Crystal orientation effects on electronic and optical properties of wurtzite ZnO/MgZnO quantum well lasers

Crystal orientation effects on electronic and optical properties of ZnO/MgZnO QW structures are investigated by taking into account the non-Markovian gain model with many-body effects. These results are compared with those for GaN-based QW structures. In a range of small crystal angles, ZnO/MgZnO QW structures have a lower internal field than GaN/AlGaN and InGaN/GaN QW structures. However, ZnO/MgZnO QW structures show a larger internal field than GaN-based QW structures at crystal angles near ${\theta =50^{\circ}}$ . The WZ ZnO/MgZnO QW structures are shown to have much larger optical gain than the GaN-based QW structures for small crystal angles. This is because WZ ZnO/MgZnO QW structures have larger matrix element and smaller effective masses than InGaN/GaN QW structures near the (0001) crystal orientation. On the other hand, in the case of the (10 ${\bar{1}}$ 0) crystal orientation, the optical gain of ZnO/MgZnO QW structures becomes smaller than that of InGaN/GaN QW structures due to the increase of the effective mass. In addition, the ZnO/MgZnO QW structures have a maximum in the optical gain near ${\theta =50^{\circ}}$ , which can be explained by the fact that the average hole effective mass increases although the matrix element at high carrier density is improved with increasing crystal angle.     

Properties of Si-doped GaN and AlGaN/GaN heterostructures grown by RF-MBE on high resistivity Fe-doped GaN

Silicon-doped GaN epilayers and AlGaN/GaN heterostructures were developed by nitrogen plasma-assisted molecular beam epitaxy on high resistivity iron-doped GaN (0001) templates and their properties were investigated by atomic force microscopy, x-ray diffraction and Hall effect measurements. In the case of high electron mobility transistors heterostructures, the AlN mole fraction and the thickness of the AlGaN barrier employed were in the range of from 0.17 to 0.36 and from 7.5 to 30 nm, respectively. All structures were capped with a 2 nm GaN layer.Despite the absence of Ga droplets formation on the surface, growth of both GaN and AlGaN by RF-MBE on the GaN (0001) surfaces followed a step-flow growth mode resulting in low surface roughness and very abrupt heterointerfaces, as revealed by XRD. Reciprocal space maps around the reciprocal space point reveal that the AlGaN barriers are fully coherent with the GaN layer.GaN layers, n-doped with silicon in the range from 10¹⁵ to 10¹⁹ cm⁻³ exhibited state of the art electrical properties, consistent with a low unintentional background doping level and low compensation ratio. The carrier concentration versus silicon cell temperatures followed an Arhenius behaviour in the whole investigated doping range. The degenerate 2DEG, at the AlGaN/GaN heteroiterfaces, exhibited high Hall mobilities reaching 1860 cm²/V s at 300 K and 10 220 cm²/V s at 77 K for a sheet carrier density of 9.6E12 cm⁻².The two dimensional degenerate electron gas concentration in the GaN capped AlGaN/GaN structures was also calculated by self-consistent solving the Schrödinger–Poisson equations. Comparison with the experimental measured values reveals a Fermi level pinning of the GaN (0001) surface at about 0.8 eV below the GaN conduction band.     

Ga-vacancy-induced room-temperature ferromagnetic and adjusted-band-gap behaviors in GaN nanoparticles

Room-temperature ferromagnetism has been found in Ga-deficient GaN grown using the direct reaction of Ga \(_{2}\) O \(_{3}\) powder with NH \(_{3}\) gas. The observed magnetism in GaN induced by Ga vacancies is investigated both experimentally and theoretically. First-principles calculations reveal that the spontaneous spin polarization is created by the 3.0  \(\mu _\mathrm{B}\) local moment for GaN and magnetism originates from the polarization of the unpaired 2 \(p\) electrons of N surrounding the Ga vacancy. At the same time, the band gap can be also adjusted by changing the Ga-vacancy concentration.     

