有機金屬分子束磊晶成長極性/半極性氮化銦磊晶薄膜之研究

Abstract

III-V族半導體中的氮化銦(Indium nitride, InN)在光電與電子元件中是具有相當有潛力的材料，如高載子遷移率、高漂移速度峰值、低電子質量與0.65 ~ 0.7 eV之能隙等優點。而目前大多數的InN樣品是使用分子束磊晶(MBE)與有機金屬氣相沉積(MOCVD)方式製備，但受限於InN本身之物理性質，要成長高品質InN是具有挑戰性的，是由於InN之熱裂解溫度約為600 ℃，無法於高溫中成長，因此目前成長高品質InN仍然是以MBE為主要鍍膜方法。 本研究中使用的磊晶方式為電漿輔助有機金屬分子束磊晶，可結合MBE與MOCVD兩者之優點成長InN，使用三甲基銦與電漿解離之氮原子做為V族與III族之來源，而影響InN結晶品質優劣的重要因子有基板種類、緩衝層、製程溫度與V/III流量比等等。因此，成長極性InN薄模與半極性InN薄膜，並分析其結構與光電特性。 極性InN薄膜成長部分，在V/III流量比於1.81時有最佳結晶品質，其(0002)與(10-12) x-ray rocking curve半高寬分別為455 arcsec與1070 arcsec。由穿透式電子顯微鏡(TEM)之分析，可知InN與GaN緩衝層之磊晶關係為: (0002)InN//(0002)GaN與[11-20]InN//[11-20]GaN，並且InN中含有高密度之基面疊差。另一方面，透過二次離子質譜儀與X光光電子光譜儀結果得知，InN薄膜中平均含有10^20 cm^-3的氫與碳；而有約10^19 cm^-3的氧，氫與碳之濃度會隨著三甲基銦流量增加而增加，且在InN薄膜表面有較高濃度的碳、氫與氧氣雜質，經表面蝕刻後，顯示減少了自由電子濃度與增加電子遷移率。光學性質部分經由光激發光(PL)之光譜得知，近能帶邊緣(NBE)發光波峰為0.692 ~ 0.735 eV，且隨著V/III流量比增加而藍移。 另外，於半極性InN之研究中，以鋁酸鑭(LaAlO3，LAO)(112)單晶做為基板，在不同溫度成長半極性InN(10-13)薄膜。當半極性InN薄膜沉積在510 ℃時有最佳結晶性，其(10-13)與(0002)半高寬分別約為1408與1830 arsec，且會有兩組180度對稱之晶域(domain)，其磊晶關係為(10-13)InN // (112)LAO與[1-210]InN// [11-1]LAO。另外，LAO與InN之間有極低晶格不匹配率([1-210]InN方向約為-7.75 %，[-3032]InN方向約為0.2 %)，電性方面，半極性InN成長溫度於510 ℃有最高之電子遷移率與最低載子濃度。光學特性經由PL光譜在10 K量測結果顯示，NBE訊號範圍約為0.72至0.81 eV。在510 ℃所沉積之InN在所有的樣品中有最強的放射訊號與窄的半高寬值。

Indium nitride is a III-V semiconductor which is potential for optoelectronics and electronics application due to its high electron mobility, high peak drift velocity, low effective electron mass and narrow bandgap of 0.65 ~0.7 eV. InN has been grown using metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). It has been known that InN has a low dissociation temperature at 600 ℃, such that the growth temperature is limited by the desorption of nitrogen and the thermal decomposition of the films. Therefore, the high-quality InN is usually obtained by using MBE. Various processing parameters may affect the quality of InN, such as substrate, buffer layer, substrate temperature, pressure, and V/III flow ratio. In this study, polar and semipolar InN films were prepared by plasma-assisted metal-organic molecular beam epitaxy (PA-MOMBE) which can have a high growth rate. Detailed characterizations of structural and optical properties of the grown polar/semipoar InN films were carried out. The results indicated that In-polar InN films grown with the V/III ratio of ∼1.81 has the smallest full width at half maximum (FWHM) value of 455 arcsec for (0002) X-ray rocking curve (XRC) andFWHMs value of 1070 arcsec for (10-12). The epitaxial relationship of InN with GaN substrate is (0002)InN//(0002)GaN and [11-20]InN//[11-20]GaN as determined by selected area electron diffraction. Additionally, secondary ion mass spectrometry and X-ray photoelectron spectroscopy results on all the deposited films showed that carbon and hydrogen of average concentration were measured in the order of magnitude of about 10^20 cm^-3 and O concentration about 1019 cm^−3. Also, the C and H concentrations increase with increasing trimethylindium flow rate. A relatively high concentration of C, H and O exists near the surface of the InN films. After etching of the InN films, a decreased carrier concentration to 3.31 × 10^19 cm^−3 can be obtained, while the corresponding electron mobility can increase to 335 cm^2/V-s. Optical properties showed that the PL spectra exhibited NBE peak in the range of 0.692 ~ 0.735 eV. Also, the peaks showed blue-shift with increasing V/III flow ratio. Semipolar InN(10-13) films were prepared on LaAlO3(112) substrate by varying the substrate temperature. The results show that semipolar InN(10-13) layers can be grown at 510 ℃ with the (0002) FWHMs value of 1830 arcsec and (10-13) XRC FWHMs value of 1408 arcsec. Also, the InN film is in epitaxy with LAO substrate with orientation relationships of InN(10-13)//LAO(112) and [1-210]InN// [11-1 ]LAO. The lattice mismatch between InN and LAO can then be estimated to be 7.75 % along the [1-210]InN direction and 0.2 % along the [-3032]InN direction. Electronic properties showed that the InN film grown at 510 ℃ exhibits the highest electron mobility of 494 cm2/V-s and lowest carrier concentration of 2.4 × 10^19 cm^-3. PL spectra at 10 K showed the peaks of near band-edge emission at energies between 0.72 - 0.81 eV. However, InN grown at 510 °C has the highest peak intensity and the narrowest FWHM of these samples which has better quality.