利用SiN_x插入层和新型图形化蓝宝石衬底提高GaN外延层质量的相关研究
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摘要
当今，虽然GaN基LED已经实现了广泛的商业化应用，但是其总体性能仍然难以达到人们预期的最好效果。主要原因之一在于缺少高质量的GaN外延层。高质量的外延层是制备高性能器件的基础，因此，如何提高外延层的整体质量一直是研究者们研究的热点问题。本论文围绕着如何提高GaN外延层整体质量的问题进行了一系列研究，得到了好的实验结果。
利用MOCVD设备在GaN外延层中原位生长SiNx插入层，通过改变SiNx的生长时间，生长位置，以及利用SiNx插入层结合变化的GaN再生长模式进行了一系列研究，得到了高质量的GaN外延层，外延层中的最小位错密度达到0.93×108cm-2。
在图形化蓝宝石衬底（PSS）上生长GaN外延层，发现了GaN在PSS图形表面的选择性生长现象。通过进一步研究发现这种现象对于GaN外延层的生长具有一定的阻碍作用，并且对于减少GaN中的位错密度具有负面影响。
考虑到湿法腐蚀具有各向异性的性质，我们创造性地利用PSS湿法再腐蚀的方法来去除GaN的选择性生长现象。实验过程中，观察到了区别于以往报道的新腐蚀区域，并得到了由新腐蚀晶面组成的新型PSS。
首次利用干法腐蚀加湿法再腐蚀方法制备的新PSS进行了GaN外延层的生长研究。表征结果表明，新PSS有效地抑制了图形表面的GaN选择性生长现象，GaN外延层可以在图形表面顺利生长。相比于普通的PSS，外延层中的位错密度减小了50%以上。同时，外延层的其它性质也得到了显著的改善。
GaN possess many prominent qualities, such as wide band gap（3.39eV）, highbreakdown field（3×10~6V/cm）, and high electron saturation velocity（2.7×10~7cm/s）,making them ideal for high-speed, high-power electronic applications andoptoelectronic devices. And GaN based LED has been used in many aspects of ourlife. Although GaN-based LEDs are commercially available, the output power stillfalls short of what is expected. One limitation is that the quality of GaN epilayer isdifficult to be perfect. Due to the lack of GaN bulk crystals, GaN epilayers usuallygrow on heterogeneous substrates, such as sapphire. The large lattice mismatch andthermal mismatch between GaN and sapphire leads to a high threading dislocation（TD）density in GaN films（about10~（10）cm-2）, which seriously deteriorate the qualityof GaN film as well as the performance of corresponding GaN-based devices.Therefore, how to decrease the TD density and improve the quality of GaN epilayer isa hotspot issues for the researchers.
In this paper, a serious of researches has been focused on improving the qualityof GaN epilayer. Such as the in-situ growth of SiNxinterlayer by metal-organicchemical vapor deposition（MOCVD）, and the optimized research of patternedsapphire substrate（PSS）. By characterization, good results were obtained. Thedetailed contents are as follows:
（1）Three SiNxinterlayers with different growth-times were grown in the GaNepilayer. The SiNxinterlayer was proved to be a porous structure. It is found that the TD density in the epilayers decreased gradually with the increased growth-times ofSiN_xinterlayer. The smallest TD density reached to0.93×10~8cm~（-2）when the growthtime of SiN_xwas6mins. The full width at half maximum（FWHM）values of（0002）and（1012）diffraction peak decreased consistently with the prolongation of SiN_xgrowth-time. The minimum FWHM values of （0002）and（1012）diffraction peaksare118arcsec and222arcsec for the GaN epilayer with SiN_xgrowth time of6mins.The surface roughness increased with the growth-times of SiN_xinterlayer, and thebiggest root mean square （RMS） value is0.248nm. The results of low-temperature（10k）photoluminescence（PL）spectra indicated the good optical quality of GaNepilayer and the smallest FWHM value of D~oX peak was1.07meV. Raman scatteringexperiments confirmed that the compressive stress in the GaN epilayers was relaxedmore effectively with the increased SiN_xgrowth time.