Modeling of polarization effects on n-GaN/i-InGaN/p-Gan solar cells with ultrathin GaN interlayers

We report the numerical study of n-GaN/i-InGaN/p-GaN solar cells on Ga-face substrates with thin GaN interlayers present in the intrinsic InGaN region. These interlayers have recently been shown to significantly increase the crystal quality of thick InGaN layers $(>\!\!\!120\,\text{ nm})$ . We find that tunneling is efficient in n-i-p structures having interlayers $\le \! 1.5\,\text{ nm}$ thick if polarization charges are sufficiently screened. If left unscreened, the large polarization charges naturally formed at the heterointerfaces degrades n-i-p performance, at a given interlayer thickness, because polarization charges increase the distance that carriers must tunnel. Simulations identify favorable parameter ranges.     

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.     

The defect structure of AlGaN/GaN superlattices grown on sapphire by the MOCVD method

High-resolution x-ray diffractometry and electron microscopy are used to study the defect structure and relaxation mechanism of elastic stresses in AlGaN/GaN superlattices grown by the MOCVD method on sapphire covered with a preliminarily deposited GaN and AlGaN buffer layer. Based on an analysis of the half-widths of three-crystal scan modes of x-ray reflections measured in different diffraction geometries, the density of different dislocation families is determined. For all the dislocation families, the density is shown to increase with the Al concentration in the solid-solution layers and depend only weakly on the superlattice period. From the electron-microscopic patterns of planar and cross sections, the types of dislocations and their distribution in depth are determined. It is shown that, in addition to high-density vertical edge and screw dislocations, which nucleate in the buffer layer and propagate through the superlattice layers, there are sloped intergrowing dislocations with a large horizontal projection and bent mixed dislocations with a Burgers vector $\left\langle {11\overline 2 3} \right\rangle $ at the interface between individual superlattice layers. The former dislocations form at the interface between the buffer layer and the superlattice and remove misfit stresses between the buffer and the superlattice as a whole, and the latter dislocations favor partial relaxation of stresses between individual superlattice layers. In samples with a high Al concentration (greater than 0.4) in AlGaN layers, there are cracks surrounded by high-density chaotic horizontal dislocations.     

增强型AlGaN/GaN高电子迁移率晶体管高温退火研究

深入研究了两种增强型AlGaN/GaN高电子迁移率晶体管(HEMT)高温退火前后的直流特性变化.槽栅增强型AlGaN/GaN HEMT在500 ℃ N₂中退火5 min后,阈值电压由0.12 V正向移动到0.57 V,器件Schottky反向栅漏电流减小一个数量级.F注入增强型AlGaN/GaN HEMT在 400 ℃ N₂中退火2 min后,器件阈值电压由0.23 V负向移动到-0.69 V,栅泄漏电流明显增大.槽栅增强型器件退火过程中Schottky有效势垒     

Theoretical analytic model for RESURF AlGaN/GaN HEMTs

In this paper, we propose a two-dimensional(2D) analytic model for the channel potential and electric field distribution of the RESURF AlGaN/GaN high electron mobility transistors(HEMTs). The model is constructed by two-dimensional Poisson's equation with appropriate boundary conditions. In the RESURF AlGaN/GaN HEMTs, we utilize the RESURF effect generated by doped negative charge in the AlGaN layer and introduce new electric field peaks in the device channels,thus, homogenizing the distribution of electric field in channel and improving the breakdown voltage of the device. In order to reveal the influence of doped negative charge on the electric field distribution, we demonstrate in detail the influences of the charge doping density and doping position on the potential and electric field distribution of the RESURF AlGaN/GaN HEMTs with double low density drain(LDD). The validity of the model is verified by comparing the results obtained from the analytical model with the simulation results from the ISE software. This analysis method gives a physical insight into the mechanism of the AlGaN/GaN HEMTs and provides reference to modeling other AlGaN/GaN HEMTs device.     

