ZnO薄膜和异质结构的光电性能研究
摘要
氧化锌(ZnO)是一种直接宽带隙半导体材料,具有很高的激子束缚能(60meV)。ZnO具有优越的光电特性,在蓝紫光发光器件、紫外探测器件和短波长激子型激光器等方面具有广阔的应用前景。目前,ZnO基材料与器件的研究已取得了较大的进展,发展ZnO基材料与器件已成为当前半导体科学技术研发的热点。
本论文针对目前ZnO材料研究所面临的基础与前沿问题,积极开展了N掺杂ZnO薄膜、ZnMgO合金以及ZnMgO/ZnO异质结构的生长与光电性质的研究,揭示了异质结界面极化效应导致的二维电子气的输运特性和光荧光性质。主要取得了以下成果：
1、研究了退火温度对氮掺杂ZnO光电性能的影响,霍尔(Hall)效应测量结果表明合适的高温退火可以导致N掺杂ZnO薄膜导电类型的转变,有利于实现N掺杂p型ZnO薄膜。研究发现MOCVD生长中非故意掺杂的碳原子在N掺杂ZnO薄膜中存在多种形态,高温退火将导致杂质碳团簇的聚集和碳团簇尺寸的显著增大,并将显著改善ZnO薄膜的表面形貌和晶体结构,有利于ZnO薄膜质量的改善和提高。
2、开展了N掺杂ZnO薄膜的光学性质的研究,首次在N掺杂ZnO薄膜中观察到具有精细结构的由多声子峰组成的绿带发光峰,由变温光致发光谱的变化规律揭示出该绿带与近带边的施主束缚激子(D0X)及其双电子伴线(TES)之间变化规律的内在关联,进一步指认该绿带发光来自于施主基态和激发态到深受主VZn之间的跃迁,并从该施主-受主复合体和晶格之间较强的电声子耦合分析解释了绿带多级声子峰的形成机理。
3、研究了ZnMgO合金的生长与性质对MOCVD生长工艺参数的依赖关系,研究表明采用ZnO衬底、高的Mg/Zn气相比、较大生长压力有助于提高Mg原子的结合效率。研究发现高Mg组分生长或较高温度生长的ZnMgO合金的结构从单一六方相向混合相甚至单一立方相转变,但进一步的高温退火技术却可以导致立方相结构向六方相为主的混合相的转变,为探索高质量ZnMgO合金的MOCVD的生长提供了一条思路。
4、研究了ZnMgO合金层的结构参数对ZnMgO/ZnO异质结的光电性质的影响,揭示了ZnMgO/ZnO异质结界面极化效应形成的二维电子气(2DEG)的输运性质对ZnMgO合金层厚度的依赖关系。研究发现ZnMgO合金层的厚度和组分分布对异质结的光致发光谱具有显著的影响,并根据激光在薄膜中的穿透深度对光致发光谱的变化给予了成功的分析和解释。首次观测到了ZnMgO/ZnO低温光致发光谱中与2DEG复合发光相关且具有较小激活能的三个发光峰,并分别指认为2DEG中电子与ZnMgO/ZnO异质界面附近不同空间位置的空穴之间的复合跃迁。
Zinc oxide (ZnO) is a direct wide band gap semiconductor with a large exciton binding energy of 60meV. Owing to its superior optoelectronic properties, ZnO is a promising candidate host for the application in violet-blue light emitting diodes, ultraviolet photodetectors and shortwavelength exciton-based lasers. Recently, the development in ZnO-based materials and devices has made significant progresses. Upon all these aspects, the researches on ZnO materials and devices have become a very active field in the modern semiconductor development.
In this thesis, focusing on the basis and forefront study of ZnO material research, nitrogen-doped ZnO films, ZnMgO alloys and growth and optoelectronic properties on ZnMgO/ZnO heterostrustures have been studied in detail. Moreover, transport properties and photoluminescence properties of two-dimensional electron gas (2DEG) formed at the interface due to polarization effects have been investigated. The main results include the following:
1、The influence of annealing temperature on optical and electrical properties of nitrogen-doped ZnO films has been investigated. High-temperature annealing is found to cause a transition of the conductive type of ZnO films verified from the Hall Effect measurement. Unintentionally doped carbon atoms are found to exist at different forms in the N-doped ZnO films grown by MOCVD growth method. High-temperature annealing also lead the aggregation of carbon clusters with its size significantly increased and notable improvement of ZnO film surface morpholohy and crystal structure attained.
