PECVD-prepared high-quality GaN films and their photoresponse properties

Jing-Yang Zhang,Ru-Zhi Wang,Meng-Qi Yang,Qi Liang

doi:10.7498/aps.71.20211922

Abstract

In this study, the high-quality GaN films are prepared by a simple, green and low-cost plasma enhanced chemical vapor deposition (PECVD) method at 950 ℃, with Ga2O3 and N2 serving as a gallium source and a nitrogen source, respectively. In order to improve the crystal quality of GaN films and ascertain the photoresponse mechanism of GaN films, the effect of the preparation temperature of GaN buffer layer on the crystal quality and photoelectric properties of GaN thin films are investigated. It is indicated that with the increase of the buffer temperature of GaN films, the crystal quality of GaN films first increases and then decreases, and the highest crystal quality is obtained at 875 ℃. When buffer layer temperature is 875 ℃, the calculated total dislocation density is 9.74 × 109 cm–2, and the carrier mobility is 0.713 cm2·V–1·s–1. The crystal quality of GaN film after being annealed is improved. The total dislocation density of GaN film decreases to 7.38 × 109 cm–2, and the carrier mobility increases to 43.5 cm2·V–1·s–1. The UV-Vis absorption spectrum results indicate that the optical band gap of GaN film is 3.35 eV. The scanning electron microscope (SEM) results indicate that GaN film (buffer layer temperature is 875 ℃) has smooth surface and compact structure. The Hall and X-ray photoelectron spectroscopy (XPS) results indicate that there are N vacancies, Ga vacancies or O doping in the GaN film, which act as deep level to capture photogenerated electrons and holes. With the bias increasing, the photoresponsivity of the GaN film photodetector gradually increases and then reaches a saturation value. This is due to the deep levels produced by vacancy or O doping. In addition, photocurrent response and recovery of GaN film are slow, which is also due to the deep levels formed by vacancy or O doping. At 5-V bias, the photoresponsivity of GaN film is 0.2 A/W, rise time is 15.4 s, and fall time is 24 s. Therefore, the high-quality GaN film prepared by the proposed green and low-cost PECVD method present a strong potential application in ultraviolet photodetector. The PECVD method developed by us provides a feasible way of preparing high-quality GaN films, and the understanding of the photoresponse mechanism of GaN films provides a theoretical basis for the wide application of GaN films.

Highlights

图 3 不同制备温度缓冲层的未退火 GaN 薄膜的 Raman 光谱图。 Figure 3 Raman spectra of unannealed GaN films with buffer layer fabricated at different temperature. 表 2 不同制备温度缓冲层的未退火 GaN 薄膜相对应的 E2(high)声子散射峰半高
图 7 缓冲层制备温度为 875°C的退火 GaN 薄膜的 Raman 光谱图。 Figure 7 Raman spectra of annealed GaN fim with buffer layer at 875 °C. 图 8 为缓冲层制备温度为 875°C的退火 GaN 薄膜的微观区域 SEM 图。从图 8a 中我们可以看出,GaN 薄膜表面没有针孔存在,表面平坦、光滑,粗糙度小。图 8b 为 GaN 薄膜的截面图,缓冲层与 GaN 薄膜生长层总厚度为 1.8 μm,GaN 薄膜内部也没有孔隙存在,生长密实,断口整齐,也说明了 GaN 薄膜具有光滑平坦的表面。结果表明,GaN 薄膜具有光滑平坦的表面和良好结晶质量。图 8c 为缓冲层制备温度为 875°C的退火 GaN 薄膜样品的实物图。样品为淡黄色,薄膜表面光滑,无针孔和黑点。
图 9 GaN 薄膜的 XPS 图谱:(a)全谱,(b) )N 1s 带,(c) Ga 2P3/2 带,(d) O 1s 带。 Figure 9 XPS spectra of annealed GaN fim with buffer layer at 875 °C: (a) general scan spectrum, (b) N 1s band, (c) Ga 2P3/2 band, (d) O 1s band