Wavelength dispersive X-ray analysis and cathodoluminescence techniques for monitoring the chemical removal of AlInN on an N-face GaN surface

This paper shows the selective etching process of an AlInN sacrificial layer, lattice-matched to GaN, on N-face GaN by an aqueous solution of 1,2-diaminoethane. Using the wavelength dispersive X-ray (WDX) spectrometers on an electron probe micro-analyser, together with an optical spectrometer and silicon CCD array added to the light microscope, and sharing the same focus as the electron microscope, cathodoluminescence spectra are collected from exactly the same spot as sampled by the WDX spectrometers. This technique allows the compositional properties of the etched AlInN layer and the optical properties of the semiconductor layers underlying the sacrificial layer to be scrutinised, verifying the etching selectivity and the efficiency of the process.     

A computational study of nonparabolic conduction band effect on quantum wire transport (e.g. GaN)

We studied the physics insight the GaN (example) quantum wire FET transistors. The model is based on the four $\mathbf{k}{\cdot } \mathbf{p}$ k · p Kane band model. We have introduced closed compact model for the Einstein relation of the diffusivity to mobility ratio (DMR) in quantum wires. The model can be applied for both wide and narrow band gaps of nonparabolic conduction band dispersion. The model is related to the optical matrix elements between conduction and valence bands. We have used 1D electrostatic to model the electron density over the maximum energy point. We have studied the effects of gate-to-source and drain-to-source voltages on the DMR by calculating the electron density using flux theory. We observed that above the threshold the non-parabolic dispersion increases the DMR. Additionally, we have studied the nonparabolic effects on the Fermi level and found that for low doping concentrations, the nonparabolic effect must be considered and an accurate calculation for the optical matrix elements is needed.     

Effects of SiNx on two-dimensional electron gas and current collapse of AlGaN/GaN high electron mobility transistors

SiN_x is commonly used as a passivation material for AlGaN/GaN high electron mobility transistors (HEMTs). In this paper, the effects of SiN_x passivation film on both two-dimensional electron gas characteristics and current collapse of AlGaN/GaN HEMTs are investigated. The SiN_x films are deposited by high- and low-frequency plasma-enhanced chemical vapour deposition, and they display different strains on the AlGaN/GaN heterostructure, which can explain the experiment results.     

Breakdown mechanisms in AlGaN/GaN high electron mobility transistors with different GaN channel thickness values

In this paper,the off-state breakdown characteristics of two different AlGaN/GaN high electron mobility transistors（HEMTs）,featuring a 50-nm and a 150-nm GaN thick channel layer,respectively,are compared.The HEMT with a thick channel exhibits a little larger pinch-off drain current but significantly enhanced off-state breakdown voltage(SV_off).Device simulation indicates that thickening the channel increases the drain-induced barrier lowering（DIBL） but reduces the lateral electric field in the channel and buffer underneath the gate.The increase of BV_off in the thick channel device is due to the reduction of the electric field.These results demonstrate that it is necessary to select an appropriate channel thickness to balance DIBL and BV_off in AlGaN/GaN HEMTs.     

The reliability of AlGaN/GaN high electron mobility transistors under step-electrical stresses

In spite of their extraordinary performance, AlGaN/GaN high electron mobility transistors (HEMTs) still lack solid reliability. Devices under accelerated DC stress tests (off-state, V_DS =0 state, and on-state step-stress) are investigated to help us identify the degradation mechanisms of the AlGaN/GaN HEMTs. All our findings are consistent with the degradation mechanism based on crystallographic-defect formation due to the inverse piezoelectric effects in Ref. [1] (Joh J and del Alamo J A 2006 IEEE IDEM Tech. Digest p. 415). However, under the on-state condition, the devices are suffering from both inverse piezoelectric effects and hot electron effects, and so to improve the reliability of the devices both effects should be taken into consideration.     

Effect of guided resonance modes on emission from GaN core–shell nanorod arrays

We model the process of incoherent emission from \(\hbox {In}_{x}\hbox {Ga}_{1-x}\hbox {N/GaN}\) quantum wells in GaN core–shell nanorod arrays using finite-difference time-domain simulations. We find that high-intensity features in the emitted field correspond to guided resonance modes near the \(\varGamma \) -point of the photonic band structure. We identify one \(\varGamma \) -point mode whose electric field intensity profile is ideal for core–shell nanorod array geometries. Using this mode, we are able to simultaneously enhance the radiative recombination rate and extraction efficiency relative to an in-filled slab. We determine the conditions on radiative and nonradiative recombination rates for which the nanorod array has a higher internal and external quantum efficiency than a reference slab. We present one nanorod array geometry where the external quantum efficiency is enhanced up to a factor of 25.     

