A micromachined, shadow-mask technology for the OMVPE fabrication of integrated optical structures

A micromachined, silicon shadow-mask technology is described which extends the capabilities of shadow-masked OMVPE for the fabrication of nonplanar micro-optical elements. The deep reactive ion etched (DRIE) shadow mask is inexpensive, reusable and produces smooth, nonplanar structures with precise control of position, shape and size. Direct fusion bonding of the mask to the substrate was found to be a reliable and reproducible method for attaching the mask to the substrate during growth. The DRIE shadow mask technology allows the deposition of microlenses with focal lengths out to 3 mm without the central flattening that was previously observed in shadow masked lenses grown under the epitaxial mask. We also describe novel applications of this technology in the fabrication of micromirrors and concentrically-variable Bragg reflectors, which should improve mode discrimination in large aperture VCSELs.     

Surface etching of 6H-SiC (0001) and surface morphology of the subsequently grown GaN via MOCVD

The correlation between surface morphological properties of the GaN epilayers and the surface conditions of 6H-SiC (0001) substrates etched in H₂, C₂H₄/H₂, and HCl/H₂ was studied. Etching 6H-SiC in H₂ produced a high quality surface with steps and terraces, while etching in HCl/H₂ produced either a rough surface with many pits and hillocks or a smooth surface similar to that etched in H₂, depending on the HCl concentration and temperature. The GaN epilayers were subsequently deposited on these etched substrates using either a low temperature GaN or a high temperature AlN buffer layer via MOCVD. The substrate surface defects increased the density and size of the “giant” pinholes (2–4 μm) on GaN epilayers grown on a LT-GaN buffer layer. Small pinholes (<100 nm) were frequently observed on the samples grown on a HT-AlN buffer layer, and their density decreased with the improved surface quality. The non-uniform GaN nucleation caused by substrate surface defects and the slow growth rate of planes of the islands were responsible for the formation of “giant” pinholes, while the small pinholes were believed to be caused by misfit dislocations.     

GaN缓冲层对生长InN薄膜的影响

采用金属有机物化学气相沉积(MOCVD)方法生长六方相InN薄膜，利用氮化镓(GaN)缓冲层技术制备了高质量薄膜，得到了其能带带隙0.7eV附近对应的光致发光光谱(PL). 通过比较未采用缓冲层，同时采用低温和高温GaN缓冲层，以及低温GaN缓冲层结合高温退火三种生长过程，发现低温GaN缓冲层结合高温退火过程能够得到更优表面形貌和晶体质量的InN薄膜，同时表征了材料的电学性质和光学性质. 通过对InN薄膜生长模式的讨论，解释了薄膜表面形貌和晶体结构的差异.     

980nm垂直腔面发射激光器的外延生长

模拟了垂直腔面发射激光器(VCSEL)的反射谱和量子阱增益谱,采用金属有机物化学气相沉积设备外延生长了980 nm的垂直腔面发射激光器,制作了氧化孔径为14 μm的内腔式氧化限制型VCSEL器件,其阈值电流为3.3mA,阈值电压为1.8V,斜率效率为0.387 W/A,室温直流电流为22.8 mA时,输出光功率为5 mW.     

薄膜太阳电池用改进型绒面结构MOCVD-ZnO:B透明导电薄膜研究

利用低压MOCVD技术在玻璃衬底上生长了改进型绒面结构ZnO:B薄膜。改进型ZnO:B薄膜包含两层薄膜,第一层采用传统工艺技术生长了类金字塔状晶粒,第二层借助相对低温生长技术获得了类球状晶粒。典型的双层生长技术获得的MOCVD-ZnO:B薄膜具有相对高的电子迁移率~27.6 cm2/Vs,主要归因于提高了晶界质量,减少了缺陷态。随着第二层修饰层厚度的增加,MOCVD-ZnO:B薄膜的绒度提高,而光学透过率有所下降。相比于传统工艺生长的ZnO薄膜,双层结构的MOCVD-ZnO:B薄膜应用于硅基薄膜太阳电池展现了较高的太阳电池转化效率。     

ZnO单晶薄膜光电响应特性

对采用MOCVD方法沉积的ZnO单晶薄膜的欧姆接触特性、光电特性进行了研究,并对比研究了射频溅射沉积SiO2抗反射膜对ZnO薄膜I-V、光电特性的影响.实验结果表明,非合金Al/ZnO金属体系与n型ZnO形成了良好的欧姆接触,溅射沉积SiO2在ZnO表面引入了载流子陷阱,影响I-V特性,延长了光响应下降时间.ZnO单晶薄膜光电导也具有时间退化现象.     

