摘要
本文利用分子束外延技术在GaAs(211)B衬底上外延CdTe(211)薄膜,系统研究不同工艺条件对CdTe 外延薄膜的表面形貌和光学性质的影响。研究表明,在一定的生长温度下,在Te气氛下生长CdTe薄膜,增加CdTe:Te的束流比,可显著降低CdTe表面金字塔缺陷的尺寸和密度,当CdTe 和Te束流比为6.5时,金字塔缺陷几乎消失,材料的表面平整度显著改善,X射线衍射(XRD)也表明CdTe晶体质量显著提高。进一步的拉曼光谱表明,随着CdTe和Te束流比的增加,Te的A1峰减弱,CdTe LO和TO声子峰强度比增强。低温光致发光光谱(PL)研究也表明随着CdTe和Te束流比的增加,Cd空位的减少可以使与杂质能级相关的深能级区域的峰强降低,与此同时和晶体质量相关的自由激子峰半峰全宽减少,材料的光学质量明显改善。该研究为探索CdTe/GaAs外延材料的理想的工艺窗口以及相关机理,并为进一步以此为缓冲层外延高质量HgCdTe材料提供基础。
Hg1-xCdxTe材料是目前最重要的红外探测材
随着外延技术的发展,CdTe薄膜的生长工艺也逐渐完善。通过优化生长工艺可以降低表面缺陷密度,比如退火、超晶格的手段也可显著降低位错缺陷密度至~1
本文利用分子束外延系统(DCA R450,Turku,Finland)在GaAs(211)B衬底上生长CdTe(211)。利用原位高能电子衍射仪(RHEED)实时监控CdTe 薄膜外延生长过程,利用X射线衍射仪(XRD,Bruker,Karlsruhe,Germany)来表征晶体质量。利用Nomarski显微镜、扫描电子显微镜(SEM,ZEISS,Jena,Germany)和原子力显微镜(AFM, Bruker, Madison, American)进行表面缺陷研究。
在本研究中,为了探索CdTe 外延薄膜在GaAs(211)衬底的工艺窗口,我们改变CdTe 外延薄膜的生长温度,特别是改变 CdTe 和Te 的束流比。
样品 | PCdTe:PTe | T/℃ | 金字塔缺陷密度/c | FWHM/arcsec |
---|---|---|---|---|
样品 01 | 3.3 | 290 |
2.4×1 | 348 |
样品 02 | 3.3 | 275 |
2.6×1 | 297 |
样品 03 | 2.2 | 265 |
4.7×1 | 226 |
样品 04 | 5.3 | 265 |
2.9×1 | 220 |
样品 05 | 6.1 | 265 |
4.9×1 | 309 |
样品 06 | 6.5 | 265 |
<1×1 | 152 |
样品 07 | 8.0 | 265 |
3.2×1 | 315 |
如

图1 (a)CdTe的XRD ω-2θ结果; (b)样品1到7的XRD摇摆曲线
Fig. 1 (a)XRD ω-2θ scan results for CdTe;(b)Sample 1 to 7 XRD rocking curve
lg | (1) |
lg | (2) |

图2 (a)~(g)为CdTe样品1至样品7的SEM照片
Fig. 2 (a)~(g)are SEM photos of CdTe samples 1~ 7
对于265 ℃生长的CdTe(样品3~7),Cd的饱和蒸气压较大更加容易脱附,因此需要更大的CdTe束流使表面组分稳定。又因为生长时采用的是CdTe固态源,CdTe(s)→Cd(g)+0.5Te2(g),PCd1.33PTe

图3 不同CdTe薄膜表面金字塔缺陷密度与束流比之间的关系
Fig. 3 Relationship between the density of pyramidal defects on the surface of different CdTe films and the beam ratio
我们进一步再通过AFM表征金字塔缺陷消失前后样品2和样品6的CdTe表面形貌,

图4 (a)、(d)分别为CdTe样品2和样品6表面的平面图,(b)、(d)分别为CdTe样品2和样品6表面的立体图,(c)、(f)分别为CdTe样品2和样品6表面上任一线上的高度变化曲线
Fig. 4 (a)and(d)are the plans of CdTe sample 2 and sample 6,(b)and(d)are the stereograms of CdTe sample 2 and sample 6, and(c)and(f)are the height change curves on any line on the surface of CdTe sample 2 and sample 6, respectively
为了进一步研究不同工艺条件下生长的CdTe薄膜晶格动力学以及相关的缺陷特性,我们利用拉曼光谱对上述样品进行了系统研究。

图 5 CdTe样品1至7的拉曼光谱
Fig. 5 Raman spectra of CdTe samples 1~ 7
样品 | Te(A1) | TO | LO |
---|---|---|---|
样品 01 |
123.2 c |
138.3 c |
164.4 c |
样品 02 |
122.3 c |
139.3 c |
165.6 c |
样品 03 |
122.6 c |
139.5 c |
165.4 c |
样品 04 |
122.9 c |
139.3 c |
165.4 c |
样品 05 |
122.6 c |
139.3 c |
165.1 c |
样品 06 | / |
139.9 c |
166.7 c |
样品 07 |
122.1 c |
138.3 c |
163.4 c |
为了进一步研究不同工艺条件下生长的CdTe外延薄膜光学性质的影响,我们对相关的CdTe外延薄膜进行低温光致发光光谱(PL)研究。

图 6 CdTe样品2和样品6在9 K下的光致发光光谱
Fig. 6 Photoluminescence spectra of CdTe sample 2 and sample 6 at 9 K
在前面研究中,通过控制生长条件,发现降低生长温度可以使XRD摇摆曲线的半峰全宽降低,但是SEM下观察到的金字塔缺陷密度并没有改变。随后降低生长温度并逐渐增加CdTe的束流会使表面金字塔缺陷密度减少至最后几乎观察不到。利用拉曼光谱和低温PL光谱分析比较缺陷在生长时出现的原因,发现不仅在拉曼光谱上有富Te峰,又在PL光谱中观察到多个Cd空位相关的发光峰。因为相同温度下Cd的饱和蒸汽压更高容易脱附,需要更大的CdTe束流,所以增加CdTe束流可以让这种缺陷消失。上述研究表明在合适的生长温度(265 ℃左右),当CdTe 和 Te束流比降低到为6.5时,CdTe 表面缺陷显著降低,相关材料的光学质量也显著提高。
本文系统研究利用分子束外延技术在GaAs 衬底上外延CdTe(211)薄膜的过程中,生长条件对薄膜表面形貌和光学性质的影响。XRD研究发现降低生长温度能提高结晶质量,而进一步的SEM和AFM的系统研究表明,增加CdTe和Te束流比,可降低表面缺陷的尺寸和密度,降低表面粗糙度。当CdTe 和 Te束流比为6.5时,金字塔缺陷几乎消失,材料的表面平整度显著改善。拉曼光谱研究表明,随着CdTe和Te束流比的增加,Te的A1峰减弱,CdTe LO和TO声子峰强度比增强。低温PL 研究也表明随着束流比的增加,晶体质量相关的自由激子峰半峰全宽减少,材料的光学质量明显改善。该研究为在GaAs(211)衬底外延CdTe高质量薄膜提供理想的工艺窗口以及相关工艺和物性的关联性提供基础。
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