Ⅲ-Ⅴ族氮化物纳米孔材料的制备和应用
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摘要
GaN基半导体材料的研究与应用是目前全球半导体研究的前沿和热点,是研制微电子器件、光电子器件的新型半导体材料,并与SiC、金刚石等半导体材料一起,被誉为是继第一代Ge、Si半导体材料、第二代GaAs、InP化合物半导体材料之后的第三代半导体材料。它具有宽的直接带隙、强的原子键、高的热导率、化学稳定性好(几乎不被任何酸腐蚀)等性质和强的抗辐照能力,在光电子、高温大功率器件和高频微波器件应用方面有着广阔的前景。
刻蚀技术是器件制备工艺中不可缺少的步骤,但由于GaN材料的抗化学腐蚀性,至今没有有效的湿法刻蚀技术。干法刻蚀(ICP)是目前器件制备中常用的刻蚀技术,但这种技术不但昂贵、选择性低,还会损伤GaN材料而影响器件特性；发展一种新的GaN基材料的湿法刻蚀技术非常重要。同时,照明市场对GaN基光电子器件需求量的增加,以及光电子器件面积的增大,急需高性能垂直结构LEDs;而它的制备需要有效的薄膜剥离技术。此外,半极性和非极性GaN基材料具有制备长波长和高性能的光电子器件巨大潜力,但材料中存在着很多缺陷一直困扰着人们。本论文针对这些GaN材料和器件研究领域的热点问题,主要开展了电化学刻蚀剥离GaN薄膜的研究；电化学刻蚀法制备多孔GaN薄膜和大面积、自支撑多孔GaN薄膜的研究；两步法生长半极性(11-22)GaN薄膜、电化学法制备半极性(11-22)多孔GaN薄膜,以及用多孔GaN作衬底生长低缺陷密度的GaN薄膜和InGaN/GaN MQWs的研究。研究发现：
在室温条件下,对n-type GaN横向选择性电化学湿法刻蚀的研究,我们发现当选择不同的刻蚀条件(偏压)和不同硅掺杂浓度的n-GaN样品时,我们可以得到三个不同的区域：没有任何刻蚀区域(Ⅰ),刻蚀形成多孔结构区域(Ⅱ)和完全刻蚀区域(Ⅲ)。这为GaN基器件制备提供了很大的可用空间。
利用n-type GaN横向选择性电化学湿法刻蚀技术,我们成功地从蓝宝石衬底上剥离了约1微米厚的大面积(1 x 1 mm2)无裂纹的GaN薄膜。为下一步制备高性能的垂直结构GaN基LEDs打下了坚实的基础。在光学应用方面,利用n-type GaN横向选择性电化学湿法刻蚀技术,我们成功地制备了GaN圆型微盘(microdisk)和DBR结构,并对GaN圆型微盘(microdisk)的光学特性进行了表征。GaN圆盘形光学微腔(微盘)(mirodisk)具有均匀分布的光学微腔模式,这些光学模的半高宽远远小于GaN带边峰的半高宽(～5nm)。GaN圆盘形光学微腔(微盘)(mirodisk)的质量因子大约为2800,激射阈值大约为7nJ。最后,我们验证了n-type GaN横向选择性电化学湿法刻蚀技术在MEMS方面的应用,制备出了GaN“桥”和悬臂梁结构,并对GaN悬臂梁结构的振动频率进行了测量和计算,测量和计算结果基本一致,GaN悬臂梁结构的中心振动频率大约在120 kHz。我们相信,n-type GaN横向选择性电化学湿法刻蚀技术将会给Ⅲ-Ⅴ氮化物材料和器件领域带来意想不到的影响。
其次,我们用电化学湿法刻蚀法制备了多孔GaN薄膜结构；研究发现外加偏压和n-GaN的硅掺杂浓度在电化学刻蚀中是两个非常重要的参数。通过改变这两个参数我们可以控制多孔GaN材料的孔尺寸、孔洞率、孔密度等特性,还可以控制电化学湿法刻蚀n-GaN的刻蚀速率。同一硅掺杂浓度的n-GaN,多孔GaN材料的孔尺寸和孔洞率随外加偏压的增加而增大,多孔GaN材料的孔密度和刻蚀速率随外加偏压是先增大,当达到某一值后开始下降。在恒定外加偏压下,多孔GaN材料的孔尺寸随α-GaN的硅掺杂浓度的增加而减小,而孔洞率、孔密度和刻蚀速率随n-GaN的硅掺杂浓度的增加而增加。我们可以在2英寸GaN样品上制备出非常均匀的多孔GaN薄膜。并且讨论了电化学法制备多孔GaN薄膜的机制,我们初步认为由于外加电场的存在使GaN薄膜表面的空穴迁移到GaN和草酸溶液界面与草酸溶液中电子复合,使GaN得到氧化变成GaOx,GaOx溶于草酸溶液中而形成GaN材料的刻蚀,关于电化学刻蚀n-GaN的机制需要更进一步深入的研究。
根据电化学刻蚀制备多孔GaN薄膜材料的特性,我们用两种不同的工艺方法制备了大面积、自支撑多孔GaN薄膜。工艺A为两步法：用均匀硅掺杂浓度的n-GaN,第一步用低外加偏压形成低孔洞率的多孔GaN层,第二步增加外加偏压形成高孔洞率的多孔GaN层,最终把低孔洞率的多孔GaN层剥离下来。工艺B中,我们用具存不同硅掺杂浓度的n-GaN层,上层为低硅掺杂浓度的n-GaN层,下层为高硅掺杂浓度的n+-GaN;在同一外加偏压下,它们分别形成不同孔洞率和孔密度的多孔GaN结构,随刻蚀时间的增加,上层多孔GaN将自动剥离下来。我们把制备的大面积、自支撑的多孔GaN薄膜转移到了不同的衬底上例如玻璃,硅和聚二甲基硅氧烷(PDMS)等。
