Atomically flat GaMnN by diffusion of Mn into GaN()

The interaction of Mn thin films on atomically flat GaN() has been investigated by Scanning Tunneling Microscopy (STM) and Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). GaN surfaces prepared by repeated sputtering/annealing cycles show various reconstructions but essentially the 6×6 one. A high density of small islands (height 0.6 nm, diameter 5 nm) nucleates upon deposition of 0.3 ML Mn. Upon annealing at 575 C, these islands coalesce into larger islands (height 4 nm, diameter 50 nm) between which the GaN surface is visible. Atomically resolved STM images between the islands show the (3×3) reconstruction of GaN. Annealing the sample further to 675 C leads to the restoration of a bare GaN surface. ToF-SIMS reveals that Mn diffused into the bulk of the sample.     

Comparative study of adsorption characteristics of Cs on the GaN （0001） and GaN （0001） surfaces

The adsorption characteristics of Cs on GaN （0001） and GaN （0001） surfaces with a coverage from 1/4 to 1 monolayer have been investigated using the density functional theory with a plane-wave uttrasoft pseudopotential method based on first-principles calculations. The results show that the most stable position of the Cs adatom on the GaN （0001） surface is at the N-bridge site for 1/4 monolayer coverage. As the coverage of Cs atoms at the N-bridge site is increased, the adsorption energy reduces. As the Cs atoms achieve saturation, the adsorption is no longer stable when the coverage is 3/4 monolayer. The work function achieves its minimum value when the Cs adatom coverage is 2/4 monolayer, and then rises with Cs atomic coverage. The most stable position of Cs adatoms on the GaN （000i） surface is at H3 site for 1/4 monolayer coverage. As the Cs atomic coverage at H3 site is increased, the adsorption energy reduces, and the adsorption is still stable when the Cs adatom coverage is 1 monolayer. The work function reduces persistently, and does not rise with the increase of Cs coverage.     

High-temperature failure of GaN LEDs related with passivation

This paper analyses the thermally-activated failure mechanisms of GaN LED test-structures related with the presence of a hydrogen rich SiN passivation layer, by comparing the electrical and optical behaviour of samples with and without passivation during thermal stress. The analysis was carried out by means of electroluminescence, cathodoluminescence, emission microscopy and current–voltage measurements. Thermal treatment induced degradation only on the samples with passivation: identified degradation modes were an efficiency decrease exponential in time, emission crowding, and a forward voltage increase. On the other side, thermal treatment did not change the behaviour of the LEDs without passivation. An interpretation for the degradation of the passivated samples is the following: as a consequence of passivation deposition, a considerable amount of hydrogen is incorporated in the passivation layer. Heating at 250 C allows this hydrogen to interact with the LED surface, thus worsening the transport properties of p-GaN and of the p-ohmic contact, and then the current and emission distribution, inducing the observed degradation and emission crowding. The activation energy of the degradation process was found to be equal to 1.3 eV. Comparison between spectral electroluminescence and cathodoluminescence measurements shows how the mechanism mentioned above is not the only ageing cause and the thermal worsening of QW confinement and/or the creation of nonradiative centers possibly contribute to the LED damage.     

GaN(0001)表面的电子结构研究

报道了纤锌矿WZ(wurtzite)GaN(0001)表面的电子结构研究.采用全势线性缀加平面波方法计算得到了GaN分波态密度,与以前实验结果一致.讨论了Ga3d对GaN电子结构的影响.利用同步辐射角分辨光电子能谱技术研究了价带电子结构.通过改变光子能量的垂直出射谱勾画沿ΓA方向的体能带结构,与计算得到的能带结构符合得较好.根据沿ΓK和ΓM方向的非垂直出射谱,确定了两个表面态的能带色散 关键词： GaN 角分辨光电发射 全势线性缀加平面波 电子结构     

Cs/GaN(0001)吸附体系电子结构和光学性质研究

采用基于第一性原理的密度泛函理论平面波超软赝势方法计算了 1/4ML Cs原子吸附 (2 × 2) GaN(0001) 表面的吸附能、能带结构、电子态密度、电荷布居数、功函数和光学性质. 计算发现, 1/4ML Cs 原子在 GaN(0001) 表面最稳定吸附位为 N 桥位, 吸附后表面仍呈现为金属导电特性, Cs原子吸附GaN(0001)表面后主要与表面 Ga 原子发生作用, Cs6s 态电子向最表面 Ga 原子转移, 引起表面功函数下降. 研究光学性质发现, Cs 原子吸附 GaN(0001) 表面后, 介电函数虚部、吸收谱、反射谱向低能方向移动.     