2、The optical properties of N-doped ZnO films has been investigated by temperature-dependent photoluminescence. For the first time, the green band with fine structures due to multi-phonon emission was observed in the N-doped ZnO films. The temperature-dependent photoluminescence spectra do reveal the correlations between the greenband and donor bound exciton (D°X) with its two-electron satellite (TES). Greenband was identified to the transitions between ground state and excited state of shallow donor to the deep acceptor Vzn·Finally, the fine structures with multi-phonon peaks observed in the green band was ascribed to the strong coupling of the electrons and the phonons in high quality N-doped ZnO films.
3、The influence of MOCVD growth parameters on ZnMgO alloys and properties have been investigated systematically. ZnO substrate, large Mg/Zn gas ratio and higher growth pressure are found to be favorable to grow high Mg composition ZnMgO alloy. High flow rate of Mg MO source or high growth temperature may cause the formation of ZnMgO alloy with mix-phase or single cubic phase. However, it's interesting to find that rapid thermal annealing may cause the cubic phase transformed to mixed phase with obvious hexagonal phase observed in ZnMgO alloy, which is in favor of high-quality ZnMgO alloys by MOCVD technique.
4、The influence of ZnMgO alloy structural parameters on the optoelectronic properties of ZnMgO/ZnO heterostrustures has been investigated. The dependence of the transport properties of two-dimentional electron gas (2DEG) formed due to polarization effects on the ZnMgO caplayer thickness has been revealed. The thickness and composition distribution of ZnMgO alloy layer shows obvious influence on photoluminescence, due to limited penetration depth of the excited laser in the ZnO based film. For the first time, three additional 2DEG-related emissions with small activation energy have been observed in low-temperature photoluminescence spectra. These emissions are reasonablely assigned to the transitions from 2DEG electrons to the photoexcited holes existed in different areas near the heterointerface respectively.
本论文针对目前ZnO材料研究所面临的基础与前沿问题,积极开展了N掺杂ZnO薄膜、ZnMgO合金以及ZnMgO/ZnO异质结构的生长与光电性质的研究,揭示了异质结界面极化效应导致的二维电子气的输运特性和光荧光性质。主要取得了以下成果：
1、研究了退火温度对氮掺杂ZnO光电性能的影响,霍尔(Hall)效应测量结果表明合适的高温退火可以导致N掺杂ZnO薄膜导电类型的转变,有利于实现N掺杂p型ZnO薄膜。研究发现MOCVD生长中非故意掺杂的碳原子在N掺杂ZnO薄膜中存在多种形态,高温退火将导致杂质碳团簇的聚集和碳团簇尺寸的显著增大,并将显著改善ZnO薄膜的表面形貌和晶体结构,有利于ZnO薄膜质量的改善和提高。
2、开展了N掺杂ZnO薄膜的光学性质的研究,首次在N掺杂ZnO薄膜中观察到具有精细结构的由多声子峰组成的绿带发光峰,由变温光致发光谱的变化规律揭示出该绿带与近带边的施主束缚激子(D0X)及其双电子伴线(TES)之间变化规律的内在关联,进一步指认该绿带发光来自于施主基态和激发态到深受主VZn之间的跃迁,并从该施主-受主复合体和晶格之间较强的电声子耦合分析解释了绿带多级声子峰的形成机理。
3、研究了ZnMgO合金的生长与性质对MOCVD生长工艺参数的依赖关系,研究表明采用ZnO衬底、高的Mg/Zn气相比、较大生长压力有助于提高Mg原子的结合效率。研究发现高Mg组分生长或较高温度生长的ZnMgO合金的结构从单一六方相向混合相甚至单一立方相转变,但进一步的高温退火技术却可以导致立方相结构向六方相为主的混合相的转变,为探索高质量ZnMgO合金的MOCVD的生长提供了一条思路。
4、研究了ZnMgO合金层的结构参数对ZnMgO/ZnO异质结的光电性质的影响,揭示了ZnMgO/ZnO异质结界面极化效应形成的二维电子气(2DEG)的输运性质对ZnMgO合金层厚度的依赖关系。研究发现ZnMgO合金层的厚度和组分分布对异质结的光致发光谱具有显著的影响,并根据激光在薄膜中的穿透深度对光致发光谱的变化给予了成功的分析和解释。首次观测到了ZnMgO/ZnO低温光致发光谱中与2DEG复合发光相关且具有较小激活能的三个发光峰,并分别指认为2DEG中电子与ZnMgO/ZnO异质界面附近不同空间位置的空穴之间的复合跃迁。
Zinc oxide (ZnO) is a direct wide band gap semiconductor with a large exciton binding energy of 60meV. Owing to its superior optoelectronic properties, ZnO is a promising candidate host for the application in violet-blue light emitting diodes, ultraviolet photodetectors and shortwavelength exciton-based lasers. Recently, the development in ZnO-based materials and devices has made significant progresses. Upon all these aspects, the researches on ZnO materials and devices have become a very active field in the modern semiconductor development.
