ZnTe薄膜和GaN基异质结构的制备及光学特性
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摘要
以ZnO、ZnSe和ZnTe为代表的Ⅱ-Ⅵ化合物半导体和以GaN材料为代表的Ⅲ-Ⅴ族氮化物半导体材料(包括AlN、GaN、InN及其合金)在光电器件的应用上获得了快速的发展。Ⅲ-Ⅴ族氮化物具有优良的光电性质,稳定的化学性质,使其可在高温、酸碱、辐射环境下使用。它们都是直接带隙材料且带隙跨越很大,可从InN的0.7eV到AlN的6.28eV,在蓝、绿光和紫外波段的光电子器件方面应用广泛。ZnTe作为宽禁带半导体材料在绿光器件,光伏器件方面有广泛的应用,它既可以作为绿光LED的优良材料,又可以作为CdTe太阳能电池的背接触层。同时ZnTe具有优良相位匹配特性,已成为最常用和最适宜的发射和探测太赫兹辐射的材料。虽然近年来在ZnTe相关材料的器件的理论研究和实际应用等方面都有了较大的发展。但在ZnTe外延材料的制备,尤其是相关器件的开发中存在着光学性能较低以及使用寿命较短等诸多问题。而这些问题主要是起因于ZnTe材料的结构质量,如难以得到质量优异的n型掺杂的ZnTe,以及如何有效减小衬底与外延层的晶格失配及热失配等。而ZnTe内部存在的杂质、缺陷和应力等则是影响材料质量和性能的主要因素。探索生长高质量ZnTe外延材料和ZnTe基异质结构等的制备工艺,探求表征其结构质量的有效手段是半导体发光领域内非常有应用价值的一个研究课题。
本论文主要以光致发光谱(PL)的测试为主,并配合其他测试手段,如X射线衍射(XRD)、原子力显微镜(AFM)和拉曼光谱等测试对ZnTe和GaN相关材料的结构特性和光学特性等进行了研究。
以二甲基锌(DMZn)和二乙基碲(DETe)为金属有机源,氢气为载气,用金属有机气相外延系统在(100)面GaAs衬底上制备出ZnTe外延层。实验前先将所用的抛光处理的GaAs衬底在有机溶剂中清洗,去除表面的油脂,然后在60℃的H2SO4+H2O2+H2O (5:1:1)的溶液中刻蚀20秒钟,最后用去离子水冲洗并用氮气吹干。为了去除GaAs衬底表面的自然氧化膜,在外延生长之前先在580℃下热退火处理30分钟。在有机源和载气引入生长室之前,MOVPE系统使用涡轮分子泵可以抽到10-7Torr量级的本底真空度。生长过程中DMZn和DETe维持在15μmol/min。样品的生长温度在390-440℃的范围内可调。为了比较衬底温度对样品性质的影响,实验中通过改变生长时间控制ZnTe外延层的厚度。实验中所用样品的厚度均为10μm左右。
对制备的ZnTe异质外延层进行了AFM、XRD和PL测试,研究了衬底温度对ZnTe外延层的形貌特征、结构性质及光学性质的影响。PL谱测试采用442nm线He-Cd激光器作为激发光源,样品放置于闭循环液氦恒温室中,测试温度范围为6-300K,激发功率在0.001-30mW之间连续可调。PL测试结果表明,衬底温度对ZnTe异质外延层的结晶质量和光学性质均有影响。对生长温度在390-440℃之间的所有ZnTe外延层,其PL谱中均为束缚激子峰I。占支配地位,且都未出现结构缺陷引起的Y线、施主受主对(DAP)及氧束缚激子(OBE)发光峰等深能级跃迁,说明外延层具有很好的光学质量。通过对比不同生长温度外延层的PL谱发现,过低或者过高的生长温度会降低外延层的质量。这是因为过低的生长温度导致反应前体的表面动能不足,容易形成缺陷从而降低结晶质量。而生长温度过高又会导致衬底中的杂质原子(As等)容易扩散进入外延层中,并且容易形成空位,同样会使外延层质量下降。因此确定中等衬底温度(420℃)是生长高质量ZnTe外延层的最佳温度。XRD测量结果表明ZnTe外延层和GaAs衬底具有相同的(100)晶面取向,二者存在着确定的外延关系。ZnTe外延层(400)面的XRC测量结果表明,生长温度在420℃左右的ZnTe外延层的XRC半高宽最小,具有最好的结晶质量。PL谱和XRC测量结果的一致说明PL光谱可以用来对外延层质量进行表征。通过PL峰随激发功率和外延层厚度变化关系,表明随着厚度增加外延层中张应力逐渐变小。
利用MOVPE方法在(100)ZnnTe衬底上制备了同质外延层,并研究了反应室压力对外延层光学性质的影响。实验结果表明,反应室压力对ZnTe同质外延层的结晶质量和光学性质有显著影响。反应室压力为500Torr时生长的的ZnTe外延层具有平整光滑的表面,说明此时处于质量输运限制生长模式和表面动能限制生长模式的过渡转换区域。低的反应室压力(300Tort)生长的ZnTe外延层,其PL谱中自由激子峰占支配地位,未出现明显的Y线、DAP发光峰及OBE等深能级跃迁,说明外延层具有很好的光学质量。而高反应室压力(700Torr)生长的ZnTe外延层,其低温PL谱中的发光成分主要是与杂质有关DAP发光及束缚激子发光,自由激子发光比较弱,这说明增大反应室压力导致ZnTe外延层的光学质量变差。这是因为在低反应室压力下,由于前体的反应速率较低,吸附在表面的反应物有足够的时间沿表面扩散,寻找合适的位置,这可以促进反应物的晶化。随着反应室压力的增加,生成外延层的前体的反应速率明显增大,前体到达表面很快发生结合,容易出现空位和并入杂质。同时,低反应室压力可以消除或减小气相中的寄生成核反应,还可以减少来自衬底的自掺杂。所有这些因素导致了低反应室压力生长的ZnTe外延层具有更好的光学特性和结晶质量。而XRC测量结果也证明了低反应室压力下外延层的结晶质量更高。另外,我们还对ZnTe衬底和ZnTe外延层的质量进行了比较,PL谱和XRC测量表明高反应室压力生长的外延层,其晶体质量比衬底差,而低反应室压力(300Torr)生长的外延层,其质量优于衬底。这说明低压MOVOPE技术是生长高质量半导体外延材料的重要途径。
