第42卷㊀第11期2021年11月
发㊀光㊀学㊀报
CHINESE JOURNAL OF LUMINESCENCE
Vol.42
No.11
Nov.2021
㊀㊀收稿日期:2021-08-30；修订日期:2021-09-10
㊀㊀基金项目：国家重点研发计划(2017YFB0404201)；国家自然科学基金(61774147,61874108)资助项目
Supported by National Key Rearch and Development Program of China(2017YFB0404201);National Natural Science Founda-tion of China(61774147,61874108)
清晨与黄昏
Article ID :1000-7032(2021)11-1739-09
Temperature Dependence and
消息范文Evolution Mechanism of Aluminum Nitride Morphologies
NIU Hui-dan 1,2,KONG Su-su 1,2,YANG Shao-yan 1,2∗,LIU Xiang-lin 1∗,
WEI Hong-yuan 1,YAO Wei-zhen 1,LI Hui-jie 1,
CHEN Qing-qing 1,2,WANG Lian-shan 1,2,WANG Zhan-guo 1
(1.Key Laboratory of Semiconductor Materials Science,Institute of Semiconductors,Chine Academy of Sciences,Beijing 100083,China;
2.Center of Materials Science and Optoelectronics Engineering,University of Chine Academy of Sciences,Beijing 100049,China)
温庭筠更漏子
∗Corresponding Authors,E-mail:sh-yyang @mi.ac;xlliu @mi.ac
Abstract :Aluminum nitride(AlN)is a significant ultra-wide bandgap miconductor material.This paper studies
the surface morphology evolution and growth mechanism of AlN grown on sapphire substrates by hydride vapor pha epitaxy(HVPE).The morphologies of AlN are controlled by the nitridation pre-treatment and the growth temperature from 750ħto 1100ħ.The results show that growth temperature played a critical role in the AlN growth of mor-phology and growth mode.The difference in nanoscale or microscale morphologies of AlN is attributed to the surface migration of Al adatoms dominated by the growth temperature and the evolution of the dislocation.Moreover,the sur-face morphology evolution leads to an inverted pyramid morphology or large V-shaped pits at the growth temperature of 900ħ.The grown V-shaped pits have {10-11}mi-polar facets and follow the three-dimensional (3D )growth
mode.The mi-polar facets AlN structure could be ud for realizing facet-controlled epitaxial of mi-polar UV-LED or other Ⅲ-nitride growth,which has prospects in optoelectronic and electronic devices.
Key words :ultra-wide bandgap miconductor;aluminum nitride;hydride vapor pha epitaxy;growth tempera-ture;surface morphology
CLC number :O482.31㊀㊀㊀Document code :A㊀㊀㊀DOI :10.37188/CJL.20210287
取款英文
温度对氮化铝表面形貌的调控及演化机理
牛慧丹1,2，孔苏苏1,2，杨少延1,2∗，刘祥林1∗，魏鸿源1，姚威振1，李辉杰1，陈庆庆1,2，汪连山1,2，王占国1
(1.中国科学院半导体研究所半导体材料科学重点实验室，北京㊀100083;
2.中国科学院大学材料与光电研究中心，北京㊀100049)
摘要：氮化铝(AlN)是一种重要的超宽禁带半导体材料㊂本文研究了采用氢化物气相外延(HVPE)方法生
长氮化铝的表面形貌演化和生长机理㊂AlN 的制备过程是氮化处理后以700~1100ħ的不同温度生长，得到四组不同温度下的表面形貌㊂结果表明，生长温度对AlN 的生长形貌和生长模式具有重要的影响㊂AlN 的生长形貌体现在纳米尺度和微米尺度的形貌差异，该结果归因于受生长温度主导的Al 原子的表面迁移能力和
1740㊀发㊀㊀光㊀㊀学㊀㊀报第42卷
位错演化㊂另外，在900ħ生长温度下得到具有倒金字塔结构的V坑形貌㊂V坑面为{10-11}半极性面，
并遵循三维(3D)生长模式㊂这种具有半极性面微观形貌的AlN可作为模板进行半极性紫外LED器件结构或其他Ⅲ族氮化物外延生长，在光电子器件和电子器件研制方面具有广阔的应用前景㊂
关㊀键㊀词：超宽禁带半导体材料；氮化铝；氢化物气相外延；生长温度；表面形貌
1㊀Introduction
AlN is a significant ultra-wide bandgap mi-conductor material with a direct bandgap of6.2eV, which has great prospects for deep ultraviolet optoe-lectronic devices and power electronic devices[1-4]. Due to the matching lattice and thermal expansion coefficient,AlN is the promising substrate material or the buffer layer for the epitaxial growth of group Ⅲ-nitrides devices[5-8].However,it is difficult to obtain high-quality single-crystalline,and crack-free AlN with low threading dislocation density(TDD). Hydride vapor pha epitaxy(HVPE)is a