电化学湿法刻蚀剥离技术制备大面积多孔GaN薄膜的另一优点是我们可以重复利用剥离后的衬底。我们在剥离大面积、自支撑多孔GaN薄膜后的衬底上生长了GaN薄膜材料,发现在剥离多孔GaN薄膜后的衬底上生长的GaN薄膜具有与常规衬底上生长的GaN薄膜同样的质量特性。
最后,我们对半极性(11-22)GaN的生长,半极性(11-22)多孔GaN的制备,以及利用多孔GaN插入层生长低缺陷密度的(11-22)GaN进行了系统的研究并发现：
两步法MOCVD生长可以在m面Al2O3衬底上获得微结构提高的半极性(11-22)GaN薄膜。m面A1203衬底在NH3气氛下的高温氮化处理是获得(11-22)GaN薄膜的关键。适当的A1N缓冲层厚度在两步法提高(11-22)GaN薄膜质量的生长过程中同样起到重要的作用。生长AlN缓冲层后,第一步高压生长(11-22)GaN形成岛状表面结构(三维生长模式),第二步低压生长增加(11-22)GaN薄膜的横向生长速率(二维生长模式),使(11-22)GaN薄膜中的一些位错线弯曲互相作用而湮灭。优化后的两步法生长可以使(11-22)GaN薄膜的XRCs半高宽降低一半左右。
电化学刻蚀法制备了半极性(11-22)纳米孔GaN薄膜。半极性(11-22)纳米孔GaN的特性与极性(0002)纳米孔GaN非常相似。同一硅掺杂浓度的(11-22)n-GaN,多孔GaN材料的孔尺寸和孔洞率随着外加偏压的增加而增大,多孔GaN材料的刻蚀速率随外加偏压的增大,先增大,当达到某一电压值后又开始下降。与PEC刻蚀获得的纳米孔(11-22)GaN相比,电化学刻蚀制备的纳米孔GaN薄膜没有面选择性,纳米孔的形成与外加偏压和样品的硅掺杂浓度有关。
用电化学法制备的半极性(11-22)多孔GaN作插入层可以生长低缺陷密度的(11-22)GaN薄膜。生长过程中,由于高温下的质量转移特性,多孔GaN层的截面结构从树枝状或平行管状变成了孔状结构。与生长在平面GaN衬底上的(11-22)GaN相比,生长在多孔GaN层上的(11-22)GaN的单位面积内的三角形坑的数目减少,位错缺陷密度降低了一个数量级左右(从1.2×1010/cm2到5.6×109/cm2),多孔GaN层有效地阻挡了位错线的传播；多孔GaN层直接阻挡位错线的传播是(11-22)GaN缺陷密度减少的主要机制。生长在多孔GaN层上的InGaN/GaN MQWs表面的单位面积内的箭头形坑的数目减少了一半左右(从-1.42×108/cm2到-7.8×107/cm2)。而且随着孔洞率的增加而逐渐减少。(11-22)InGaN/GaN MQWs样品的光致发光峰值波长为-472 nm；PL峰值强度随着多孔GaN层孔洞率的增加而增加,与生长在平面GaN衬底上的MQWs相比,最大可达到4倍的增强。这是因为1)(11-22)InGaN/GaNMQWs样品中缺陷密度的减少、材料质量的提高；2)纳米孔结构对(11-22)InGaN/GaN MQWs光致发光的散射；以及3)纳米孔结构的低折射率对(11-22)InGaN/GaN MQWs光致发光的反射作用。
GaN-based semiconductor is a very important material for microelectronics and optoelectronics applications, and, as well as SiC and diamond, called the third generation semiconductor materials, following first generation-Si and second generation-GaAs. Special properties ofⅢ-nitride materials, such as Wide bandgap, strong atom bond, high thermal conductivity, and chemical inertness, are attracting the applications in the fields of optoelectronics, high temperature, high power, and high frequency devices.