In this thesis, focusing on the basis and forefront study of ZnO material research, nitrogen-doped ZnO films, ZnMgO alloys and growth and optoelectronic properties on ZnMgO/ZnO heterostrustures have been studied in detail. Moreover, transport properties and photoluminescence properties of two-dimensional electron gas (2DEG) formed at the interface due to polarization effects have been investigated. The main results include the following:
1、The influence of annealing temperature on optical and electrical properties of nitrogen-doped ZnO films has been investigated. High-temperature annealing is found to cause a transition of the conductive type of ZnO films verified from the Hall Effect measurement. Unintentionally doped carbon atoms are found to exist at different forms in the N-doped ZnO films grown by MOCVD growth method. High-temperature annealing also lead the aggregation of carbon clusters with its size significantly increased and notable improvement of ZnO film surface morpholohy and crystal structure attained.
2、The optical properties of N-doped ZnO films has been investigated by temperature-dependent photoluminescence. For the first time, the green band with fine structures due to multi-phonon emission was observed in the N-doped ZnO films. The temperature-dependent photoluminescence spectra do reveal the correlations between the greenband and donor bound exciton (D°X) with its two-electron satellite (TES). Greenband was identified to the transitions between ground state and excited state of shallow donor to the deep acceptor Vzn·Finally, the fine structures with multi-phonon peaks observed in the green band was ascribed to the strong coupling of the electrons and the phonons in high quality N-doped ZnO films.
3、The influence of MOCVD growth parameters on ZnMgO alloys and properties have been investigated systematically. ZnO substrate, large Mg/Zn gas ratio and higher growth pressure are found to be favorable to grow high Mg composition ZnMgO alloy. High flow rate of Mg MO source or high growth temperature may cause the formation of ZnMgO alloy with mix-phase or single cubic phase. However, it's interesting to find that rapid thermal annealing may cause the cubic phase transformed to mixed phase with obvious hexagonal phase observed in ZnMgO alloy, which is in favor of high-quality ZnMgO alloys by MOCVD technique.
4、The influence of ZnMgO alloy structural parameters on the optoelectronic properties of ZnMgO/ZnO heterostrustures has been investigated. The dependence of the transport properties of two-dimentional electron gas (2DEG) formed due to polarization effects on the ZnMgO caplayer thickness has been revealed. The thickness and composition distribution of ZnMgO alloy layer shows obvious influence on photoluminescence, due to limited penetration depth of the excited laser in the ZnO based film. For the first time, three additional 2DEG-related emissions with small activation energy have been observed in low-temperature photoluminescence spectra. These emissions are reasonablely assigned to the transitions from 2DEG electrons to the photoexcited holes existed in different areas near the heterointerface respectively.
引文
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