Summary

PECVD 法制备高结晶 GaN 薄膜及其光电响应性能研究*

在本工作中,采用一种简单、绿色、低成本的等离子增强化学气相沉积 (PECVD)法,在 950 °C下成功制备了高结晶质量的 GaN 薄膜。为了提高 GaN 薄膜结晶质量和弄清 GaN 薄膜光响应机制,我们研究了 GaN 缓冲层制备温度对 GaN 薄膜结晶质量和光电性能的影响。研究表明,随着 GaN 缓冲层制备温度的增加,GaN 薄膜的结晶质量先提高后降低,在缓冲层温度为 875 °C 时,结晶质量最高,此时计算得出的总位错密度为 9.74 109 cm-2,载流子迁移率为 0.713 cm2/V·s。经过退火后,GaN 薄膜的总位错密度降低到 7.38 109 cm-2,载流子迁移率增大到 43.5 cm2/V·s,此时 GaN 薄膜光响应度为 0.20 A/W,光响应时间为 15.4 s,恢复时间为 24 s,可应用于紫外光探测器。通过 Hall 测试和 XPS 分析得出,GaN 薄膜内部存在着 N 空位、Ga 空位或 O 掺杂,它们作为深阱能级束缚和复合光生电子和空穴,使得光响应度与偏压呈抛物线关系;另外,空位和 O 掺杂形成的深阱能级也是导致 GaN 薄膜的光电流响应和恢复缓慢的根本原因。. 改善 GaN 薄膜的缓冲层表面形貌,能有效改善 GaN 薄膜的结晶质量。例如, Yang 等人[9]在 AlN 为缓冲层上制备了结晶质量良好的 GaN 薄膜,研究得出, AlN 缓冲层厚度的增加,AlN 薄膜的表面粗糙度明显减小,从而使得 GaN 薄膜表面粗糙度从 11.5 nm 减小到 2.3 nm,氮化镓薄膜的结晶质量和表面形貌都得到了改善。Okuno 等人[10]采用金属有机化学气相沉积法在氧化铝衬底上首先生长 AlN 缓冲层,然后在缓冲层上生长 GaN 薄膜,研究得出,AlN 缓冲层的表面粗糙度小,GaN 薄膜的结晶质量会提高。还有 Bak 等人[11]研究表明,随着 AlN 缓冲层粗糙度变大,GaN 薄膜的结晶质量是先提高后降低的。. 图 3 为不同制备温度缓冲层的未退火 GaN 薄膜的 Raman 光谱图。各未退火 GaN 薄膜样品的 E2(high)声子散射峰都对应为 574 cm-1, 通过经验公式σ= /4.3(cm-1.GPa-1)计算得出内应力大小为 1.4GPa[18]。研究表明,GaN 薄膜 Raman 光谱的 E2(high)声子散射峰半高宽与其结晶质量有关,E2(high)声子散射峰半高宽越宽,GaN 薄膜结晶质量越差[19,20,21]。因此,我们列出了各未退火 GaN 薄膜样品的 E2(high)声子散射峰半高宽,具体数值如表 2 所示。根据表 2 的对应关系,我们可以看出, 随着 GaN 薄膜缓冲层温度的增加,E2(high)声子散射峰半高宽先从 18.3cm-1 减小到 10 cm-1,然后增加到 25.2 cm-1;说明 GaN 薄膜随着缓冲层制备温度的增加,结晶质量先增加,然后降低。. 图 3 不同制备温度缓冲层的未退火 GaN 薄膜的 Raman 光谱图。 Figure 3 Raman spectra of unannealed GaN films with buffer layer fabricated at different temperature. 图 3 不同制备温度缓冲层的未退火 GaN 薄膜的 Raman 光谱图。 Figure 3 Raman spectra of unannealed GaN films with buffer layer fabricated at different temperature. 表 2 不同制备温度缓冲层的未退火 GaN 薄膜相对应的 E2(high)声子散射峰半高

GaN 薄膜样品

Ga LMM

GaN 薄膜制备方法

Fall curve Fitted