Etching technology is a critical step for the GaN-based devices fabrication. But so far, there is still no effective wet etching technique for GaN-based materials due to its chemical inertness. Dry etching (ICP) is normally used to fabricate GaN-based devices. But this technique is not only expensive and low selectivity but also damages the GaN materials during process. Developing a new wet etching technique is very important for the reducing cost and damage of the GaN-based devices. Meanwhile, solid state lighting market exploring needs large area, vertical LEDs, which needs liftoff technique to.remove the substrates. In addition, nonpolar and semipolar GaN materials will push optoelectronics devices to longer wavelength and better performance. But the high defect density of nonpolar and semipolar GaN inhabits its development. In this dissertation, based on the problems mentioned above, we investigate the electrochemical etching GaN liftoff, fabrication of nanoporous GaN and large area, free standing GaN by electrochemical etching, semipolar (11-22) GaN MOCVD growth by two-step growth, and defect reduction of semipolar (11-22) GaN using a nanoporous GaN interlayer. The main conclusions are summarized below.
We studied the conductivity based selective etching of GaN at room temperature. Three regions are identified including no etching region, porous GaN formation region, and the complete removal or electropolishing region with increasing conductivity or applied voltage, which provides flexibility for the fabrication of GaN devices. Based on this etching technique, we lifted off a large area (>1×1 mm2), crack free GaN thin film with a thichness of～1μm, which is large enough for large area LEDs (1×1mm2); a few photonic applications (microdisks and DBRs) and GaN beams and cantilever were demonstrated. The simplicity of EC etching and its compatibility with conventional GaN structures allow us to anticipate a variety of useful device applications in the future.