阶梯AlGaN外延新型Al_(0.25)Ga_(0.75)N/GaNHEMTs击穿特性分析

为了优化AlGaN/GaN HEMTs器件表面电场,提高击穿电压,本文首次提出了一种新型阶梯AlGaN/GaN HEMTs结构.新结构利用AlGaN/GaN异质结形成的2DEG浓度随外延AlGaN层厚度降低而减小的规律,通过减薄靠近栅边缘外延的AlGaN层,使沟道2DEG浓度分区,形成栅边缘低浓度2DEG区,低的2DEG使阶梯AlGaN交界出现新的电场峰,新电场峰的出现有效降低了栅边缘的高峰电场,优化了AlGaN/GaN HEMTs器件的表面电场分布,使器件击穿电压从传统结构的446 V,提高到新结构的640 V.为了获得与实际测试结果一致的击穿曲线,本文在GaN缓冲层中设定了一定浓度的受主型缺陷,通过仿真分析验证了国际上外延GaN缓冲层时掺入受主型离子的原因,并通过仿真分析获得了与实际测试结果一致的击穿曲线.     

Neutron irradiation effects on AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) were exposed to 1 MeV neutron irradiation at a neutron fluence of 1 × 10¹⁵ cm^-2. The dc characteristics of the devices, such as the drain saturation current and the maximum transconductance, decreased after neutron irradiation. The gate leakage currents increased obviously after neutron irradiation. However, the rf characteristics, such as the cut-off frequency and the maximum frequency, were hardly affected by neutron irradiation. The AlGaN/GaN heterojunctions have been employed for the better understanding of the degradation mechanism. It is shown in the Hall measurements and capacitance-voltage tests that the mobility and concentration of two-dimensional electron gas (2DEG) decreased after neutron irradiation. There was no evidence of the full-width at half-maximum of X-ray diffraction (XRD) rocking curve changing after irradiation, so the dislocation was not influenced by neutron irradiation. It is concluded that the point defects induced in AlGaN and GaN by neutron irradiation are the dominant mechanisms responsible for performance degradations of AlGaN/GaN HEMT devices.     

Identification of \mathrm{\ensuremath J^{\pi} = 19/2^+} and \mathrm{\ensuremath 23/2^+} isomeric states in 127Sb

The nucleus $\ensuremath {\rm ^{127}Sb}$ , which is on the neutron-rich periphery of the $\ensuremath \beta$ -stability region, has been populated in complex nuclear reactions involving deep-inelastic and fusion-fission processes with $\ensuremath {\rm {}^{136}Xe}$ beams incident on thick targets. The previously known isomer at 2325 keV in $\ensuremath {\rm {}^{127}Sb}$ has been assigned spin and parity $\ensuremath 23/2^+$ , based on the measured $\ensuremath \gamma$ - $\ensuremath \gamma$ angular correlations and total internal conversion coefficients. The half-life has been determined to be 234(12) ns, somewhat longer than the value reported previously. The 2194 keV state has been assigned $\ensuremath J^{\pi} = 19/2^+$ and identified as an isomer with $\ensuremath T_{1/2} = 14(1) {\rm ns}$ , decaying by two $\ensuremath E2$ branches. The observed level energies and transition strengths are compared with the predictions of a shell model calculation. Two $\ensuremath 15/2^+$ states have been identified close in energy, and their properties are discussed in terms of mixing between vibrational and three-quasiparticle configurations.     