（2）The effect of combining porous SiN_xinterlayer with changed GaN regrowthmodes on improving the quality of the GaN epilayers was studied. Two different GaNregrowth conditions were used after SiN_xdeposition:（a）the regrowth mode of GaNtransformed form3D to2D, which labeled as sample A;（b）the GaN regrowth startedfrom the growth of nucleation layer and annealed at high temperature, then thefollowing growth conditions of GaN epilayer were the same with that of sample A,which labeled as sample B. It is found that the RMS values of the two samples arenearly the same. The effect of the two samples on decreasing the TD density is worsethan that of the above section. While for the aspect of optical quality, the smallestFWHM value of D~oX peak was0.804meV for sample A, which was the smallest inour study. In other hand, the compressive stress in the GaN epilayers of sample B was0.606GP, indicating that the condition of sample B was best for relaxing the stress.
（3）The SiN_xinterlayer was grown after the growth and annealing of GaNnucleation layer. The surface of the epilayer was very rough. The FWHM values of（0002） and（1012） diffraction peaks were145arcsec and299arcsec, and the TDdensity was1.18×10~8cm~（-2）, which was the smallest with the same SiN_xgrowth-time.The optical quality of the epilayer is good, the FWHM value of D~oX peak was1.24 meV. The result of Raman scattering experiments indicated that the compressivestress in GaN epilayers was relaxed effectively.
Due to the sticking coefficients of Ga and N to GaN and SiN_xare about1and0,respectively, when GaN is deposited on the surface that partially covered with SiN_x, itwill grow firstly on the exposed surface of GaN rather than on SiN_x. Along with theincreased growth time, the GaN will grow laterally over the SiN_xregion, just like theprocess of GaN-growth in ELOG, and SiN_xacts as the mask. Thus, the TD densitywas decreased and the stress was relaxed. Furthermore, the SiN_xis preferentiallyformed at the TD cores because of the presence of N-dangling bonds for Si-Nformation, so the SiN_xinterlayer can also blocks some dislocations from entering theupper epilayers. The introduction of in-situ SiN_xhas the advantage of maskless,one-step processing, and the possible contaminations associated with the ex-situmethods can also be eliminated. These advantages are of great importance forimproving the performance of GaN-based devices.
（4）Relevant knowledge of patterned sapphire substrate（PSS）was introducedand the growth mechanism of GaN grown on the PSS was also investigated. Thespecial selective growth phenomenon of GaN was found on the surface of PSS, andthe distribution morphology of GaN changed significantly after the recrystallization athigh temperature. It is thought that the slope of PSS prepared by ICP etchingcomposes of two types of crystal planes, the different arrangement of aluminum andoxygen atoms of corresponding crystal planes leaded to the selective growthphenomenon. A model of the pattern was proposed and the investigation in atom-levelwas carried on. Moreover, the influence of the selective growth phenomenon on thequality of GaN epilayer was also studied. Many big GaN islands were found in theselective zones on the pattern surface, which could hinder the growth of GaN on thepattern and introduce many TD in the epilayer. The effect of PSS on reducing the TDswas influenced negatively.
（5）Wet etching was chosen as a method to suppress the selective growthphenomenon as well as the negative influence of which on the quality of GaN epilayer.During the etching process of NaOH, a porous structure of cone-shaped pattern was found, and the special morphology kept unchanged with increased etching time.When etched by KOH, it is found that the etching rate was very fast. The cone-shapedpattern transformed to a small hexagonal pyramid structure after20S. Mixture of98%H_2SO_4and85%H_3PO_4（H_2SO_4:H_3PO_4=3:1） was chosen as the corrodent,finally. During the etching process, three kinds of etching zones were found. Twoetching zones appeared first and vanished with increasing etching time. The other oneexposed later and expanded gradually. Finally, the cone-shaped pattern with anarcuate slope transformed to a hexagonal pyramid. The calculated orientation of thecrystallographic planes in the two etching zones were {1103} and {43127}, whichwere different from the previous reports.
（6）Anew polyhedral PSS prepared during the wet etching was chosen to growthe GaN epilayer. It is found that the GaN nucleation layer was evenly distributed onthe polyhedral pattern. After recrystallization, though the nucleation layer transformedinto many small GaN grains, the distribution of them was uniform. These observationresults indicated that the selective growth of GaN had been suppressed successfully.And there were no big GaN islands on the pattern surface during the sequent growthof GaN, the epilayer could growth smoothly on the pattern region. The statisticalvalue of TD density was about1.38×10~8cm~(-2)in epilayer grown on the new PSS,which was much smaller than that in epilayer grown on the cone-shaped PSS（about3.4×10~8cm~(-2)）. In addition, the characterization results indicated that the opticalquality was further improved and the residual compressive stress was relaxed moreeffectively by the new PSS.
