Recombination processes in structures with GaN/ALN quantum dots

Mechanisms of the generation and the radiative and nonradiative recombination of carriers in structures with GaN quantum dots in the AlN matrix are studied experimentally and theoretically. Absorption, stationary and nonstationary photoluminescence of quantum dots at different temperatures are investigated. It is found that the photoluminescence intensity considerably decreases with the temperature while the photoluminescence kinetics weakly depends on the temperature. The photoluminescence kinetics is shown to be determined by radiative recombination inside quantum dots. A mechanism of nonradiative recombination is proposed, according to which the main reason for the thermal quenching of photoluminescence is nonradiative recombination of charge carriers, generated by optical transitions between quantum dots and wetting layer states.     

Light Absorption of GaN Nanowire Array Cathode with Alkali Metal Nanoparticle Modified on the Surface

Since the adsorption of alkali metals is necessary for the negative electron affinity (NEA) of the photocathode, light absorption models of GaN nanowire (NW) arrays with alkali metal (Li, Na, K, and Cs) nanoparticles (NPs) modified on the NW surface based on the finite difference time domain (FDTD) method are constructed. The absorption spectra of hemispherical, spherical alkali metal NPs adsorbed on the outer surface of the NW, and spherical alkali metal embedded on the inner surface and center of the NW are studied. When the ratio of NW diameter to period (D/P) is greater than 0.5, the adsorption of alkali metal NPs cannot improve the absorption of GaN NW arrays. Alkali metal decoration can cause the absorption gain of NW arrays and optical loss of NPs, so the diameter and spacing of alkali metal NPs need to be balanced. When Li NPs are embedded in NW, plasmons can enhance the generation of electron-hole pairs, making GaN NWA obtain higher optical absorption and quantum efficiency. Therefore, the method of Li and Cs NPs embedded in GaN NW can provide a reference for the process NEA design, which will contribute to the development of the ultraviolet photocathode with high absorption characteristics.     

GaN肖特基二极管的正向电流输运和低频噪声行为

首先测量了GaN肖特基二极管的正向变温电流-电压特性,研究了其电流输运机制,然后分析了在不同注入电流条件下的低频噪声行为.结果表明:1)在正向高电压区,热发射机制占主导,有效势垒高度约为1.25 eV;2)在正向低偏压区(V <0.8 V),与位错相关的缺陷辅助隧穿电流占主导,有效势垒高度约为0.92 eV (T=300 K);3)在极小电流(I <1μA)和极低频率(f <10 Hz)下,洛伦兹型噪声才会出现;电子的渡越时间取决于多个缺陷对电子的不断捕获和释放过程,典型时间常数约为30 ms (I=1μA);4)在更高频率和电流下,低频1/f噪声占主导;电流的输运主要受到势垒高度的随机波动的影响,所对应的系数约为1.1.     

掺杂GaN/AlN超晶格第一性原理计算研究

第一性原理计算方法在解释实验现象和预测新材料结构及其性质上有着重要作用. 因此, 通过基于密度泛函理论的第一性原理的方法, 本文系统地研究了Mg和Si掺杂闪锌矿和纤锌矿两种晶体结构的GaN/AlN超晶格体系中的能量稳定性以及电学性质. 结果表明: 在势阱层(GaN 层)中, 掺杂原子在体系中的掺杂形成能不随掺杂位置的变化而发生变化, 在势垒层(AlN层)中也是类似的情况, 这表明对于掺杂原子来说, 替代势垒层(或势阱层)中的任意阳离子都是等同的; 然而, 相比势阱层和势垒层的掺杂形成能却有很大的不同, 并且势阱层的掺杂形成能远低于势垒层的掺杂形成能, 即掺杂元素(Mg_Ga, Mg_Al, Si_Ga和Si_Al)在势阱区域的形成能更低, 这表明杂质原子更易掺杂于结构的势阱层中. 此外, 闪锌矿更低的形成能表明: 闪锌矿结构的超晶格体系比纤锌矿结构的超晶格体系更易于实现掺杂; 其中, 闪锌矿结构中, 负的形成能表明: 当Mg原子掺入闪锌矿结构的势阱层中会自发引起缺陷. 由此, 制备以闪锌矿结构超晶格体系为基底的p型半导体超晶格比制备n型半导体超晶格需要的能量更低并且更为容易制备. 对于纤锌矿体系来说, 制备p型和n型半导体的难易程度基本相同. 电子态密度对掺杂体系的稳定性和电学性质进一步分析发现, 掺杂均使得体系的带隙减小, 掺杂前后仍然为第一类半导体. 综上所述, 本文内容为当前实验中关于纤锌矿结构难以实现p型掺杂问题提供了一种新的技术思路, 即可通过调控相结构实现其p型掺杂.     