Secondly, we fabricated nanoporous GaN by a simply electrochemical etching. Applied voltage and Si doping concentration of n-GaN are two very important parameters for the electrochemical etching; the properties of nanoporous GaN, such as pore size, porosity, pore density and etch rate, can be tuned by changing these two parameters. Pore size and porosity of nanoporous GaN increase with applied voltage increasing for the same Si-doped GaN, and pore density and etch rate increase first, and decrease when applied voltage is larger than a certain value. However, pore size of nanaoporous GaN decrease, and porosity, pore density and etch rate increase with the Si doping concentration of n-GaN increasing at the same applied voltage. we obtained 2 inch uniform nanoporous GaN sample. For the electrochemical etching mechanism, we think that the EC etching in GaN involves a two-step process of oxidation of GaN (into GaOx) and the dissolution of GaOx by the oxalic acid; the holes in GaN and the electrons in oxalic acid solution (OA) recombined at the interface between GaN and OA due to present of electrical field, some GaN are oxidized into GaOx, GaOx dissolved in the OA solution to form nanoporous structures. More works should be done about the electrochemical etching mechanisms in the future.
Based on the study of nanoporous GaN, we present a new scheme in splitting and lifting-off GaN using nanoporous (NP) GaN medium by a simple and robust electrochemical (EC) etching process. This procedure can be considered an implementation of the "smart-cut" principle using nanoscale wet etching and is compatible to wafer-level scaling up. The NP GaN produced by the EC etching offers a new way to selectively weaken the mechanical strength of GaN, making it possible to split and separate epitaxial GaN layer. The use of the nanoetching leads to a flexible process in forming columnar pores during the initial vertical drilling, followed by localized isotropic etching deep in the layer to create lift-off. This procedure can be applied to almost all semiconductors but is especially pertinent to GaN with its given its chemical inertness. We demonstrate that large area (≥1 cm2), free-standing GaN layers, with a thickness from 0.5 to a few microns, can be separated in less than 20 minutes, and the mono-crystallinity of the lift-off GaN layers is well preserved by this process. After NP GaN liftoff, we can transfer it onto different substrates, such as glass, Si, and polydimethylsiloxane (PDMS).
Another advantage of this liftoff technique is that the substrate can be reused after liftoff free standing GaN, which we demonstrated in this dissertation.
Last, we investigated the growth of semipolar (11-22) GaN by two-step growth on m-plane sapphire, and using a nanoporous GaN interlayer to reduce the defect density of semipolar (11-22) GaN.
A pure (11-22) GaN has been attained with a great reproducibility by adopting an appropriate nitridation for m-sapphire. A two-step growth approach is introduced to substantially improve (11-22) GaN quality evaluated by a comprehensive x-ray analysis. With the insertion of an islanding growth step, the FWHMs of the ori-axis (11-22) XRCs decreased by-55%, and the FWHM plots became less steep. The growth mode changes from 3D to 2D making the dislocation bending, interaction and annihilation.
Semipolar (11-22) nanoporous GaN was fabricated by the electrochemical etching. The properties of semipolar (11-22) nanoporous GaN is very similar with the (0002) nanoporous GaN. Pore size and porosity of semipolar (11-22) nanaoporous GaN increase with the applied voltage increasing at the same Si doping concentration of n-GaN; etch rate increases first, then decreases when applied voltage is larger than a certain value. There is no plane selectivity for electrochemical etching comparing to semipolar (11-22) nanoporous GaN by PEC etching; nanoporous structure formation is only related to applied voltage and Si doping concentration of n-GaN.
A new method is presented toward the reduction of microstructural defects during the growth of (1122) semipolar GaN on sapphire. The use of a nanoporous and monocrystalline interlayer, produced by a simple electrochemical (EC) etching process, facilitates homogeneous lateral growth. TEM results show that the partial dislocation density above the NP GaN interlayer is nearly reduced by a factor of two. The direct blocking of dislocations is the main mechanism of defect reduction of semipolar (11-22) GaN using a nanoporous interlayer. The InGaN/GaN quantum wells grown on NP GaN exhibit a three-time enhancement in photoluminescence (PL) intensity over that on planar semipolar GaN templates. The improvement in PL intensity is likely due to 1) improved material quality,2) the presence of NP layers for diffusive scattering underneath the MQWs, and 3) the reduction of index of refraction in the NP layer. The surface pit density of InGaN/GaN MQWs grown on nanoporous GaN layer reduced by a factor of 2 (from～1.42×108/cm2 to～7.8×107/cm2), which confirmed the defects reduction of semipolar GaN.