The discovery and suppression of selective GaN growth and the big GaN islandswas of great importance for further improving the function of PSS. The new methodused in our study for preparing the new PSS provided new ideas for the fabrication ofPSS. The new PSS could improve the quality of GaN epilayer markedly, which hasthe positive significance for the development of the industry of GaN-based device.
利用MOCVD设备在GaN外延层中原位生长SiNx插入层，通过改变SiNx的生长时间，生长位置，以及利用SiNx插入层结合变化的GaN再生长模式进行了一系列研究，得到了高质量的GaN外延层，外延层中的最小位错密度达到0.93×108cm-2。
在图形化蓝宝石衬底（PSS）上生长GaN外延层，发现了GaN在PSS图形表面的选择性生长现象。通过进一步研究发现这种现象对于GaN外延层的生长具有一定的阻碍作用，并且对于减少GaN中的位错密度具有负面影响。
考虑到湿法腐蚀具有各向异性的性质，我们创造性地利用PSS湿法再腐蚀的方法来去除GaN的选择性生长现象。实验过程中，观察到了区别于以往报道的新腐蚀区域，并得到了由新腐蚀晶面组成的新型PSS。
首次利用干法腐蚀加湿法再腐蚀方法制备的新PSS进行了GaN外延层的生长研究。表征结果表明，新PSS有效地抑制了图形表面的GaN选择性生长现象，GaN外延层可以在图形表面顺利生长。相比于普通的PSS，外延层中的位错密度减小了50%以上。同时，外延层的其它性质也得到了显著的改善。
GaN possess many prominent qualities, such as wide band gap（3.39eV）, highbreakdown field（3×10~6V/cm）, and high electron saturation velocity（2.7×10~7cm/s）,making them ideal for high-speed, high-power electronic applications andoptoelectronic devices. And GaN based LED has been used in many aspects of ourlife. Although GaN-based LEDs are commercially available, the output power stillfalls short of what is expected. One limitation is that the quality of GaN epilayer isdifficult to be perfect. Due to the lack of GaN bulk crystals, GaN epilayers usuallygrow on heterogeneous substrates, such as sapphire. The large lattice mismatch andthermal mismatch between GaN and sapphire leads to a high threading dislocation（TD）density in GaN films（about10~（10）cm-2）, which seriously deteriorate the qualityof GaN film as well as the performance of corresponding GaN-based devices.Therefore, how to decrease the TD density and improve the quality of GaN epilayer isa hotspot issues for the researchers.
In this paper, a serious of researches has been focused on improving the qualityof GaN epilayer. Such as the in-situ growth of SiNxinterlayer by metal-organicchemical vapor deposition（MOCVD）, and the optimized research of patternedsapphire substrate（PSS）. By characterization, good results were obtained. Thedetailed contents are as follows:
（1）Three SiNxinterlayers with different growth-times were grown in the GaNepilayer. The SiNxinterlayer was proved to be a porous structure. It is found that the TD density in the epilayers decreased gradually with the increased growth-times ofSiN_xinterlayer. The smallest TD density reached to0.93×10~8cm~（-2）when the growthtime of SiN_xwas6mins. The full width at half maximum（FWHM）values of（0002）and（1012）diffraction peak decreased consistently with the prolongation of SiN_xgrowth-time. The minimum FWHM values of （0002）and（1012）diffraction peaksare118arcsec and222arcsec for the GaN epilayer with SiN_xgrowth time of6mins.The surface roughness increased with the growth-times of SiN_xinterlayer, and thebiggest root mean square （RMS） value is0.248nm. The results of low-temperature（10k）photoluminescence（PL）spectra indicated the good optical quality of GaNepilayer and the smallest FWHM value of D~oX peak was1.07meV. Raman scatteringexperiments confirmed that the compressive stress in the GaN epilayers was relaxedmore effectively with the increased SiN_xgrowth time.
（2）The effect of combining porous SiN_xinterlayer with changed GaN regrowthmodes on improving the quality of the GaN epilayers was studied. Two different GaNregrowth conditions were used after SiN_xdeposition:（a）the regrowth mode of GaNtransformed form3D to2D, which labeled as sample A;（b）the GaN regrowth startedfrom the growth of nucleation layer and annealed at high temperature, then thefollowing growth conditions of GaN epilayer were the same with that of sample A,which labeled as sample B. It is found that the RMS values of the two samples arenearly the same. The effect of the two samples on decreasing the TD density is worsethan that of the above section. While for the aspect of optical quality, the smallestFWHM value of D~oX peak was0.804meV for sample A, which was the smallest inour study. In other hand, the compressive stress in the GaN epilayers of sample B was0.606GP, indicating that the condition of sample B was best for relaxing the stress.