Effect of shroud flow on high quality InxGa1−xN deposition in a production scale multi-wafer-rotating-disc reactor

High quality InGaN thin films and InGaN/GaN double heterojunction (DH) structures have been epitaxially grown on c-sapphire substrates by MOCVD in a production scale multi-wafer-rotating-disc reactor between 770 to 840°C. We observed that shroud flow (majority carrier gas in the reaction chamber) is the key to obtaining high quality InGaN thin films. High purity H₂ as the shroud flow results in poor crystal quality and surface morphology but strong photolumines-cence (PL) at room temperature. However, pure N₂ as the shroud flow results in high crystal quality InGaN with an x-ray full width at half maximum (FWHM)_InGaN(0002) of 7.5 min and a strong room temperature PL peaking at 400 nm. In addition, InGaN/GaN single heterojunction (SH) and DH structures both have excellent surface morphology and sharp interfaces. The full width at half maximum of PL at 300K from an InGaN/GaN DH structure is about 100 meV which is the best reported to date. A high indium mole fraction in InGaN of 60% and high quality zinc doped InGaN depositions were also achieved.     

Improved selective growth of InP around dry-etched mesas by metalorganic chemical vapor deposition at low growth temperature

A comparative study has been carried out regarding selective embedding growth of InP by metalorganic chemical vapor deposition (MOCVD) around dry-etched mesas, using two types of reactors: a conventional horizontal type and a highspeed rotating-susceptor type. In the case of the conventional horizontal-type MOCVD, overgrowth on the mask was observed when the growth temperature was low (600°C). On the other hand, an almost planar grown surface without such overgrowth was achieved by using the high-speed rotating-susceptor MOCVD for a wide range of growth temperatures, especially even at a low growth temperature of 580°C. Regarding the high-speed rotating-susceptor MOCVD, we have also investigated the effects of dopants on the growth behaviors and have found a remarkable difference between n-type S-doped and p-type Zn-doped InP in the growth behaviors. The mechanism for suppressing overgrowth in case of the high-speed rotating-susceptor MOCVD, as well as the cause for the different effects between the dopants, are discussed.     

Direct growth of CdTe on (100), (211), and (111) Si by metalorganic chemical vapor deposition

CdTe epilayers were grown directly on (100), (211), and (111) silicon substrates by metalorganic chemical vapor deposition (MOCVD). The crystallinity and the growth orientation of the CdTe film were dependent on the surface treatment of the Si substrate. The surface treatment consisted of exposure of the Si surface to diethyltelluride (DETe) at temperatures over 600°C prior to CdTe growth. Direct growth of CdTe on (100) Si produced polycrystalline films whereas (lll)B single crystals grew when Si was exposed to DETe prior to CdTe growth. On (211) Si, single crystal films with (133)A orientation was obtained when grown directly; but produced films with (211)A orientation when the Si surface was exposed to DETe. On the other hand, only (lll)A CdTe films were possible on (111) Si, both with and without Te source exposure, although twinning was increased after exposure. The results indicate that the exposure to a Te-source changes the initial growth stage significantly, except for the growth on (111) Si. We propose a model in which a Te atom replaces a Si atom that is bound to two Si atoms.     

Growth and characterization of inTISb for IR-detectors

Epitaxial In_1-xTl_xSb films with compositions up to x = 0.1 have been demonstrated using the metalorganic chemical vapor deposition technique on InSb and GaAs substrates. A specially designed high-temperature source delivery system was used for the low vapor pressure cyclopentadienylthallium source. Tl-compositions in the deposited films were measured by Rutherford backscattering spectroscopy which confirmed the incorporation of up to 10% Tl. Room temperature infrared transmission spectra of InTISb exhibited considerable absorption beyond 7 μm. Photoconductive detectors were fabricated in InTISb films grown on semi-insulating GaAs. Spectral response measurements showed substantial photoresponse at 8.5 to 14 μm. In spite of the large lattice-mismatch (≈14%) between InTISb and GaAs, photoconductive detectors exhibited black-body detectivities (D^* _bb) of 5.0 × 10⁸ cm-Hz^1/2W⁻¹ at 40K.