刻蚀技术是器件制备工艺中不可缺少的步骤,但由于GaN材料的抗化学腐蚀性,至今没有有效的湿法刻蚀技术。干法刻蚀(ICP)是目前器件制备中常用的刻蚀技术,但这种技术不但昂贵、选择性低,还会损伤GaN材料而影响器件特性；发展一种新的GaN基材料的湿法刻蚀技术非常重要。同时,照明市场对GaN基光电子器件需求量的增加,以及光电子器件面积的增大,急需高性能垂直结构LEDs;而它的制备需要有效的薄膜剥离技术。此外,半极性和非极性GaN基材料具有制备长波长和高性能的光电子器件巨大潜力,但材料中存在着很多缺陷一直困扰着人们。本论文针对这些GaN材料和器件研究领域的热点问题,主要开展了电化学刻蚀剥离GaN薄膜的研究；电化学刻蚀法制备多孔GaN薄膜和大面积、自支撑多孔GaN薄膜的研究；两步法生长半极性(11-22)GaN薄膜、电化学法制备半极性(11-22)多孔GaN薄膜,以及用多孔GaN作衬底生长低缺陷密度的GaN薄膜和InGaN/GaN MQWs的研究。研究发现：
在室温条件下,对n-type GaN横向选择性电化学湿法刻蚀的研究,我们发现当选择不同的刻蚀条件(偏压)和不同硅掺杂浓度的n-GaN样品时,我们可以得到三个不同的区域：没有任何刻蚀区域(Ⅰ),刻蚀形成多孔结构区域(Ⅱ)和完全刻蚀区域(Ⅲ)。这为GaN基器件制备提供了很大的可用空间。
利用n-type GaN横向选择性电化学湿法刻蚀技术,我们成功地从蓝宝石衬底上剥离了约1微米厚的大面积(1 x 1 mm2)无裂纹的GaN薄膜。为下一步制备高性能的垂直结构GaN基LEDs打下了坚实的基础。在光学应用方面,利用n-type GaN横向选择性电化学湿法刻蚀技术,我们成功地制备了GaN圆型微盘(microdisk)和DBR结构,并对GaN圆型微盘(microdisk)的光学特性进行了表征。GaN圆盘形光学微腔(微盘)(mirodisk)具有均匀分布的光学微腔模式,这些光学模的半高宽远远小于GaN带边峰的半高宽(～5nm)。GaN圆盘形光学微腔(微盘)(mirodisk)的质量因子大约为2800,激射阈值大约为7nJ。最后,我们验证了n-type GaN横向选择性电化学湿法刻蚀技术在MEMS方面的应用,制备出了GaN“桥”和悬臂梁结构,并对GaN悬臂梁结构的振动频率进行了测量和计算,测量和计算结果基本一致,GaN悬臂梁结构的中心振动频率大约在120 kHz。我们相信,n-type GaN横向选择性电化学湿法刻蚀技术将会给Ⅲ-Ⅴ氮化物材料和器件领域带来意想不到的影响。
其次,我们用电化学湿法刻蚀法制备了多孔GaN薄膜结构；研究发现外加偏压和n-GaN的硅掺杂浓度在电化学刻蚀中是两个非常重要的参数。通过改变这两个参数我们可以控制多孔GaN材料的孔尺寸、孔洞率、孔密度等特性,还可以控制电化学湿法刻蚀n-GaN的刻蚀速率。同一硅掺杂浓度的n-GaN,多孔GaN材料的孔尺寸和孔洞率随外加偏压的增加而增大,多孔GaN材料的孔密度和刻蚀速率随外加偏压是先增大,当达到某一值后开始下降。在恒定外加偏压下,多孔GaN材料的孔尺寸随α-GaN的硅掺杂浓度的增加而减小,而孔洞率、孔密度和刻蚀速率随n-GaN的硅掺杂浓度的增加而增加。我们可以在2英寸GaN样品上制备出非常均匀的多孔GaN薄膜。并且讨论了电化学法制备多孔GaN薄膜的机制,我们初步认为由于外加电场的存在使GaN薄膜表面的空穴迁移到GaN和草酸溶液界面与草酸溶液中电子复合,使GaN得到氧化变成GaOx,GaOx溶于草酸溶液中而形成GaN材料的刻蚀,关于电化学刻蚀n-GaN的机制需要更进一步深入的研究。
根据电化学刻蚀制备多孔GaN薄膜材料的特性,我们用两种不同的工艺方法制备了大面积、自支撑多孔GaN薄膜。工艺A为两步法：用均匀硅掺杂浓度的n-GaN,第一步用低外加偏压形成低孔洞率的多孔GaN层,第二步增加外加偏压形成高孔洞率的多孔GaN层,最终把低孔洞率的多孔GaN层剥离下来。工艺B中,我们用具存不同硅掺杂浓度的n-GaN层,上层为低硅掺杂浓度的n-GaN层,下层为高硅掺杂浓度的n+-GaN;在同一外加偏压下,它们分别形成不同孔洞率和孔密度的多孔GaN结构,随刻蚀时间的增加,上层多孔GaN将自动剥离下来。我们把制备的大面积、自支撑的多孔GaN薄膜转移到了不同的衬底上例如玻璃,硅和聚二甲基硅氧烷(PDMS)等。
电化学湿法刻蚀剥离技术制备大面积多孔GaN薄膜的另一优点是我们可以重复利用剥离后的衬底。我们在剥离大面积、自支撑多孔GaN薄膜后的衬底上生长了GaN薄膜材料,发现在剥离多孔GaN薄膜后的衬底上生长的GaN薄膜具有与常规衬底上生长的GaN薄膜同样的质量特性。
最后,我们对半极性(11-22)GaN的生长,半极性(11-22)多孔GaN的制备,以及利用多孔GaN插入层生长低缺陷密度的(11-22)GaN进行了系统的研究并发现：
两步法MOCVD生长可以在m面Al2O3衬底上获得微结构提高的半极性(11-22)GaN薄膜。m面A1203衬底在NH3气氛下的高温氮化处理是获得(11-22)GaN薄膜的关键。适当的A1N缓冲层厚度在两步法提高(11-22)GaN薄膜质量的生长过程中同样起到重要的作用。生长AlN缓冲层后,第一步高压生长(11-22)GaN形成岛状表面结构(三维生长模式),第二步低压生长增加(11-22)GaN薄膜的横向生长速率(二维生长模式),使(11-22)GaN薄膜中的一些位错线弯曲互相作用而湮灭。优化后的两步法生长可以使(11-22)GaN薄膜的XRCs半高宽降低一半左右。