promising method ud to synthesize groupⅢ-nitride materials on foreign substrates[9-10].Many advantages include simple equipment,rapid growth,large-area growth, and reducing dislocation density.However,some problems still need to be overcome to improve the AlN epitaxial growth quality.For example,the tem-perature of HVPE growth was limited to1200ħdue to the melt temperature of quartz tube reactor[11-12]. Many rearchers have focud on the crystal-line quality and surface morphology of AlN[13-18]. The growth condition卡通小鱼
s have a remarkable influence on the properties and morphologies of groupⅢ-ni-tride materials for electrical and optoelectronic de-vices[19-21].Among them,V-shaped pit defects have been found to improve the electroluminescence prop-erties ofⅢ-nitride light-emitting diodes(LEDs)[22]. Rathkanthiwar et al.demonstrated that the V-shaped pit morphological defects influenced the dark current and spectral responsivity for AlGaN UV-B photode-tectors with the AlN buffer layer[23].Li por-ted that V-shaped pits in AlGaN MQWs on AlN tem-plates might affect optical influences for DUV emis-sion[24].The V-shaped pit is a common structural defect in groupⅢ-nitrides epitaxial layer,which is like an inverted hexagonal pyramid with six{10-11}side facets extending from the(0001)plane[25]. This surface pit is usually obtained under high tem-perature or lowⅤ/Ⅲratio,originating from the threading dislocations.It is important to figure out the evolution of the V-shaped pits on the AlN surface and the behavior of how to ari the{10-11}mi-polar facets.
In this work,the surface morphology evolution of AlN growth was investigated bad on different growth temperatures from750ħto1100ħ.The AlN grew on c-plane sapphire substrates via HVPE equipment.By tuning the growth temperature,the AlN epitaxial layers were different in nanoscale to microscale morphologies.The V-shaped pits were generated by the evolution of dislocation and the co-alescence of the grains at900ħ.A surface diffu-sion-bad mechanism explained the formation of
the microscopic structure.During the AlN epitaxial growth process,the migration of Al adatoms would significantly influence the quality and surface mor-phology of AlN growth.Due to the lower surface mobility of Al adatoms,the short diffusion length of Al adatoms resulted in3D islands growth on the AlN surface morphology.Moreover,the growth model has a transition from3D to2D mode in a higher tem-perature by enhancing the diffusion length of Al ada-toms.
2㊀Experiments
c1驾照科目一模拟考试题
2.1㊀Materials Growth
The AlN samples were grown in our homemade HVPE horizontal growth equipment.The HVPE re-actor consisted of two independent heated zones:the source zone and the growth zone.At first,HCl gas flew over metal Al boat forming AlCl3as Al source in the source zone at550ħ.Next,ammonia(NH3) gas was ud as the N source.AlN was grown on2-inch sapphire wafers with the c-plane surface at the
㊀第11期㊀㊀NIU Hui-dan,et al.:Temperature Dependence and Evolution Mechanism of Aluminum Nitride Morphologies1741㊀
high-temperature growth zone.During the AlN growth process,nitrogen(N2)was introduced to the reactor as carrier gas.The growth zone for AlN was limited to a range from750ħto1100ħ.The to-tal gas flow rate was3600cm3/min(standard cubic centimeter per minute)to keep the pressure at1.33ˑ104Pa(100Torr).