GaN及其相关化合物以其在蓝光到近紫外波段的光电器件以及高功率、高频和高温晶体管等电子器件的广泛应用而备受关注。但因为缺乏高质量的GaN晶体和外延层,对基本的带边发射,如带带发射、激子发射以及能带到杂质的发射等复合跃迁,仍然没有完全掌握。因此制备高质量GaN外延层及相关结构,并对其发光机理进行深入研究,对提高GaN基发光电器件的理论知识和设计能力都有重要的意义。
采用MOVPE方法在c面蓝宝石衬底上制备了GaN外延层及AlN/GaN异质结构。利用X射线衍射、激光拉曼谱和PL光谱等表征手段对制备的样品结构、应力及光学性质进行了研究。对GaN外延层,研究了带边发光峰随激发功率和温度的变化规律,明确了带边发光峰的种类及起源。GaN外延层中,低温下中性施主束缚激子发光占主导地位,同时还可以观察到清晰的自由激子精细结构,窄的激子峰半高宽表明GaN外延层具有很高的结晶质量。随着温度的升高,束缚激子发生热解离,其强度迅速降低,而自由激子具有较大的结合能,其强度下降得较慢。温度在150K以上,PL谱中自由激子发光占支配地位。通过PL光谱中峰位与外延层残余应力的关系,对GaN外延层中的应力进行了估算,结果表明外延层中由晶格失配引起的应力已经弛豫掉,可以不用考虑。外延层中的残余应力主要是源于衬底与外延层的热膨胀系数差异引起的压应力。这个结果与通过拉曼测量估算出的外延层应力符合的很好。
对AlN/GaN异质结构,研究了AlN厚度的变化对该结构的电学、光学性质以及应力的影响。霍尔效应测量结果显示,AlN厚度较小(3nm)的AlN/GaN异质结构中,室温下的载流子面密度和霍尔迁移率分别为1013cm-2和1720cm2/V.s,1OK时的霍尔迁移率高达9500cm/V.s。而AlN厚度增大为6nm后,该结构的载流子面密度虽有增大,但其霍尔迁移率却明显降低。这是因为AlN厚度增大会导致应力释放引起结构质量的下降。对A1N厚度较小(3nm)的AlN/GaN异质结构,其PL谱中主要是GaN带边激子发光,而对AlN厚度较大(6nm)的AlN/GaN异质结构,其PL谱中除GaN带边激子发光外,还出现了较强的紫外发光带UVL,与该发光带对应的受主能级可能与GaN层中镓空位有关,这说明AlN的厚度影响了GaN层的光学质量。拉曼谱的测量结果表明AlN厚度较大(6nm)的AlN/GaN异质结构中存在更小的压应力,说明该样品中出现了应力的释放。因此要制备高载流子面密度和高迁移率的AlN/GaN异质结构,应选择合适的AlN垒层厚度。
Both Ⅲ-Ⅴ nitride semiconductor material (including AlN, GaN, InN and their alloys) represented by GaN and Ⅱ-Ⅵ compound semiconductor such as ZnO, ZnSe and ZnTe, have obtained a rapid development in the application of optoelectronic devices.Ⅲ-Ⅴ nitride can be used in high temperature, acid, alkali and adiation environment due to the excellent optoelectronic properties and chemical stability. They are direct band gap material and cover a large band gap range (from InN of0.7to AlN of6.28eV). As a result, they have a wide range of applications in blue, green and ultraviolet optoelectronic device. As a broad band gap semiconductor ZnTe is widely used in green devices and photovoltaic devices. It can serve as an excellent material for a green LED and the back contact