电化学刻蚀法制备了半极性(11-22)纳米孔GaN薄膜。半极性(11-22)纳米孔GaN的特性与极性(0002)纳米孔GaN非常相似。同一硅掺杂浓度的(11-22)n-GaN,多孔GaN材料的孔尺寸和孔洞率随着外加偏压的增加而增大,多孔GaN材料的刻蚀速率随外加偏压的增大,先增大,当达到某一电压值后又开始下降。与PEC刻蚀获得的纳米孔(11-22)GaN相比,电化学刻蚀制备的纳米孔GaN薄膜没有面选择性,纳米孔的形成与外加偏压和样品的硅掺杂浓度有关。
用电化学法制备的半极性(11-22)多孔GaN作插入层可以生长低缺陷密度的(11-22)GaN薄膜。生长过程中,由于高温下的质量转移特性,多孔GaN层的截面结构从树枝状或平行管状变成了孔状结构。与生长在平面GaN衬底上的(11-22)GaN相比,生长在多孔GaN层上的(11-22)GaN的单位面积内的三角形坑的数目减少,位错缺陷密度降低了一个数量级左右(从1.2×1010/cm2到5.6×109/cm2),多孔GaN层有效地阻挡了位错线的传播；多孔GaN层直接阻挡位错线的传播是(11-22)GaN缺陷密度减少的主要机制。生长在多孔GaN层上的InGaN/GaN MQWs表面的单位面积内的箭头形坑的数目减少了一半左右(从-1.42×108/cm2到-7.8×107/cm2)。而且随着孔洞率的增加而逐渐减少。(11-22)InGaN/GaN MQWs样品的光致发光峰值波长为-472 nm；PL峰值强度随着多孔GaN层孔洞率的增加而增加,与生长在平面GaN衬底上的MQWs相比,最大可达到4倍的增强。这是因为1)(11-22)InGaN/GaNMQWs样品中缺陷密度的减少、材料质量的提高；2)纳米孔结构对(11-22)InGaN/GaN MQWs光致发光的散射；以及3)纳米孔结构的低折射率对(11-22)InGaN/GaN MQWs光致发光的反射作用。
GaN-based semiconductor is a very important material for microelectronics and optoelectronics applications, and, as well as SiC and diamond, called the third generation semiconductor materials, following first generation-Si and second generation-GaAs. Special properties ofⅢ-nitride materials, such as Wide bandgap, strong atom bond, high thermal conductivity, and chemical inertness, are attracting the applications in the fields of optoelectronics, high temperature, high power, and high frequency devices.
Etching technology is a critical step for the GaN-based devices fabrication. But so far, there is still no effective wet etching technique for GaN-based materials due to its chemical inertness. Dry etching (ICP) is normally used to fabricate GaN-based devices. But this technique is not only expensive and low selectivity but also damages the GaN materials during process. Developing a new wet etching technique is very important for the reducing cost and damage of the GaN-based devices. Meanwhile, solid state lighting market exploring needs large area, vertical LEDs, which needs liftoff technique to.remove the substrates. In addition, nonpolar and semipolar GaN materials will push optoelectronics devices to longer wavelength and better performance. But the high defect density of nonpolar and semipolar GaN inhabits its development. In this dissertation, based on the problems mentioned above, we investigate the electrochemical etching GaN liftoff, fabrication of nanoporous GaN and large area, free standing GaN by electrochemical etching, semipolar (11-22) GaN MOCVD growth by two-step growth, and defect reduction of semipolar (11-22) GaN using a nanoporous GaN interlayer. The main conclusions are summarized below.
