半导体氮化镓(GaN)材料具有直接带隙、禁带宽度大、热导系数高、击穿电场强度高以及电子迁移率高等优异特性[1], 被广泛应用于电子器件和光电子器件中, 如高电子迁移率晶体管(High Electron Mobility Transistor, HEMT)[2]、发光二极管(Light Emitting Diode, LED)[3⇓-5]、激光二极管(Laser Diode, LD)[6]和紫外探测器(Ultraviolet Photodetector, UV PD)[7]等。目前通过多种外延方式均可获得GaN单晶材料, 其中金属有机化学气相淀积(Metal Organic Chemical Vapor Deposition, MOCVD)具有外延质量好、反应速率快、重复性高和产量大等优点[8], 是制备GaN最常用的方法。
使用MOCVD制备GaN时, 常用异质外延的方式, 其中蓝宝石衬底以成本低、尺寸小和工艺成熟等特点, 成为外延GaN的主要衬底之一。而GaN与蓝宝石衬底之间存在较大的晶格失配和热失配, 由此产生的大量位错会使器件性能退化。因此, 制备高质量GaN时, 常需要氮化铝(AlN)材料作为成核层。AlN成核层为外延GaN提供了与衬底取向相同的成核中心, 同时释放了GaN与衬底间的晶格失配应力和热膨胀失配应力[9], 有利于后续GaN材料的二维(2D)生长。AlN成核层的常用制备方法有MOCVD的两步生长法和磁控溅射法[10], 也可使用分子束外延法[11]。等离子体增强原子层沉积(Plasma-enhanced Atomic Layer Deposition, PEALD)技术也可用于制备AlN薄膜材料[12⇓-14], 其特点是能在真空和低温环境下实现单原子层沉积[15], 从而获得低杂质浓度、高质量的AlN薄膜。这种方法制备的AlN材料已经用于多种半导体器件中[16⇓-18], 但是目前少有将PEALD制备的AlN作为成核层外延生长单晶GaN的报道。本研究提出在蓝宝石上使用PEALD制备AlN来外延生长单晶GaN, 探究不同厚度的AlN作为成核层对GaN外延层质量的影响, 寻找AlN作为成核层的最佳厚度。
1 实验方法
1.1 PEALD制备AlN
本实验使用ALD-150PE设备在c面蓝宝石衬底上沉积AlN, 设置反应温度为200 ℃, 开启射频电源, 沉积工艺如图1所示, 具体步骤如下: 通入0.03 s的TMA→通入30 s的Ar进行吹扫→通入10 s的流量为20 sccm的N2-plasma→通入20 s的Ar进行吹扫。
图1
图1
PEALD工艺制备AlN的示意图
Fig. 1
Schematic diagram of plasma-enhanced atomic layer deposition (PEALD) process of AlN
设置沉积循环周期数, 便可获得不同厚度的AlN。在两寸蓝宝石衬底上循环沉积125、208、292和333周AlN, 分别记为样品1、样品2、样品3和样品4。其中样品1、样品2和样品3用于外延GaN, 样品4用来测试AlN的沉积速率。
1.2 MOCVD外延GaN
MOCVD是一种化学气相沉积技术, 由1968年Manasevit[24]提出的制备化合物半导体单晶薄膜材料的方法发展而来。对于氮化物的制备, MOCVD采用H2或N2作为载气, 把源气瓶中的金属有机化合物(如三甲基镓(TMGa)、三甲基铟(TMIn)和TMA等)传输到反应室, 并与同时到达的NH3在衬底表面发生化学反应, 生成所需氮化物, 剩余气体则通入尾气处理系统。
本工作使用VEECO K465I型号的MOCVD设备, 在沉积AlN后的样品1、样品2和样品3上进行GaN的外延生长, 具体外延工艺条件: 反应温度为1080 ℃, 压力为26660 Pa; 使用TMGa作为反应Ga源, 流量为630 sccm; 使用NH3作为反应N源, 流量为822 sccm; 使用H2和N2作为载气, 流量分别为1644和740 sccm。
利用MOCVD的原位检测系统，可以在反应过程中检测薄膜生长速率等参数, 最终制得6 μm 厚的GaN，样品1、样品2和样品3外延得到的GaN分别记为样品a、样品b和样品c。
2 沉积AlN层表征与讨论
2.1 PEALD制备AlN速率验证
图2
2.2 AlN形貌表征