layer CdTe solar cells. Meanwhile, because of excellent phase matching characteristics, ZnTe has become the most common and appropriate material of terahertz emitter and receiver. In recent years, the theoretical research and practical application of ZnTe-related materials develop very quickly. However, both the preparation of ZnTe epitaxial material and the application of the related devices exist many difficulties such as low optical performance, short service life and so on. All these problems are mainly caused by the quality of ZnTe material, such as difficult to obtain n-type ZnTe of good quality and reduce lattice and thermal mismatch between substrate and epilayer. Therefore, it is an important issue to explore the growth technology of high quality ZnTe and ZnTe-based heterostructure and seek characterization means of the structural quality on semiconductor optoelectronic field.
In this paper, photoluminescence is used as the main measurement means with other measurement methods for the supplementary to investigate structural and optical properties of ZnT and GaN, such as X-ray diffraction (XRD), atomic force microscopy (AFM) and Raman spectroscopy.
ZnTe epilayers were grown by the home-built MOVPE at atmospheric pressure on (100) GaAs substrates with different growth temperatures. DMZn and DETe were used as source materials and H2as carrier gas. Polished (100) GaAs substrates were chemically cleaned, degreased in organic solvents, and then etched in a solution composed of H2SO4+H2O2+H2O (5:1:1) for about20s at60℃followed by deionized water rinse. In order to remove the native oxide on the surface of GaAs substrate, it was cleaned by thermal annealing at580℃for30min before the growth. The growth system was evacuated to the order of10-7Torr with a turbomolecular pump before the source materials and the carrier gas were bled into the chamber. The transport rates of DMZn and DETe were kept at15μmol/min. The substrate temperature changed from390to440℃. To discuss the substrate temperature dependence, the thickness of all ZnTe epilayers in our experiment was controlled at around10μmby changing growth time.