We studied the conductivity based selective etching of GaN at room temperature. Three regions are identified including no etching region, porous GaN formation region, and the complete removal or electropolishing region with increasing conductivity or applied voltage, which provides flexibility for the fabrication of GaN devices. Based on this etching technique, we lifted off a large area (>1×1 mm2), crack free GaN thin film with a thichness of～1μm, which is large enough for large area LEDs (1×1mm2); a few photonic applications (microdisks and DBRs) and GaN beams and cantilever were demonstrated. The simplicity of EC etching and its compatibility with conventional GaN structures allow us to anticipate a variety of useful device applications in the future.
Secondly, we fabricated nanoporous GaN by a simply electrochemical etching. Applied voltage and Si doping concentration of n-GaN are two very important parameters for the electrochemical etching; the properties of nanoporous GaN, such as pore size, porosity, pore density and etch rate, can be tuned by changing these two parameters. Pore size and porosity of nanoporous GaN increase with applied voltage increasing for the same Si-doped GaN, and pore density and etch rate increase first, and decrease when applied voltage is larger than a certain value. However, pore size of nanaoporous GaN decrease, and porosity, pore density and etch rate increase with the Si doping concentration of n-GaN increasing at the same applied voltage. we obtained 2 inch uniform nanoporous GaN sample. For the electrochemical etching mechanism, we think that the EC etching in GaN involves a two-step process of oxidation of GaN (into GaOx) and the dissolution of GaOx by the oxalic acid; the holes in GaN and the electrons in oxalic acid solution (OA) recombined at the interface between GaN and OA due to present of electrical field, some GaN are oxidized into GaOx, GaOx dissolved in the OA solution to form nanoporous structures. More works should be done about the electrochemical etching mechanisms in the future.