Large N approach to Kaon decays and mixing 28 years later: \Delta I=1/2 rule, \hat{B}_K,and \Delta M_K

We review and update our results for $K\rightarrow \pi \pi $ decays and $K^0$ – $\bar{K}^0$ mixing obtained by us in the 1980s within an analytic approximate approach based on the dual representation of QCD as a theory of weakly interacting mesons for large $N$ , where $N$ is the number of colors. In our analytic approach the Standard Model dynamics behind the enhancement of $\hbox {Re}A_0$ and suppression of $\hbox {Re}A_2$ , the so-called $\Delta I=1/2$ rule for $K\rightarrow \pi \pi $ decays, has a simple structure: the usual octet enhancement through the long but slow quark–gluon renormalization group evolution down to the scales $\mathcal{O}(1\, {\hbox { GeV}})$ is continued as a short but fast meson evolution down to zero momentum scales at which the factorization of hadronic matrix elements is at work. The inclusion of lowest-lying vector meson contributions in addition to the pseudoscalar ones and of Wilson coefficients in a momentum scheme improves significantly the matching between quark–gluon and meson evolutions. In particular, the anomalous dimension matrix governing the meson evolution exhibits the structure of the known anomalous dimension matrix in the quark–gluon evolution. While this physical picture did not yet emerge from lattice simulations, the recent results on $\hbox {Re}A_2$ and $\hbox {Re}A_0$ from the RBC-UKQCD collaboration give support for its correctness. In particular, the signs of the two main contractions found numerically by these authors follow uniquely from our analytic approach. Though the current–current operators dominate the $\Delta I=1/2$ rule, working with matching scales $\mathcal{O}(1 \, {\hbox { GeV}})$ we find that the presence of QCD-penguin operator $Q_6$ is required to obtain satisfactory result for $\hbox {Re}A_0$ . At NLO in $1/N$ we obtain $R=\hbox {Re}A_0/\hbox {Re}A_2= 16.0\pm 1.5$ which amounts to an order of magnitude enhancement over the strict large $N$ limit value $\sqrt{2}$ . We also update our results for the parameter $\hat{B}_K$ , finding $\hat{B}_K=0.73\pm 0.02$ . The smallness of $1/N$ corrections to the large $N$ value $\hat{B}_K=3/4$ results within our approach from an approximate cancelation between pseudoscalar and vector meson one-loop contributions. We also summarize the status of $\Delta M_K$ in this approach.     

Growth by molecular beam epitaxy of AlGaN/GaN high electron mobility transistors on Si-on-polySiC

We report on the growth by molecular beam epitaxy of AlGaN/GaN high electron mobility transistors (HEMTs) on Si(111)/ SiO₂/polySiC substrates. The structural, optical, and electrical properties of these films are studied and compared with those of heterostructures grown on thick Si(111) substrates. Field effect transistors have been realized, and they demonstrate the potentialities of III–V nitrides grown on these advanced substrates.     

Enhanced effect of side-Ohmic contact processing on the 2DEG electron density and electron mobility of In0.17Al0.83N/AlN/GaN heterostructure field-effect transistors

From the capacitance–voltage curves and current–voltage characteristics of the In_0.17Al_0.83N/AlN/GaN heterostructure field-effect transistors (HFETs) with side-Ohmic contacts and normal-Ohmic contacts, two-dimensional electron gas (2DEG) electron mobility was calculated. It is found that the polarization Coulomb field scattering (PCF) is closely related to the normal-Ohmic contact processing, and the PCF was weakened by side-Ohmic contact processing in In_0.17Al_0.83N/AlN/GaN HFETs, similar to that in AlGaN/AlN/GaN HFET devices. Further, due to the stronger spontaneous polarization in the thinner In_0.17Al_0.83N barrier layer, the influence of the gate bias on the PCF in In_0.17Al_0.83N/AlN/GaN HFETs is greater than that in AlGaN/AlN/GaN HFETs. As a result, the PCF in In_0.17Al_0.83N/AlN/GaN HFETs with side-Ohmic contacts is stronger than that in AlGaN/AlN/GaN HFETs with side-Ohmic contacts. Moreover, the 2DEG electron density in the In_0.17Al_0.83N/AlN/GaN HFETs with side-Ohmic contacts is increased by more than twice compared with the 2DEG electron density in the In_0.17Al_0.83N/AlN/GaN HFETs with normal-Ohmic contacts.