低覆盖度的Au/GaN(0001)界面的同步辐射研究

利用同步辐射光电子能谱研究了低覆盖度Au在GaN(0001)表面的初始生长模式,肖特基势垒高 度以及界面的电子结构.结果表明，Au沉积初始阶段有界面的化学反应,随后呈三维岛状生长 .由光电子能谱实验确定的肖特基势垒高度为14 eV. 通过对界面价带谱和Au 4f芯能级谱 的分析,确定了界面化学反应的存在.利用线性缀加平面波方法计算了GaN(0001)和Au的价带 态密度并分析了化学反应产生的机理,认为在初始阶段界面形成了Au_Ga合金. 关键词： 同步辐射 光电子能谱 Au/GaN欧姆接触 态密度     

Theoretical study of hydrogen adsorption on the GaN(0 0 0 1) surface

Ab initio density functional theory, using the B3LYP hybrid functional with all-electron basis sets, has been applied to the adsorption of H on the (0 0 0 1) surface of wurtzite GaN. For bulk GaN, good agreement is obtained with photoemission and X-ray emission data for the valence band and for the Ga 3d and N 2s shallow core levels. A band gap of E_g = 4.14 eV is computed vs the experimental value (at 0 K) of 3.50 eV. A simple model, consisting of a (2 × 2) structure with 3/4-monolayer (ML) of adsorbed H, is found to yield a density of states in poor agreement with photoemission data for H adsorbed on surfaces prepared by ion bombardment and annealing. A new model, consisting of co-adsorbed Ga (1/4 ML) and H (1/2 ML), is proposed to account for these data.     

GROWTH MODE AND SURFACE RECONSTRUCTION OF GaN(0001) THIN FILMS ON 6H-SiC(0001)

Two-dimensional growth of GaN thin films on an atomically flat C-face 6H-SiC(0001) surface prepared by ultra-high vacuum Si-etching is observed when using an AlN buffer layer in N plasma-assisted molecular beam epitaxy. Scanning tunneling microscopy and reflection high energy electron diffraction observations reveal a series of Ga-stabilized reconstructions which are consistent with those reported for an N-polar GaN(0001) film. The result, including the effect observed previously for GaN thin film on Si-terminated 6H-SiC(0001), agrees with the polarity assignment of heteroepitaxial wurtzite GaN films on polar 6H-SiC substrates, i.e., GaN film grown on SiC(0001) is <0001> oriented (N-face) while that on SiC(0001) is <0001> oriented (Ga-face).     

斜切蓝宝石衬底MOCVD生长GaN薄膜的透射电镜研究

利用MOCVD技术在斜切角度为0.3°的c面蓝宝石衬底上生长了非故意掺杂 GaN 薄膜, 并采用透射电子显微镜对材料的质量和材料内部缺陷进行了分析. 研究发现斜切蓝宝石衬底上外延的GaN材料中,位错在距离衬底0.8 μm附近大量湮灭, 同时位错扎堆出现.基于上述现象, 提出了斜切衬底上GaN材料中位错的湮灭机制, 解释了斜切衬底能够提高GaN晶体质量的原因.     