The detailed growth process is as follows.Heat treatment in hydrogen was conducted at1100ħfor 10min,removing contaminants from the surface of sapphire substrates.Then,the nitridation process was pretreated by NH3gas flow for3min.During the growth step,the reaction chamber was heated to the t temperature.The NH3gas flow was t at 500cm3/min,and the HCl gas flow was t at100 cm3/min to grow the AlN epitaxial layer for60min, in which theⅤ/Ⅲratio is about15.When the growth process finished,the reactor was allowed to cool down naturally to room temperature flowing NH3 and N2gas.
2.2㊀Materials Characterization
The morphology of samples was studied using scanning electron microscopy(SEM:FEI NOVA NanoSEM650).The crystal structures were charac-terized by X-ray diffraction analysis(XRD:Rigaku SmartLab,Cu Kα)and Raman spectroscopy(Lab-RAM HR Evolution,532nm solid state lar as the excitation source)at room temperature.3㊀Results and Discussion
3.1㊀Growth Process and Mechanism of AlN AlN samples were grown at various tempera-tures from750ħto1100ħafter pre-treatment ni-tridation,the morphologies shown in Fig.1.When the growth temperature was750ħ,the AlN dis-played a high-density nanoneedle array perpendicu-lar to the sapphire substrate,as shown in Fig.1(a). The diameter of nanoneedles gradually decread along with the c-axis orientation.While the tempera-ture was800ħ,the AlN nanocolumns were found, as shown in Fig.1(b).The diameter of AlN was significantly larger compared with the sample of750ħ.However,both AlN of750ħand800ħwere still one-dimensional nanoscale structures.When the growth temperature incread to900ħ,the AlN changed to hexagonal micro-pillars,as shown in Fig.1(c),forming a V-shaped pit on the top of the micro-pillar simultaneously.While the temperature further reached1000ħ,the AlN merged as a sin-gle-crystalline thick film,as shown in Fig.1(d). The growth conditions of AlN under1050ħand 1100ħwere also provided in Fig.1(e)and(f). The AlN growth in HVPE is a combination of atom transportation and surface reaction.In a large Ⅴ/Ⅲratio growth system,the diffusion of Al pre-cursor plays an important role in the growth of AlN[26]
.
Fig.1㊀SEM images of the AlN samples grown at different temperatures.(a)AlN nanoneedles were grown at750ħ.(b)AlN nanocolumns were grown at800ħ.(c)AlN micro-pillars with hexagonal V-shaped
pits were grown at900ħ.(d)AlN islands merged at1000ħ.(e)AlN islands were grown at1050ħ.(f)AlN islands were grown at1100ħ.
1742㊀发㊀㊀光㊀㊀学㊀㊀报第42卷
A surface diffusion-bad mechanism[27-28]was corre-lated with the diffusion length of Al adatoms,which was directly influenced by growth temperature. When the growth temperature was higher,the diffu-sion length of Al adatoms should be farther.The chemical reaction rate on the surface was very fast actually,in which the precursors reaching the sub-strate were reacting quickly and actively to grow. Hence,the AlN growth was dependent on the mass transport,namely the transportation rate of the pre-cursors to the substrate surface.