Mn掺杂GaN稀磁半导体的电磁特性研究

采用基于密度泛函理论的第一性原理平面波超软赝势法计算了不同浓度Mn掺杂GaN(Ga1-xMnxN,x=0.0625和0.1250)的晶格常数、能带结构和态密度,分析比较了掺杂前后GaN的电子结构和磁性.结果表明:Mn掺入后体系仍为直接带隙半导体,带隙宽度随Mn含量的增加逐步增大.Mn掺杂GaN均使得N2p与Mn3d轨道杂化,产生自旋极化杂质带,自旋向上的能带占据费米面,掺杂后的Ga1-xMnxN表现为半金属铁磁性,适合自旋注入;随着Mn掺杂浓度的增加,体系的半金属性有所增强.     

Scintillation and optical behavior of GaN nanowires in the presence of low-energy X-ray photons: A Geant4 simulation

GaN nanowires were studied as a possible candidate for use in the future development of X-ray scintillator detectors with high spatial resolution using a Monte Carlo simulation. The assessment was performed from the optical response perspective. The ability of such a nanowire to act as scintillating fibers was simulated using Geant4 code. The optimal dimensions of the nanowire (considering practical limitations) to achieve the best optical guiding effect were also determined. Moreover, the energy response of the nanowire was investigated for the design of a suitable external photodetector. To more accurately study the proposed structure, a scintillating screen containing approximately 2,000,000 nanowires was simulated in a porous anodized alumina membrane (PAAM) with a hexagonal arrangement. Using these specifications, the spatial resolution and efficiency of the detector were precisely calculated. The results showed that the spatial resolution and efficiency of the detector were <1 μm and ∼17%, respectively.     

Analysis of carrier transport properties in GaN/Al0.3Ga0.7N multiple quantum well nanostructures

Analysis of carrier transport properties in GaN based multiple quantum well nanostructure has been carried out with an applied bias. Effect of an applied bias and aluminium mole composition in the barrier on the scattering rate, capture time and escape rate has been investigated. The scattering rate was found to be decreased with an increase of applied bias voltage and aluminium mole composition. Capture time shows oscillatory nature with variations in mole composition of aluminium under biasing conditions. The escape rate was found to be increasing from 0.01 ps⁻¹ to 0.69 ps⁻¹ with applied bias voltage.     

Improved linearity performance of AlGaN/GaN MISHFET over conventional HFETs: An optimization study for wireless infrastructure applications

In this work an extensive study on the linearity distortion behaviour of AlGaN/GaN MISHFET is performed and compared with those of conventional HFET structures. The contribution of higher order terms in a Taylor series expansion of small signal drain current is considered by evaluating higher order transconductance coefficients. Linearity figures of merit such as input intercept power, intermodulation distortion and other higher order harmonics have been investigated for both the structures. The impact of gate biasing on the linearity is also examined. The influence of critical technology parameters such as gate length, doping density, dopant layer thickness and gate insulator thickness is investigated to optimize the MISHFET structure for better linearity characteristics. Improved linearity performance has been observed for insulated gate structure proving its superiority over conventional HFETs for RF wireless and low noise applications. The results obtained reveal that by careful optimization of technology parameters, a suitably designed MISHFET architecture is more linear than its conventional counterparts.     

Structural and optical investigation of nonpolar a-plane GaN grown by metal-organic chemical vapour deposition on r-plane sapphire by neutron irradiation

Nonpolar (1120) a-plane GaN films are grown by metal-organic chemical vapour deposition (MOCVD) on r-plane (1102) sapphire.The samples are irradiated with neutrons under a dose of 1 × 10 15 cm 2.The surface morphology,the crystal defects and the optical properties of the samples before and after irradiation are analysed using atomic force microscopy (AFM),high resolution X-ray diffraction (HRXRD) and photoluminescence (PL).The AFM result shows deteriorated sample surface after the irradiation.Careful fitting of the XRD rocking curve is carried out to obtain the Lorentzian weight fraction.Broadening due to Lorentzian type is more obvious in the as-grown sample compared with that of the irradiated sample,indicating that more point defects appear in the irradiated sample.The variations of line width and intensity of the PL band edge emission peak are consistent with the XRD results.The activation energy decreases from 82.5 meV to 29.9 meV after irradiation by neutron.     

Threading dislocation density comparison between GaN grown on the patterned and conventional sapphire substrate by high resolution X-ray diffraction

GaN epifilms are grown on the patterned sapphire substrates (PSS) (0001) and the conventional sapphire substrates (CSS) (0001) by metal-organic chemical vapor deposition (MOCVD) using a novel two-step growth. High resolution X-ray diffraction (HR-XRD) is used to investigate the threading dislocation (TD) density of the GaN epifilms. The TD density is calculated from the ω-scans full width at half maximum (FWHM) results of HR-XRD. The edge dislocation destiny of GaN grown on the PSS is 2.7×108 cm-2, which is less than on the CSS. This is confirmed by the results of atomic force microscopy (AFM) measurement. The lower TD destiny indicates that the crystalline quality of the GaN epifilms grown on the PSS is improved compared to GaN epifilms grown on the CSS. The residual strains of GaN grown on the PSS and CSS are compared by Raman Scattering spectra. It is clearly seen that the residual strain in the GaN grown on PSS is lower than on the CSS.