Morphology, structural and optical properties of epitaxial layers were studied. For PL measurements, the samples were mounted in a closed-cycle He cryostate and the temperature was controlled from6to300K. The442nm line of a He-Cd laser was used as an excitation light source with the spot size of～250μm, and the excitation power changed from0.1μW to30mW. The PL results show that the substrate temperature affects both crystalline quality and optical properties of of ZnTe heteroepitaxial layer. The PL spectra of ZnTe epitaxial layer grown at between390and440℃are dominated by sharp excitonic emission at around2.371eV. Meanwhile, Y line, DAP emission peak and OBE transition are absent in all ZnTe epilayers. It indicates ZnTe epilayers have a good optical quality. However, by comparing the spectra of the epilayers, it is found that a too low (390℃) or too high (440℃) substrate temperature deteriorates epilayer quality. This is because the substrate temperature, especially the too low substrate temperature (390℃), may lead to the formation of defects or the inclusion of impurities easily. Moreover, the too high substrate temperature (440℃) also results in the diffusion of the As atom into the epilayer and the inclusion of impurities easily. The As atoms are found to mainly distribute near the epilayer/substrate interface. In contrast, a moderate substrate temperature (around420℃) is considered to be suitable to obtain high-quality ZnTe epilayer due to its narrowest linewidths of the excitonic emission peaks and strongest emission intensity among the epilayers. Further, XRC measurement results are consistent with PL measurement results. It indicates PL measurements can be used to characterize the quality of the epitaxial layer. The energy of PL peaks changes with the variation of excitation power and epitaxial layer thickness, which manifests that the stress in ZnTe epilayer reduces with increasing epilayer thickness.
ZnTe homoepitaxial layers were prepared on (100) ZnTe substrate by MOVPE and the effect of reactor pressure on the optical properties of the epilayers were investigated. Experimental results show that reactor pressure have a significant effect on crystal quality and optical properties of the ZnTe homoepitaxial layer. The PL spectra of ZnTe epilayer grown at low reactor pressure (300Torr) were dominated by the free exciton recombination. The Y line, DAP emission peak and OBE deep-level transition were not observed in the spectra, showing that epitaxial layers are of good optical quality. For ZnTe epilayers grown at high reactor pressure (700Torr), the PL spectra were dominated by impurity-related emissions. The reactor pressure dependences of the PL spectrum and XRC of the ZnTe epilayer manifest that reducing reactor pressure results in the enhancement of the free excitonic emission relative to the impurity-related emissions as well as the decrease of XRC linewidth, indicating that the reducing reactor pressure can improve the PL property and crystallinity of ZnTe epilayer. The facts can be explained as that the adsorbed precursors have enough time to diffuse over the surface to find its proper position at low reactor pressure due to the low reaction rate of the precursors, thus facilitating recrystallization of the adsorbed precursors. At the same time, the growth process at low reactor pressure also results in the elimination of parasitic nucleations in the gas phase and the reduction of autodoping. In addition, we also compared the quality of ZnTe substrate and the homoepitaxial layer. The results show that ZnTe epitaxial layer grown at reactor pressure of700Torr has poorer quality compared with the substrate. However, the low reactor pressure (300Torr) is appropriate for the homoepitaxial growth with good crystal quality.
GaN and related heterostructures have been studied in last decade due to their possible applications for optoelectronic devices performing in the spectral region from the blue to near-UV and in electronic devices such as high power, high temperature and high frequency transistor. Great efforts have recently been devoted to the understanding of its optical properties. However, the investigation on its fundamental band edge transitions, such as the band-to-band, exciton, and band-to-impurity transitions, are still not well understood due to the lack of high quality GaN crystals and epilayers. Therefore, it is vital to understand the fundamental properties of GaN and the design of optoelectronic devices through the preparation of high quality GaN related structure and the investigation of the luminescence mechanism.
GaN epitaxial layers and AIN/GaN heterostructures were fabricated on c-plane sapphire substrates by MOVPE. We studied the properties of the sample using PL spectra, Raman spectroscopy and X-ray diffraction. According to power and temperature dependences of PL specrta of GaN epitaxial layer, we defined the origin of near band edge emission peaks. At low temperature, neutral donor bound exciton emission dominates the PL spectrum and free exciton fine structure can be observed, showing that GaN epitaxial layer has high crystal quality. Bound excitons release with increasing temperature, so its strength rapidly reduce. At the same time, the intensity of free exciton declines slowly than that of bound exciton due to larger binding energie. Above150K, free exciton dominated the PL spectra of GaN epilayer. We also estimate the stress in GaN epilayer and the results indicate that GaN epitaxial layer exists compressive stress derived from the thermal mismatch between the substrate and the epitaxial layer. Hall measurement results show that the2DEG density and Hall mobility of AIN/GaN heterostructure with3nm-AIN layer at room temperature are1013cm-2and1720cm2/V.s, respectively. At10K Hall mobility is as high as9500cm/V.s. For AIN/GaN heterostructure with6nm-AIN layer, although the2DEG density increases, the Hall mobility reduces significantly. This may be because the quality of AIN/GaN heterostructure deteriorates with increasing thickness of AlN layer. We also studied the effects of the thickness of AlN layer on optical properties of GaN layer. For AIN/GaN heterostructure with3nm-AIN layer, the PL spectrum is dominated by GaN band edge exciton emissions. However, AIN/GaN heterostructure with6nm-AIN layer, in addition to GaN band edge exciton emissions, strong DAP emission appears in the PL spectrum, showing the thickness of AlN layer affects the optical quality of GaN layer. The measurement results have an important significance to improve the design and performance of AIN/GaN-based electronic devices.
