| In recent years, GaN was widely used in manufacture of optoelectronic devices because of its excellent photoelectric properties. Si based GaN optoelectronic devices were specially advantageous in realizing integration in devices, however, the large lattice mismatch and thermal mismatch between GaN and Si had seriously hindered their developments and applications. In previous work, using Si-NPA as substrate, we prepared GaN/Si-NPA nanoheterostructure array by CVD technique, and realized yellow and NIR light emission by electroluminescence. In this paper, we studied the rule of reflectance spectrum, photoluminescence, character of Ⅰ-Ⅴ, electroluminescence changed by the preparation conditions of growing temperature, pressure of Ammonia of buffer layer and annealing, aiming to realize the optimizing of preparation conditions of GaN/Si-NPA. The main contents and results are as follows:1. Controllable preparation of GaN/Si-NPA, morphology and structureThe GaN/Si-NPA nanoheterostructure array was synthesized on Si-NPA using the catalyst of Pt through the method of CVD. We achieved the controllable morphologies of GaN/Si-NPA through changing the growing temperature, pressure of Ammonia of buffer layer and annealing. SEM images showed, after growing film of GaN, we could see clearly that the samples kept the array structure of Si-NPA. The morphologies of GaN were mainly nanograins and nanowires. When the growing temperature or pressure of Ammonia of buffer layer was low, the morphology of GaN was nanoparticles; with the growing temperature or pressure of Ammonia of buffer layer increasing, the morphologies of GaN were nanoparticles and nanowires. XRD patternes showed that the structure of GaN was wurtzite, but there were larger half high width and other peaks which demonstrated that the properties of GaN crystal was not very good. Annealing treatment did not significantly change the morphologies of GaN, but it improved the crystalline of GaN.2. The reflectance spectrum of GaN/Si-NPA The reflectivity of the GaN/Si-NPA increased gradually with growing temperature or pressure of Ammonia of buffer layer increased. In the visible light range, the range of the maximum reflectance of GaN/Si-NPA was from6.72%to22.57%. When the growing temperature or pressure of Ammonia of buffer layer was low, the quantity of GaN on substrate was less, after the photons getting to sample, a small part of them were reflected by the GaN film, large of them penetrated the GaN film to substrate and were absorbed, so the reflectivity was low; When the growing temperature or pressure of Ammonia of buffer layer increased, the quantity of GaN on substrate increased as well, a small part were absorbed by substrate, most of the photons were reflected by GaN, so the reflectance increased.3. The photoluminescence of GaN/Si-NPAThe pattern displayed that there were two luminous peaks, one was about at369nm, which was the intrinsic peak of GaN, another was about at550nm, which was the yellow luminous peak, two of them increased with the the growing temperature or pressure of Ammonia of buffer layer increased. Tests showed that the luminous of GaN/Si-NPA was consisted of two parts, namely substrate and GaN film. When the growing temperature or pressure of Ammonia of buffer layer was low, the quantity of GaN on substrate was less, excitation light could through GaN to Si-NPA which had the strong PL, at this time, the result was the role of GaN film and substrate together. With the growing temperature or pressure of Ammonia of buffer layer increased, the quantity of GaN on substrate increased as well, excitation light was completely absorbed by GaN film, the strength of intrinsic and yellow light emission peak of GaN increased. After annealing, the strength of intrinsic peak of GaN enhanced and the yellow light emission decreased, which showed that the crystallization of GaN improved and the number of defect reduced.4. The I-V and carrier transport mechanism of GaN/Si-NPAThe nanoheterojunction of n-GaN/p-Si-NPA showed good rectifying effect. After analysis, we thought that the rectification properties were from the heterojunction formed by GaN nanoparticles and silicon nanocrystals existed in substrate Si-NPA. Before and after annealing, the samples had the rectification properties. but after annealing, the leakage current of samples decreased significantly, which indicated that annealing could reduce the number of defects and improve the quality of the interface of samples. At the same time, increasing the growing temperature or pressure of Ammonia of buffer layer could also achieve the result of reducing leakage current of heterojunction and improving the quality of the interface. Studying the transport mechanism of carrier of heterojunction indicated that, before and after annealing, there were two ways of transport mechanism of carrier in the heterojunction, which were ohms transmission in low voltage and the space charge limited current transmission in high voltage. The transport mechanism changed from one to another mean that the defects of electronic level of heterojunction were filled.5. The electroluminescence and analysis of mechanism of GaN/Si-NPAAs test results showed, under the lower forward biased, when the loop had a current, the all samples had a wide yellow luminous peak, the strength enhanced with the growing temperature or pressure of Ammonia of buffer layer increased. When the forward biased raised, GaN/Si-NPA began to appear two electroluminescent peaks, which were yellow light emission peak at about550nm and NIR light emission peak at about820nm. With the applied voltage increased, the strength of yellow peak increased firstly and then tended to be stable, while the strength of near infrared peak increased straightly. After analysis we knew that the yellow emission was shaped by electrons which transited in different defect levels formed by defect in GaN crystal and the NIR light emission produced by electrons transited from conduction band of GaN to the valence band of Si-NPA. |
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