Based on the study of nanoporous GaN, we present a new scheme in splitting and lifting-off GaN using nanoporous (NP) GaN medium by a simple and robust electrochemical (EC) etching process. This procedure can be considered an implementation of the "smart-cut" principle using nanoscale wet etching and is compatible to wafer-level scaling up. The NP GaN produced by the EC etching offers a new way to selectively weaken the mechanical strength of GaN, making it possible to split and separate epitaxial GaN layer. The use of the nanoetching leads to a flexible process in forming columnar pores during the initial vertical drilling, followed by localized isotropic etching deep in the layer to create lift-off. This procedure can be applied to almost all semiconductors but is especially pertinent to GaN with its given its chemical inertness. We demonstrate that large area (≥1 cm2), free-standing GaN layers, with a thickness from 0.5 to a few microns, can be separated in less than 20 minutes, and the mono-crystallinity of the lift-off GaN layers is well preserved by this process. After NP GaN liftoff, we can transfer it onto different substrates, such as glass, Si, and polydimethylsiloxane (PDMS).
Another advantage of this liftoff technique is that the substrate can be reused after liftoff free standing GaN, which we demonstrated in this dissertation.
Last, we investigated the growth of semipolar (11-22) GaN by two-step growth on m-plane sapphire, and using a nanoporous GaN interlayer to reduce the defect density of semipolar (11-22) GaN.
A pure (11-22) GaN has been attained with a great reproducibility by adopting an appropriate nitridation for m-sapphire. A two-step growth approach is introduced to substantially improve (11-22) GaN quality evaluated by a comprehensive x-ray analysis. With the insertion of an islanding growth step, the FWHMs of the ori-axis (11-22) XRCs decreased by-55%, and the FWHM plots became less steep. The growth mode changes from 3D to 2D making the dislocation bending, interaction and annihilation.
Semipolar (11-22) nanoporous GaN was fabricated by the electrochemical etching. The properties of semipolar (11-22) nanoporous GaN is very similar with the (0002) nanoporous GaN. Pore size and porosity of semipolar (11-22) nanaoporous GaN increase with the applied voltage increasing at the same Si doping concentration of n-GaN; etch rate increases first, then decreases when applied voltage is larger than a certain value. There is no plane selectivity for electrochemical etching comparing to semipolar (11-22) nanoporous GaN by PEC etching; nanoporous structure formation is only related to applied voltage and Si doping concentration of n-GaN.
A new method is presented toward the reduction of microstructural defects during the growth of (1122) semipolar GaN on sapphire. The use of a nanoporous and monocrystalline interlayer, produced by a simple electrochemical (EC) etching process, facilitates homogeneous lateral growth. TEM results show that the partial dislocation density above the NP GaN interlayer is nearly reduced by a factor of two. The direct blocking of dislocations is the main mechanism of defect reduction of semipolar (11-22) GaN using a nanoporous interlayer. The InGaN/GaN quantum wells grown on NP GaN exhibit a three-time enhancement in photoluminescence (PL) intensity over that on planar semipolar GaN templates. The improvement in PL intensity is likely due to 1) improved material quality,2) the presence of NP layers for diffusive scattering underneath the MQWs, and 3) the reduction of index of refraction in the NP layer. The surface pit density of InGaN/GaN MQWs grown on nanoporous GaN layer reduced by a factor of 2 (from～1.42×108/cm2 to～7.8×107/cm2), which confirmed the defects reduction of semipolar GaN.
引文
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