本论文主要以光致发光谱(PL)的测试为主,并配合其他测试手段,如X射线衍射(XRD)、原子力显微镜(AFM)和拉曼光谱等测试对ZnTe和GaN相关材料的结构特性和光学特性等进行了研究。
以二甲基锌(DMZn)和二乙基碲(DETe)为金属有机源,氢气为载气,用金属有机气相外延系统在(100)面GaAs衬底上制备出ZnTe外延层。实验前先将所用的抛光处理的GaAs衬底在有机溶剂中清洗,去除表面的油脂,然后在60℃的H2SO4+H2O2+H2O (5:1:1)的溶液中刻蚀20秒钟,最后用去离子水冲洗并用氮气吹干。为了去除GaAs衬底表面的自然氧化膜,在外延生长之前先在580℃下热退火处理30分钟。在有机源和载气引入生长室之前,MOVPE系统使用涡轮分子泵可以抽到10-7Torr量级的本底真空度。生长过程中DMZn和DETe维持在15μmol/min。样品的生长温度在390-440℃的范围内可调。为了比较衬底温度对样品性质的影响,实验中通过改变生长时间控制ZnTe外延层的厚度。实验中所用样品的厚度均为10μm左右。
对制备的ZnTe异质外延层进行了AFM、XRD和PL测试,研究了衬底温度对ZnTe外延层的形貌特征、结构性质及光学性质的影响。PL谱测试采用442nm线He-Cd激光器作为激发光源,样品放置于闭循环液氦恒温室中,测试温度范围为6-300K,激发功率在0.001-30mW之间连续可调。PL测试结果表明,衬底温度对ZnTe异质外延层的结晶质量和光学性质均有影响。对生长温度在390-440℃之间的所有ZnTe外延层,其PL谱中均为束缚激子峰I。占支配地位,且都未出现结构缺陷引起的Y线、施主受主对(DAP)及氧束缚激子(OBE)发光峰等深能级跃迁,说明外延层具有很好的光学质量。通过对比不同生长温度外延层的PL谱发现,过低或者过高的生长温度会降低外延层的质量。这是因为过低的生长温度导致反应前体的表面动能不足,容易形成缺陷从而降低结晶质量。而生长温度过高又会导致衬底中的杂质原子(As等)容易扩散进入外延层中,并且容易形成空位,同样会使外延层质量下降。因此确定中等衬底温度(420℃)是生长高质量ZnTe外延层的最佳温度。XRD测量结果表明ZnTe外延层和GaAs衬底具有相同的(100)晶面取向,二者存在着确定的外延关系。ZnTe外延层(400)面的XRC测量结果表明,生长温度在420℃左右的ZnTe外延层的XRC半高宽最小,具有最好的结晶质量。PL谱和XRC测量结果的一致说明PL光谱可以用来对外延层质量进行表征。通过PL峰随激发功率和外延层厚度变化关系,表明随着厚度增加外延层中张应力逐渐变小。
利用MOVPE方法在(100)ZnnTe衬底上制备了同质外延层,并研究了反应室压力对外延层光学性质的影响。实验结果表明,反应室压力对ZnTe同质外延层的结晶质量和光学性质有显著影响。反应室压力为500Torr时生长的的ZnTe外延层具有平整光滑的表面,说明此时处于质量输运限制生长模式和表面动能限制生长模式的过渡转换区域。低的反应室压力(300Tort)生长的ZnTe外延层,其PL谱中自由激子峰占支配地位,未出现明显的Y线、DAP发光峰及OBE等深能级跃迁,说明外延层具有很好的光学质量。而高反应室压力(700Torr)生长的ZnTe外延层,其低温PL谱中的发光成分主要是与杂质有关DAP发光及束缚激子发光,自由激子发光比较弱,这说明增大反应室压力导致ZnTe外延层的光学质量变差。这是因为在低反应室压力下,由于前体的反应速率较低,吸附在表面的反应物有足够的时间沿表面扩散,寻找合适的位置,这可以促进反应物的晶化。随着反应室压力的增加,生成外延层的前体的反应速率明显增大,前体到达表面很快发生结合,容易出现空位和并入杂质。同时,低反应室压力可以消除或减小气相中的寄生成核反应,还可以减少来自衬底的自掺杂。所有这些因素导致了低反应室压力生长的ZnTe外延层具有更好的光学特性和结晶质量。而XRC测量结果也证明了低反应室压力下外延层的结晶质量更高。另外,我们还对ZnTe衬底和ZnTe外延层的质量进行了比较,PL谱和XRC测量表明高反应室压力生长的外延层,其晶体质量比衬底差,而低反应室压力(300Torr)生长的外延层,其质量优于衬底。这说明低压MOVOPE技术是生长高质量半导体外延材料的重要途径。
GaN及其相关化合物以其在蓝光到近紫外波段的光电器件以及高功率、高频和高温晶体管等电子器件的广泛应用而备受关注。但因为缺乏高质量的GaN晶体和外延层,对基本的带边发射,如带带发射、激子发射以及能带到杂质的发射等复合跃迁,仍然没有完全掌握。因此制备高质量GaN外延层及相关结构,并对其发光机理进行深入研究,对提高GaN基发光电器件的理论知识和设计能力都有重要的意义。
采用MOVPE方法在c面蓝宝石衬底上制备了GaN外延层及AlN/GaN异质结构。利用X射线衍射、激光拉曼谱和PL光谱等表征手段对制备的样品结构、应力及光学性质进行了研究。对GaN外延层,研究了带边发光峰随激发功率和温度的变化规律,明确了带边发光峰的种类及起源。GaN外延层中,低温下中性施主束缚激子发光占主导地位,同时还可以观察到清晰的自由激子精细结构,窄的激子峰半高宽表明GaN外延层具有很高的结晶质量。随着温度的升高,束缚激子发生热解离,其强度迅速降低,而自由激子具有较大的结合能,其强度下降得较慢。温度在150K以上,PL谱中自由激子发光占支配地位。通过PL光谱中峰位与外延层残余应力的关系,对GaN外延层中的应力进行了估算,结果表明外延层中由晶格失配引起的应力已经弛豫掉,可以不用考虑。外延层中的残余应力主要是源于衬底与外延层的热膨胀系数差异引起的压应力。这个结果与通过拉曼测量估算出的外延层应力符合的很好。