TiO2分子在GaN(0001)表面吸附的理论研究

采用基于密度泛函理论的平面波超软赝势法,计算了TiO₂分子在GaN(0001)表面的吸附成键过程、吸附能量和吸附位置. 计算结果表明不同初始位置的TiO₂分子吸附后,Ti在fcc或hcp位置,两个O原子分别与表面两个Ga原子成键,Ga-O化学键表现出共价键特征,化学结合能达到7.932-7.943eV,O-O连线与GaN[1120]方向平行,与实验观测(100)[001] TiO₂//(0001)[1120]GaN一致. 通过动力学过程计算分析,TiO₂分子吸附过程经历了物理吸附、化学吸附与稳定态形成的过程,稳定吸附结构和优化结果一致. 关键词： GaN(0001)表面 2分子')" href="#">TiO₂分子 密度泛函理论 吸附     

Structural properties of GaN(0001) epitaxial layers revealed by high resolution X-ray diffraction

High-resolution X-ray diffraction has been used to analyze GaN(0001) epitaxial layers on sapphire substrates. Several structural properties of GaN, including the lattice constants, strains, and dislocation densities are revealed by the technique of X-ray dffraction (XRD). Lattice constants calculated from the omega/2theta scan are c=0.5185 nm and a=0.3157 nm. Also, the in-plane strain is -1.003%, while out of the plane, the epitaxial film is almost relaxed. Several methods are used to deduce the mosaicity a...     

Desorption induced GaN quantum dots on (0001) AlN by MOVPE

GaN quantum dots (QDs) are realized on (0001) AlN templates by growing a thin GaN layer on an AlN buffer layer and applying a subsequent desorption step without ammonia present. A growth interruption (GRI), which is commonly applied after the GaN growth allowing for QD formation, is systematically investigated regarding the temperature, duration and initial GaN coverage. Without GRI the initial GaN layer exhibits a two‐dimensional nonuniform growth at the step edges. In this study, the surface morphology only changes significantly if the GRI is performed without ammonia exposure. Thus, an initial two‐dimensional GaN layer can be shaped into three‐dimensional nanostructures. Presented coverage studies by atomic force microscopy (AFM) show desorption as the main driving force for island evolution. By tailoring the growth parameters, GaN QDs can be achieved. Uncapped GaN samples exhibit QDs with 1.2 nm in height and 30 nm in diameter. Additionally, capped GaN QDs exhibit excitonic luminescence lines at about 4.3 eV with FWHM down to 2 meV and an excitonic fine structure splitting of 7 meV. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

XPS study of the formation of ultrathin GaN film on GaAs(1 0 0)

The nitridation of GaAs(1 0 0) surfaces has been studied using XPS spectroscopy, one of the best surface sensitive techniques. A glow discharge cell was used to produce a continuous plasma with a majority of N atomic species. We used the Ga3d and As3d core levels to monitor the chemical state of the surface and the coverage of the species. A theoretical model based on stacked layers allows to determine the optimal temperature of nitridation. Moreover, this model permits the determination of the thickness of the GaN layer. Varying time of nitridation from 10 min to 1 h, it is possible to obtain GaN layers with a thickness between 0.5 nm and 3 nm.     

GaN(0001)表面电子结构和光学性质的第一性原理研究

采用基于第一性原理的密度泛函理论平面波超软赝势方法计算了(2×2)GaN(0001)清洁表面的能带结构、态密度、表面能、功函数和光学性质.发现弛豫后GaN(0001)表面的能带结构发生较大变化,表面呈现金属导电特性,导带底附近存在明显的表面态,在偶极矩的作用下表面电荷发生转移,Ga端面为正极性表面;计算获得了GaN(0001)表面的表面能和功函数分别为2.1J.m^-2和4.2eV;比较分析了GaN(0001)表面和体相GaN的光学性质,发现两者存在较大差异.     