使用NX20原子力显微镜(Atomic Force Microscope, AFM) 观察沉积AlN后样品的表面形貌, 其1 μm×1 μm尺寸的3D形貌如图3所示。沉积的AlN会在蓝宝石衬底上形成一种岛状的特殊形貌, 这与PEALD工艺制备AlN的报道相同[20]。通过对比三组样品的AFM图, 可以观察到随着沉积厚度增大, 样品表面的小岛形貌变化明显。沉积12.5 nm厚AlN时, 小岛高度和体积较小, 大小和分布较不均匀, 表明开始形成小岛形状; 沉积20.8 nm厚AlN时, 小岛高度和体积适中, 大小和分布较为均匀, 且小岛数量最多; 沉积29.2 nm厚AlN时, 小岛高度和体积较大, 大小和分布较不均匀, 这与小岛之间的合并有关。
图3
图3
AlN的表面形貌
Fig. 3
Surface topographies of AlN
AFM 3D images of (a) sample 1, (b) sample 2 and (c) sample 3
2.3 AlN晶体质量表征
图4
3 外延GaN表征与讨论
3.1 GaN形貌表征
对完成了外延生长GaN的三组样品进行形貌测试, 使用Sigma-300扫描电子显微镜(Scanning Electron Microscopy, SEM)观察三组样品的表面形貌。在2k倍镜下观察三组样品表面形貌, 如图5所示。样品b表面平滑, 而样品a和样品c表面没有合并, 呈现台阶和颗粒状。样品a具有更大的晶粒, 样品c表面布满较小的颗粒, 说明样品a和样品c都有多晶化的趋势。
图5
图5
GaN的SEM照片
Fig. 5
SEM images of GaN
(a) Sample a; (b) Sample b; (c) Sample c
图6
结合图3和图6的AFM图像以及图5的SEM图像结果分析, 样品b具有最好的表面形貌, 说明PEALD制备的20.8 nm厚度的AlN成核层最有利于GaN生长, 获得的GaN表面最平坦。GaN表面形貌的差异与沉积AlN提供的成核机理有关[28], 样品a和样品c表面未合并, 呈现颗粒状。根据AlN薄膜的表面形貌分析可知, 对于样品c, 由于沉积的AlN较厚(29.2 nm), 在蓝宝石衬底上提供了较大的成核岛且成核岛间距较大, 使GaN更倾向于在这些大岛上生长, GaN的三维(3D)生长模式得到增强。对于样品a, 由于沉积的AlN较薄(12.5 nm), 在蓝宝石衬底上刚开始形成成核岛, 成核岛体积较小且数量不多, 成核岛的大小和分布不太均匀, 使GaN生长介于2D生长模式和3D生长模式之间, 因此其表面没有连接起来形成统一的平面。对于样品b, 由于沉积AlN的厚度合适(20.8 nm), 在蓝宝石衬底上形成大小适中、数量最多, 且大小和分布十分均匀的成核岛, 给外延的GaN提供了非常合适的成核点, 因而获得的GaN表面最平坦。
3.2 GaN光学表征
为了研究外延GaN的应力状态, 使用Alpha300RS设备对三组样品进行了拉曼(Raman)测试, 结果如图7所示。无应变GaN的E2(high)峰的频率为567.6 cm-1, 当GaN受到压应力时该散射峰的波数变大[29]。图7可知样品a、样品b和样品c的E2(high)峰频率分别为568.7、569.9和570.4 cm-1; 半峰全宽(Full Width at Half Maximum, FWHM)分别为4.78、3.64和5.57 cm-1。三组样品的E2(high)峰频率都大于567.6 cm-1, 说明所有样品均处于压应力状态, 这是蓝宝石与GaN的热膨胀系数不同所导致的。蓝宝石的热膨胀系数较大, 会对外延的GaN产生压应力[30]。对比样品a、样品b和样品c, 发现沉积AlN越薄, 外延GaN所受到压应力越小。因此调节沉积AlN层的厚度可以改变外延GaN的应力大小。样品b的E2(high)峰的FWHM最小, 表明样品b的结晶质量最好。沿着GaN的[0001]晶向入射Raman光时, 测试光谱中除了有明显的E2(high)峰, 还会在735 cm-1附近出现A1(LO)散射峰。对比三组样品, 从样品b的Raman图谱中还能观察到明显的A1(LO)散射峰, 频率为736.2 cm-1, FWHM为7.74 cm-1, 这也证明样品b具有最好的结晶质量[30-31]。
图7
图7
在不同厚度AlN成核层外延GaN的Raman光谱
Fig. 7