对AlN/GaN异质结构,研究了AlN厚度的变化对该结构的电学、光学性质以及应力的影响。霍尔效应测量结果显示,AlN厚度较小(3nm)的AlN/GaN异质结构中,室温下的载流子面密度和霍尔迁移率分别为1013cm-2和1720cm2/V.s,1OK时的霍尔迁移率高达9500cm/V.s。而AlN厚度增大为6nm后,该结构的载流子面密度虽有增大,但其霍尔迁移率却明显降低。这是因为AlN厚度增大会导致应力释放引起结构质量的下降。对A1N厚度较小(3nm)的AlN/GaN异质结构,其PL谱中主要是GaN带边激子发光,而对AlN厚度较大(6nm)的AlN/GaN异质结构,其PL谱中除GaN带边激子发光外,还出现了较强的紫外发光带UVL,与该发光带对应的受主能级可能与GaN层中镓空位有关,这说明AlN的厚度影响了GaN层的光学质量。拉曼谱的测量结果表明AlN厚度较大(6nm)的AlN/GaN异质结构中存在更小的压应力,说明该样品中出现了应力的释放。因此要制备高载流子面密度和高迁移率的AlN/GaN异质结构,应选择合适的AlN垒层厚度。
Both Ⅲ-Ⅴ nitride semiconductor material (including AlN, GaN, InN and their alloys) represented by GaN and Ⅱ-Ⅵ compound semiconductor such as ZnO, ZnSe and ZnTe, have obtained a rapid development in the application of optoelectronic devices.Ⅲ-Ⅴ nitride can be used in high temperature, acid, alkali and adiation environment due to the excellent optoelectronic properties and chemical stability. They are direct band gap material and cover a large band gap range (from InN of0.7to AlN of6.28eV). As a result, they have a wide range of applications in blue, green and ultraviolet optoelectronic device. As a broad band gap semiconductor ZnTe is widely used in green devices and photovoltaic devices. It can serve as an excellent material for a green LED and the back contact layer CdTe solar cells. Meanwhile, because of excellent phase matching characteristics, ZnTe has become the most common and appropriate material of terahertz emitter and receiver. In recent years, the theoretical research and practical application of ZnTe-related materials develop very quickly. However, both the preparation of ZnTe epitaxial material and the application of the related devices exist many difficulties such as low optical performance, short service life and so on. All these problems are mainly caused by the quality of ZnTe material, such as difficult to obtain n-type ZnTe of good quality and reduce lattice and thermal mismatch between substrate and epilayer. Therefore, it is an important issue to explore the growth technology of high quality ZnTe and ZnTe-based heterostructure and seek characterization means of the structural quality on semiconductor optoelectronic field.
In this paper, photoluminescence is used as the main measurement means with other measurement methods for the supplementary to investigate structural and optical properties of ZnT and GaN, such as X-ray diffraction (XRD), atomic force microscopy (AFM) and Raman spectroscopy.