The AlN growth followed the vapor-solid(VS) growth model[29]including three stages of gas trans-port,gas-pha reaction and surface reaction.In the nitridation pre-treatment process,the O atoms at the sapphire surface were substituted by the N atoms from NH3.In the gas transport process,the Al source of AlCl3reacted from HCl vapor with Al was carried by N2gas.And then,AlCl3and NH3would mix to flow in the high-temperature growth zone[30]. Macromolecules[AlN]n(n=integer)prent with Al N bond as the backbone developed into nano-clusters for gas-pha reaction.The nanoclusters
waved
in gas-pha were adsorbed or aggregated to sub-strate surface,forming a certain density of randomly distributed nuclei. Subquently,the N or Al atoms or their mole-cules were absorbed into the nuclei for the2D step and3D island growth models.In the surface ada-toms diffusion process,the adatoms migration deter-mined the size of isolated island and the microcosmic morphology.Hence,the surface reaction decided the growth conditions of AlN.Gas-pha and sur-face reactions were mutually reinforcing steps for the AlN growth.And the growth temperature had a powerful influence on the two steps.Such as the low-temperature growth,the macromolecules[AlN]n have a higher polymerization degree in the gas pha[31-32].So the size of nuclei stopped growing larger in the initial nucleation process due to the poor surface diffusion mobility of Al adatoms. During the epitaxial growth process,the migra-tion of Al adatoms would significantly influence the quality and surface morphology of AlN growth.The difference in growth rates along2D step growth and 3D island growth resulted the various micro-nano structures of AlN.The temperature provided the thermal motivations for the nuclei to coverage and form grains,which influenced the size and density of crystal grains[33].At a relatively low temperature,the migration of Al atoms was around the limited area of the substrate surface due to the strong binding ener-gy of Al N bonds.The Al N bond strength is the intrinsic molecular property,regardless of tempera-ture.When the growth condition was at a high tem-perature,the larger thermal motivation resulted in the enlarged diameter of nuclei and the lo
wer nucle-ation density.In addition,the whole AlN tended to transition from3D islands to2D layer epitaxy.The schematic illustration of AlN growth about the VS growth model is shown in Fig.2(a).There formed AlN nanoneedles at750ħ.The density of nuclei on substrate surface and the lateral growth of AlN were not sufficient to coalesce into a whole film.The growth temperature over800ħin the reaction chamber was high enough to motivate the AlN nuclei to gather into hexagonal grains.The Raman spectra of AlN samples with different growth temperatures are shown in Fig.2(b),forming the AlN micro-nano structures.It displays the c-facet strain of AlN samples.With the temperature increasing,the E2 (high)mode near655cm-1was right shift at900ħcompared to the result at800ħ.The shift is due to the in-plane compression biaxial stress of AlN that leads to the changes in phonon frequencies[34-35]. Hence at750ħand800ħ,the nanometers diame-ter surface is relatively small,relieving the stress caud by lattice mismatch with the sapphire sub-strate.With the further increa of temperature to 900ħ,the compressive stress was produced when the AlN pillars were aggregating into large islands, and then the top surface was split into V-shaped pits.
In contrast,AlN grown at1000ħexhibited continuous film morphology.Some irregular mound-like structures on the substrate were caud by the large viscosity coefficient of Al atoms becau the empty Al N bonds were stronger to catch Al atoms.
全国青少年网络文明公约
㊀第11期㊀㊀NIU Hui-dan,et al.:Temperature Dependence and Evolution Mechanism of Aluminum Nitride Morphologies 1743
㊀Fig.2㊀(a)Schematic illustration of the VS growth model for nitridation pre-treatment method in different growth temperatures,
forming the AlN micro-nanostructures.(b)Raman spectra of AlN samples with different growth temperatures.
The anisotropic crystal structure and chemical bonds determined the competitiveness of the preferred growth.When the mobility of Al atoms was larger at 1000ħ,the Al atoms easily enhanced the lateral growth of AlN to form crystal grains.Then the grains
sharing a similar crystal orientation tended to con-verge with each other.The combination of adjacent
grains was motivated by reducing broken bonds of the interface to lower the whole surface energy.The corresponding X-ray diffraction (XRD )patterns are in Fig.3,showing a prominent(0002)peak of the wurtzite AlN along with an obvious c -axis orienta-tion.So the growth behavior about the preferred ori-entation is along {0001}direction of AlN epilayers at different growth temperatures.According to Van der Drift s competition mechanism for growth orien-
tation,the [0001]direction of AlN,perpendicular to the substrate,has the fastest growth rate [36].Due to their different dangling band density and crystal lattice,this growth behavior was mainly controlled by kinetics.Also,the high temperature could in-crea the diffusion length of N,Al atoms,and their clusters towards the surface energy equilibrium.The orientated AlN grains,perpendicular to the
sapphire
Fig.3㊀XRD pattern of AlN samples at varied growth temperatures with int of FWHM.(a)750ħ.(b)800ħ.(c)900ħ.
(d)1000ħ.(e)1050ħ.(f)1100ħ.
