，则出现了严重的应变 弛豫现象. 关键词： 0.22Ga_0.78N/GaN异质结')" href="#">Al_0.22Ga_0.78N/GaN异质结 应变 3 N₄钝化')" href="#">Si₃ N₄钝化 高温XRD     

Structural,optical, and electrical properties of boron-doped ZnO1-xSx thin films deposited by MOCVD

Boron-doped ZnO_1-xS_x (ZnO_1-xS_x:B) thin films were fabricated by metalorganic chemical vapor deposition (MOCVD). We investigated the structural, optical, and electrical properties of the ZnO_1-xS_x:B thin films. X-ray diffraction patterns showed that, except for the ZnO:B (x?=?0) and ZnS:B (x?=?1) thin films, the ZnO_1-xS_x:B thin films exhibit amorphous characters. Optical transmittance spectra were analyzed to estimate the band gaps of the thin films with different S content. All thin films showed direct band gaps ranging from 3.34?eV (ZnO:B) to 3.49?eV (ZnS:B). The influence of sulfur content on carrier concentration, electrical resistivity, and Hall mobility of the ZnO_1-xS_x:B thin films were analyzed from Hall effect measurements measured at temperatures ranging from liquid nitrogen temperature to room temperature. The ZnO_1-xS_x:B thin films exhibited n-type electrical conductivity except for ZnS:B, which was not measurable in this study due to its high resistivity (>100?Ω?cm).     

高温AlN插入层对AlGaN/GaN异质结材料和HEMTs器件电学特性的影响

研究了在GaN缓冲层中插入40 nm厚高温AlN层的GaN外延层和AlGaN/GaN异质结材料, AlN插入层可以增加GaN层的面内压应力并提高AlGaN/GaN高电子迁移率晶体管（HEMTs）的电学特性. 在精确测量布拉格衍射角的基础上定量计算了压应力的大小. 增加的压应力一方面通过增强GaN层的压电极化电场, 提高了AlGaN/GaN异质结二维电子气（2DEG）面密度, 另一方面使AlGaN势垒层对2DEG面密度产生的两方面影响相互抵消. 同时, 这种AlN插入层的采用降低了GaN与AlGaN层之间的 关键词： 高温AlN插入层 AlGaN/GaN异质结 二维电子气 应力     

Improved device performance of recessed-gate AlGaN/GaN HEMTs by using in-situ N2O radical treatment

The N₂O radicals in-situ treatment on gate region has been employed to improve device performance of recessed-gate AlGaN/GaN high-electron-mobility transistors (HEMTs). The samples after gate recess etching were treated by N₂O radicals without physical bombardment. After in-situ treatment (IST) processing, the gate leakage currents decreased by more than one order of magnitude compared to the sample without IST. The fabricated HEMTs with the IST process show a low reverse gate current of 10^-9 A/mm, high on/off current ratio of 10⁸, and high f_T×L_g of 13.44 GHz· μm. A transmission electron microscope (TEM) imaging illustrates an oxide layer with a thickness of 1.8 nm exists at the AlGaN surface. X-ray photoelectron spectroscopy (XPS) measurement shows that the content of the Al-O and Ga-O bonds elevated after IST, indicating that the Al-N and Ga-N bonds on the AlGaN surface were broken and meanwhile the Al-O and Ga-O bonds formed. The oxide formed by a chemical reaction between radicals and the surface of the AlGaN barrier layer is responsible for improved device characteristics.