ZnTe epilayers were grown by the home-built MOVPE at atmospheric pressure on (100) GaAs substrates with different growth temperatures. DMZn and DETe were used as source materials and H2as carrier gas. Polished (100) GaAs substrates were chemically cleaned, degreased in organic solvents, and then etched in a solution composed of H2SO4+H2O2+H2O (5:1:1) for about20s at60℃followed by deionized water rinse. In order to remove the native oxide on the surface of GaAs substrate, it was cleaned by thermal annealing at580℃for30min before the growth. The growth system was evacuated to the order of10-7Torr with a turbomolecular pump before the source materials and the carrier gas were bled into the chamber. The transport rates of DMZn and DETe were kept at15μmol/min. The substrate temperature changed from390to440℃. To discuss the substrate temperature dependence, the thickness of all ZnTe epilayers in our experiment was controlled at around10μmby changing growth time.
Morphology, structural and optical properties of epitaxial layers were studied. For PL measurements, the samples were mounted in a closed-cycle He cryostate and the temperature was controlled from6to300K. The442nm line of a He-Cd laser was used as an excitation light source with the spot size of～250μm, and the excitation power changed from0.1μW to30mW. The PL results show that the substrate temperature affects both crystalline quality and optical properties of of ZnTe heteroepitaxial layer. The PL spectra of ZnTe epitaxial layer grown at between390and440℃are dominated by sharp excitonic emission at around2.371eV. Meanwhile, Y line, DAP emission peak and OBE transition are absent in all ZnTe epilayers. It indicates ZnTe epilayers have a good optical quality. However, by comparing the spectra of the epilayers, it is found that a too low (390℃) or too high (440℃) substrate temperature deteriorates epilayer quality. This is because the substrate temperature, especially the too low substrate temperature (390℃), may lead to the formation of defects or the inclusion of impurities easily. Moreover, the too high substrate temperature (440℃) also results in the diffusion of the As atom into the epilayer and the inclusion of impurities easily. The As atoms are found to mainly distribute near the epilayer/substrate interface. In contrast, a moderate substrate temperature (around420℃) is considered to be suitable to obtain high-quality ZnTe epilayer due to its narrowest linewidths of the excitonic emission peaks and strongest emission intensity among the epilayers. Further, XRC measurement results are consistent with PL measurement results. It indicates PL measurements can be used to characterize the quality of the epitaxial layer. The energy of PL peaks changes with the variation of excitation power and epitaxial layer thickness, which manifests that the stress in ZnTe epilayer reduces with increasing epilayer thickness.