（3）The SiN_xinterlayer was grown after the growth and annealing of GaNnucleation layer. The surface of the epilayer was very rough. The FWHM values of（0002） and（1012） diffraction peaks were145arcsec and299arcsec, and the TDdensity was1.18×10~8cm~（-2）, which was the smallest with the same SiN_xgrowth-time.The optical quality of the epilayer is good, the FWHM value of D~oX peak was1.24 meV. The result of Raman scattering experiments indicated that the compressivestress in GaN epilayers was relaxed effectively.
Due to the sticking coefficients of Ga and N to GaN and SiN_xare about1and0,respectively, when GaN is deposited on the surface that partially covered with SiN_x, itwill grow firstly on the exposed surface of GaN rather than on SiN_x. Along with theincreased growth time, the GaN will grow laterally over the SiN_xregion, just like theprocess of GaN-growth in ELOG, and SiN_xacts as the mask. Thus, the TD densitywas decreased and the stress was relaxed. Furthermore, the SiN_xis preferentiallyformed at the TD cores because of the presence of N-dangling bonds for Si-Nformation, so the SiN_xinterlayer can also blocks some dislocations from entering theupper epilayers. The introduction of in-situ SiN_xhas the advantage of maskless,one-step processing, and the possible contaminations associated with the ex-situmethods can also be eliminated. These advantages are of great importance forimproving the performance of GaN-based devices.
（4）Relevant knowledge of patterned sapphire substrate（PSS）was introducedand the growth mechanism of GaN grown on the PSS was also investigated. Thespecial selective growth phenomenon of GaN was found on the surface of PSS, andthe distribution morphology of GaN changed significantly after the recrystallization athigh temperature. It is thought that the slope of PSS prepared by ICP etchingcomposes of two types of crystal planes, the different arrangement of aluminum andoxygen atoms of corresponding crystal planes leaded to the selective growthphenomenon. A model of the pattern was proposed and the investigation in atom-levelwas carried on. Moreover, the influence of the selective growth phenomenon on thequality of GaN epilayer was also studied. Many big GaN islands were found in theselective zones on the pattern surface, which could hinder the growth of GaN on thepattern and introduce many TD in the epilayer. The effect of PSS on reducing the TDswas influenced negatively.
（5）Wet etching was chosen as a method to suppress the selective growthphenomenon as well as the negative influence of which on the quality of GaN epilayer.During the etching process of NaOH, a porous structure of cone-shaped pattern was found, and the special morphology kept unchanged with increased etching time.When etched by KOH, it is found that the etching rate was very fast. The cone-shapedpattern transformed to a small hexagonal pyramid structure after20S. Mixture of98%H_2SO_4and85%H_3PO_4（H_2SO_4:H_3PO_4=3:1） was chosen as the corrodent,finally. During the etching process, three kinds of etching zones were found. Twoetching zones appeared first and vanished with increasing etching time. The other oneexposed later and expanded gradually. Finally, the cone-shaped pattern with anarcuate slope transformed to a hexagonal pyramid. The calculated orientation of thecrystallographic planes in the two etching zones were {1103} and {43127}, whichwere different from the previous reports.
（6）Anew polyhedral PSS prepared during the wet etching was chosen to growthe GaN epilayer. It is found that the GaN nucleation layer was evenly distributed onthe polyhedral pattern. After recrystallization, though the nucleation layer transformedinto many small GaN grains, the distribution of them was uniform. These observationresults indicated that the selective growth of GaN had been suppressed successfully.And there were no big GaN islands on the pattern surface during the sequent growthof GaN, the epilayer could growth smoothly on the pattern region. The statisticalvalue of TD density was about1.38×10~8cm~(-2)in epilayer grown on the new PSS,which was much smaller than that in epilayer grown on the cone-shaped PSS（about3.4×10~8cm~(-2)）. In addition, the characterization results indicated that the opticalquality was further improved and the residual compressive stress was relaxed moreeffectively by the new PSS.
The discovery and suppression of selective GaN growth and the big GaN islandswas of great importance for further improving the function of PSS. The new methodused in our study for preparing the new PSS provided new ideas for the fabrication ofPSS. The new PSS could improve the quality of GaN epilayer markedly, which hasthe positive significance for the development of the industry of GaN-based device.
引文
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