Raman spectra of epitaxial GaN on AlN nucleation layers with different thicknesses
为了研究GaN样品的光学特性, 使用EtaMax光致发光(Photoluminescence, PL)设备对三组GaN进行测试。在激光波长为266 nm, 功率为8 mW的条件下, 三组样品测试结果如图8所示。样品a、样品b和样品c的FWHM分别为14.39、10.94和14.21 nm。样品2的FWHM最小, 表明其具有最好的光学特性, 这与Raman测试结果相符合。
图8
图8
不同厚度AlN成核层的GaN的PL光谱图
Fig. 8
PL spectra of GaN with different thicknesses of AlN nucleation layers
3.3 GaN晶体质量表征
为了研究不同AlN厚度对外延GaN结晶质量的影响, 使用χ'Pert3 MRD高分辨X射线衍射(High Resolution X-ray Diffraction, HRXRD)仪对样品进行测试, 获得GaN材料的摇摆曲线。在纤锌矿结构中, 螺位错密度(Screw Dislocation Density, Dscrew)与(002)晶面上摇摆曲线FWHM的平方成正比, 刃位错密度(Edge Dislocation Density, Dedge)与(102)晶面上摇摆曲线FWHM的平方成正比, 总位错密度(Total Dislocation Density, Dtotal)为二者之和, 具体表达式如式(1)所示[32]。
式(1)中b(002)和b(102)分别是(002)晶面和(102)晶面的伯格斯矢量(Burgers Vector), 取值分别为5.185×10-8和3.189×10-8 cm。对样品b进行测试, 结果如图9所示。样品b的(002)晶面摇摆曲线FWHM为681 arcsec, 计算得Dscrew为9.30×108 cm-2；(102)晶面摇摆曲线FWHM为1265 arcsec, 计算得Dedge为8.48×109 cm-2, 所以Dtotal为9.41×109 cm-2。
图9
图9
样品b不同晶面的摇摆曲线
Fig. 9
Rocking curves of sample b on different crystal planes
(a) (002); (b) (102)
对样品a和样品c进行测试, 结果如图10所示。由于样品a和样品c表面呈现颗粒状, 所以二者(002)晶面摇摆曲线的FWHM特别大, 分别为12197和21034 arcsec, 根据式(1)计算得到Dscrew分别为2.98×1011和8.87×1011 cm-2, 比样品b的Dscrew高了3个数量级, 说明样品b的晶体质量远优于样品a和样品c。此外, 样品a和样品c的(102)晶面摇摆曲线无法测出, 也说明样品a和样品c晶体质量不佳。总之, 三组样品的HRXRD的测试结果表明样品b的GaN晶体质量最好。
图10
图10
样品a(a)和样品c(b)(002)晶面的摇摆曲线
Fig. 10
Rocking curves of (002) crystal plane for sample a (a) and sample c (b)
4 结论
本研究在蓝宝石衬底上使用PEALD设备沉积的AlN作为成核层外延了单晶GaN，实验中制备了三组不同AlN厚度的样品作为对照，探究了厚度对外延GaN的影响。在对AlN的测试中发现，沉积AlN厚度的变化会导致AlN表面成核岛的大小和分布情况发生变化，进一步导致后续外延GaN的质量发生变化。测试结果表明，对于较薄(12.5 nm)和较厚(29.2 nm)的AlN成核层，AlN成核岛的尺寸差异较大并且分布不均匀，不利于外延单晶GaN，最终导致其上外延GaN的表面尚未合并。当AlN成核层厚度为20.8 nm时，AlN成核岛的尺寸合适并分布均匀，有利于外延单晶GaN，其上外延的GaN表明平坦，Rms为0.272 nm，并且具有更好的光学特性，其晶体质量更是得到极大提升，Dscrew相比于另外两组样品降低了3个数量级。总之，本研究实现了在PEALD设备沉积AlN上单晶GaN的外延生长，该技术为获取高质量氮化物了提供了一种新的方法。
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