Comparative study of hexagonal and cubic GaN growth by RF-MBE

The epitaxial growth of both cubic and hexagonal GaN epilayers is considered here with the aim of comparing their physical properties. In particular, the growth mechanisms at the first stages of growth will be dealt with together with the quality of the growth front. The optical characteristics of the epilayers will be compared by reference to the structure of the defects present within the different types of layers.     

KOH热湿腐蚀法准确估算GaN的位错密度

实验目的在于解决GaN外延层中六方形腐蚀坑起源问题存在的分歧,并利用腐蚀法准确地估计GaN的位错密度.大量对熔融KOH腐蚀GaN过程中表面形貌的演化以及温度和时间对腐蚀结果影响的实验结果表明,位错类型与腐蚀坑三维形状相对应,而与腐蚀坑大小无关,极性是GaN不同种类位错的腐蚀坑具有不同形状的决定性因素.使所有缺陷都显示出来所需的腐蚀温度和时间呈反比关系.腐蚀法估算GaN位错密度的准确性取决于优化的腐蚀条件和合理的微观观测方法.     

掺Si对MOCVD生长的GaN单晶膜的黄带发射及生长速率的影响

对ＭＯＣＶＤ生长ＧａＮ：Ｓｉ薄膜进行了研究,研究表明随ＳｉＨ４／ＴＭＧａ流量比增大,ＧａＮ：Ｓｉ单晶膜的电子浓度增大,迁移率下降,Ｘ射线双晶衍射峰半高宽增加,同时这发射强度得到了大大的提高,并报导了随ＳｉＨ４／ＴＭＧａ流量比增大,ＧａＮ：Ｓｉ的生长速率降低的现象,研究结果还表明,预反应对ＧａＮ：Ｓｉ单晶膜黄带发射影响很大,预反应的减小可以使黄带受到抑制。     

Characterisation of dislocations, nanopipes and inversion domains in GaN by transmission electron microscopy

Transmission electron microscopy is used to analyse a range of defects observed in hexagonal GaN films grown on sapphire and GaN substrates by metalorganic chemical vapour deposition. Large angle convergent beam electron diffraction is used to analyse the Burgers vectors of dislocations and to show that hollow tubes, or nanopipes, are associated with screw dislocations having Burgers vectors±c. Weak-beam electron microscopy shows that dislocations are dissociated into partials in the (0001) basal plane, but that threading segments are generally undissociated. The presence of high densities of inversion domains in GaN/sapphire films is confirmed using convergent beam electron diffraction and the atomic structure of the {

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} inversion domain boundary is determined by an analysis of displacement fringes seen in inclined domains.     

Improvement of luminescence efficiency and photostability in polymer thin films

The photodegradation mechanism and luminescence efficiency of a series of thin polymer films prepared under a variety of conditions was studied. The conjugated polymer, poly(m-phenylene-co-2,5-dioctoxy-p-phenylenevinylene) (PmPV), is shown by infra-red spectroscopy to degrade via the chain scission of the carbon double bond along the polymer backbone. This causes a reduction in conjugation length and a blue shift in its absorption and photoluminescence (PL) spectra. To reduce photodegradation effects, films were prepared using argon (Ar) gas and were investigated in air and an oxygen free environment. The initial PL intensity increased by over 70% for Ar treated films. The PL decay in air was bi-exponential in nature, with a sharp initial decay linked to atmospheric oxygen, and a longer second decay linked to oxygen embedded in the film. The increase in both PL efficiency and degradation lifetime, coupled with device encapsulation, should significantly improve the performance of electroluminescent devices.     

Study of Zn diffusion in n-type GaSb by cathodoluminescence and scanning tunneling spectroscopy

We report here the studies carried out in zinc diffused n-type GaSb by cathodoluminescence (CL) microscopy and by scanning tunneling spectroscopy. Samples with different diffusion profiles measured by secondary ion mass spectrometry (SIMS) were obtained. CL plan-view observations show high homogeneity in the diffused layers. Cross-sectional measurements of the Zn diffused layers were performed by current imaging tunneling spectroscopy (CITS). The junction border was revealed clearly in the CITS images and conductance spectra recorded at differents points of the layers provided information on the local surface band gaps and the conductive behaviour. The results were related to the diffusion profiles and were found to agree with diffusion models suggested previously.     

Quantitative characterization of pores in transparent ceramics by coupling electron microscopy and confocal laser scanning microscopy

This work is dedicated to the implementation of a new characterization method of porosity in transparent ceramics. This quantitative method couples scanning electron microscopy with confocal laser scanning microscopy. From this original method, the volume fraction of pores has been determined for different sintered Nd:YAG specimens with an accuracy of about 10%. This technique appears to be promising because it leads to both the pore size distribution and the residual porosity for fully dense samples. For example, it becomes possible to reach porosity levels ranging from 0.09% to 0.0004% for transparent Nd:YAG specimens which could not be measured by using conventional techniques. Finally, correlations between the residual porosity of these full dense samples and their optical properties could be established.     

Enhanced visible luminescence and modification in morphological properties of cadmium oxide nanoparticles induced by annealing

Cadmium oxide nanoparticles synthesised by a simple sol–gel synthesis method showed luminescence properties in the visible region of the electromagnetic spectrum. Both green and blue emissions were observed in photoluminescence spectra. We have investigated luminescence properties by changing the synthesis conditions. An enhanced luminescence of CdO nanoparticles was realised when these particles were annealed at different temperatures. Cadmium interstitial vacancies and oxygen vacancies played an important role in luminescence properties. X-ray diffraction confirmed annealing-induced changes in morphological properties. A good correlation between all the experimental results was obtained. Optical properties were investigated by diffuse reflectance spectra and photoluminescence spectra. Structural properties were investigated by high-resolution transmission electron microscopy.     

Temperature dependence of strain in Al0.22Ga0.78N/GaN heterostructures with and without Si3N4 passivation

The high-temperature characteristics of strain in Al_0.22Ga_0.78N layers, with and without a Si₃N₄ passivation layer, were investigated at temperatures from room temperature to 813 K by means of high-resolution X-ray diffraction. The results show that a small temperature-dependent strain relaxation occurs in the unpassivated Al_0.22Ga_0.78N layers when the temperature exceeds 523 K. After passivating, an additional tensile strain and an initial increase of the in-plane tensile strain with increasing temperature were observed in Al_0.22Ga_0.78N layers, and at higher temperatures the in-plane tensile strain only decreases slightly in the 100-nm-thick Al_0.22Ga_0.78N layer, but a pronounced temperature-dependent strain relaxation occurs in the 50-nm-thick one due to the fact that the thickness of the Al_0.22Ga_0.78N layer is close to the critical thickness, and hence the increase of tensile strain induced by passivation will result in partial strain relaxation via the formation of cracks or the glide motion and multiplication of dislocations.