CO adsorption on Ni(1 0 0): Evidences for a weakly bound phase by HREELS measurements

The vibrational properties of CO on Ni(1 0 0) were investigated by high resolution electron energy loss spectroscopy. Loss measurements were performed as a function of temperature and CO exposures. At room temperature, regardless of CO coverage, we found stretching energies at 250 meV and 240 meV, assigned to CO at atop and bridge sites, respectively. At low temperature and for CO exposures lower than 0.6 L, the loss spectra showed a single stretching peak at 240 meV which is assigned to CO at bridge sites. For higher doses, a new intense peak at 260 meV appeared in the loss spectra. The rise of this new loss strongly influenced the intensity of the peak at 240 meV suggesting the occurrence of dipole couplings between adjacent CO molecules. This unusual high stretching frequency leads us to assign the peak at 260 meV to the stretching vibration of CO molecules which are weakly bonded to the Ni surface. We suggest that the formation of this phase is due to an electronic effect arising from a reduced back donation of electrons into the empty π orbital of CO.     

X-ray topographic imaging of (Al, Ga)N/GaN based electronic device structures on SiC

Structural defects and their impact on the performance, lifetime and reliability of electronic devices are of permanent interest for crystal growers and device manufacturers. This is especially true for epitaxial (Al, Ga)N/GaN based high electron mobility transistor (HEMT) structures on 4H-SiC (0 0 0 1) substrates. This work points out how micropipes, dislocations and grain boundaries present in a 4H-SiC (0 0 0 1) wafer and subsequently overgrown with an (Al, Ga)N-GaN-HEMT layer sequence show up in X-ray topographic images and two-dimensional XRD maps. Using X-ray topography in transmission geometry, micropipes and other structural defects are localized non-destructively below structured metallization layers with a spatial resolution of a few tens of micrometers.     

Olefin metathesis reaction on GaN (0 0 0 1) surfaces

Proof-of-concept reactions were performed on GaN (0 0 0 1) surfaces to demonstrate surface termination with desired chemical groups using an olefin cross-metathesis reaction. To prepare the GaN surfaces for olefin metathesis, the surfaces were hydrogen terminated with hydrogen plasma, chlorine terminated with phosphorous pentachloride, and then terminated with an alkene group via a Grignard reaction. The olefin metathesis reaction then bound 7-bromo-1-heptene. The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy, and water contact angle measurements following each step in the reaction scheme. The XPS data was used to qualitatively identify surface chemical species and to quantitatively determine molecular surface coverage. The bromine atom in 7-bromo-1-heptene served as a heteroatom for identification with XPS. The reaction scheme resulted in GaN substrates with a surface coverage of 0.10 monolayers and excellent stability towards oxidation when exposed to oxygen plasma.     

Band alignment between NiO_x and nonpolar/semipolar GaN planes for selective-area-doped termination structure

Band alignment between NiO_x and nonpolar GaN plane and between NiO_x and semipolar GaN plane are measured by x-ray photoelectron spectroscopy. They demonstrate that the maximum value of the valence band in the unintentional-doped a-plane, m-plane, and r-plane GaN are comparable to each other, which means that all the substrates are of n-type with similar background carrier concentrations. However, the band offset at the NiO_x/GaN interface presents obvious crystalline plane dependency although they are coated with the same NiO_x films. By fitting the Ga 3 d spectrum obtained from the NiO_x/GaN interface, we find that relatively high Ga–O content at the interface corresponds to a small band offset. On the one hand, the high Ga–O content on the GaN surface will change the growth mode of NiO_x. On the other hand, the affinity difference between Ga and O forms a dipole which will introduce an extra energy band bending.     

DLTS characterization of silicon nitride passivated AlGaN/GaN heterostructures

Passivating the ungated surface of AlGaN/GaN HEMTs with silicon nitride (SiN) is effective in improving the microwave output power performances of these devices. However, very little information is available about surface states in GaN-based HEMTs after SiN passivation. In this work we investigate AlGaN/GaN HEMTs structures having either metal–semiconductor or metal–SiN–semiconductor gate contacts. In short gate devices conductance DLTS measurements point out a hole-like peak that shows an anomalous behaviour and can be ascribed to surface states in the access regions of the device. In insulated gate HEMTs a band of levels is detected and ascribed to surface states, whose energy ranges from 0.14 to 0.43 eV. Capacitance–voltage measurements allow us to point out the existence of a second band of interface states deeper in energy than the former one. This band is responsible for slow transients observed in the characteristics of the insulated gate FAT-HEMT.