ZnTe homoepitaxial layers were prepared on (100) ZnTe substrate by MOVPE and the effect of reactor pressure on the optical properties of the epilayers were investigated. Experimental results show that reactor pressure have a significant effect on crystal quality and optical properties of the ZnTe homoepitaxial layer. The PL spectra of ZnTe epilayer grown at low reactor pressure (300Torr) were dominated by the free exciton recombination. The Y line, DAP emission peak and OBE deep-level transition were not observed in the spectra, showing that epitaxial layers are of good optical quality. For ZnTe epilayers grown at high reactor pressure (700Torr), the PL spectra were dominated by impurity-related emissions. The reactor pressure dependences of the PL spectrum and XRC of the ZnTe epilayer manifest that reducing reactor pressure results in the enhancement of the free excitonic emission relative to the impurity-related emissions as well as the decrease of XRC linewidth, indicating that the reducing reactor pressure can improve the PL property and crystallinity of ZnTe epilayer. The facts can be explained as that the adsorbed precursors have enough time to diffuse over the surface to find its proper position at low reactor pressure due to the low reaction rate of the precursors, thus facilitating recrystallization of the adsorbed precursors. At the same time, the growth process at low reactor pressure also results in the elimination of parasitic nucleations in the gas phase and the reduction of autodoping. In addition, we also compared the quality of ZnTe substrate and the homoepitaxial layer. The results show that ZnTe epitaxial layer grown at reactor pressure of700Torr has poorer quality compared with the substrate. However, the low reactor pressure (300Torr) is appropriate for the homoepitaxial growth with good crystal quality.
GaN and related heterostructures have been studied in last decade due to their possible applications for optoelectronic devices performing in the spectral region from the blue to near-UV and in electronic devices such as high power, high temperature and high frequency transistor. Great efforts have recently been devoted to the understanding of its optical properties. However, the investigation on its fundamental band edge transitions, such as the band-to-band, exciton, and band-to-impurity transitions, are still not well understood due to the lack of high quality GaN crystals and epilayers. Therefore, it is vital to understand the fundamental properties of GaN and the design of optoelectronic devices through the preparation of high quality GaN related structure and the investigation of the luminescence mechanism.
GaN epitaxial layers and AIN/GaN heterostructures were fabricated on c-plane sapphire substrates by MOVPE. We studied the properties of the sample using PL spectra, Raman spectroscopy and X-ray diffraction. According to power and temperature dependences of PL specrta of GaN epitaxial layer, we defined the origin of near band edge emission peaks. At low temperature, neutral donor bound exciton emission dominates the PL spectrum and free exciton fine structure can be observed, showing that GaN epitaxial layer has high crystal quality. Bound excitons release with increasing temperature, so its strength rapidly reduce. At the same time, the intensity of free exciton declines slowly than that of bound exciton due to larger binding energie. Above150K, free exciton dominated the PL spectra of GaN epilayer. We also estimate the stress in GaN epilayer and the results indicate that GaN epitaxial layer exists compressive stress derived from the thermal mismatch between the substrate and the epitaxial layer. Hall measurement results show that the2DEG density and Hall mobility of AIN/GaN heterostructure with3nm-AIN layer at room temperature are1013cm-2and1720cm2/V.s, respectively. At10K Hall mobility is as high as9500cm/V.s. For AIN/GaN heterostructure with6nm-AIN layer, although the2DEG density increases, the Hall mobility reduces significantly. This may be because the quality of AIN/GaN heterostructure deteriorates with increasing thickness of AlN layer. We also studied the effects of the thickness of AlN layer on optical properties of GaN layer. For AIN/GaN heterostructure with3nm-AIN layer, the PL spectrum is dominated by GaN band edge exciton emissions. However, AIN/GaN heterostructure with6nm-AIN layer, in addition to GaN band edge exciton emissions, strong DAP emission appears in the PL spectrum, showing the thickness of AlN layer affects the optical quality of GaN layer. The measurement results have an important significance to improve the design and performance of AIN/GaN-based electronic devices.
引文
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