Growth and electronic properties of ZnO epilayers by plasma-assisted molecular beam epitaxy

ZnO thin films were grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy (MBE). The crystalline properties of the layers as measured by x-ray diffraction were found to improve with lower growth temperatures, where the full-width at half-maximum (FWHM) of the x-ray rocking curves was shown to be in the range of 100 to 1,100 arcsec. The electronic properties were found to improve for higher growth temperatures, with n-type carrier concentration and electron mobility in the range of 1×10¹⁷ −5×10¹⁸ cm⁻³ and 80–36 cm²/Vs, respectively. Photoluminescence (PL) measurements indicated that growth at higher temperatures provided superior band edge radiative emission, while growth at lower temperatures resulted in significant deep level radiative emission centered at 2.35 eV. Photoconductive decay measurements exhibit a slow decay indicating the presence of hole traps, where Zn vacancies are believed to be the source of both the slow decay and the deep level emission observed in PL spectra.     

Reflectance and photoreflectance for in-situ monitoring of the molecular beam epitaxial growth of CdTe and Hg-based materials

Epitaxial growth of Hg-based semiconductors by molecular beam epitaxy (MBE) and metalorganic MBE (MOMBE) has progressed sufficiently to shift emphasis to the control of factors limiting the yield of both materials and devices. This paper reports on anex-situ study to evaluate the suitability of reflectance and photoreflectance (PR) asin-situ characterization techniques for the growth of CdTe and HgCdTe. Photoreflectance yields information about CdTe layers, with largest utility for doped and multi-layer structures. However, caution must be taken in interpretation of the spectra since the near-bandedge PR spectra consists of multiple transitions and the E₁ transition energy is very sensitive to the sample history. Photoreflectance appears to be of limited utility for HgCdTe single layer growth with x<0.4. However, reflectance measurements of the E₁ peak can be used to determine composition in HgCdTe single layers with an accuracy Δx = ±0.01, which can be useful for growth control. A tight binding model was used to calculate the E₁ peak energy as a function of bandgap for HgCdTe and HgTe/CdTe superlattices. Comparisons are made with experimental observations. Surface interdiffusion in HgTe-CdTe superlattices was probed using reflectance measurements.     

Gas-source molecular beam epitaxial growth and characterization of InNxP1−x on InP

We report a study of N incorporation into InP using a radio frequency (rf) N plasma source. Very streaky reflection high-energy electron diffraction patterns are observed for InN_xP_1−x (x <1%) grown on InP, indicating layer-by-layer growth of the film. The sharp x-ray diffraction peak and the clear Pendelloesung fringes in the high-resolution x-ray rocking curves reveal the high crystalline quality and uniformity of the film. They also suggest the smoothness of the interface between InP and InN_xP_1−xand of the surface of the InN_xP_1−x layer. This is further confirmed by scanning electron microscopy on these samples, where featureless surface is obtained. The formation of an InNP alloy is confirmed by x-ray θ-2θ diffraction measurement where no phase separation is observed. Different ways to increase the N composition in InNP were explored. At a fixed N₂ flow-rate fraction, lowering the growth temperature increases the N composition in InNP. Raising the rf power or using a larger beam exit aperture will also increase the N incorporation as a result of the availability of more active N species.     

Uniform low defect density molecular beam epitaxial HgCdTe

This paper describes recent advances in MBE HgCdTe technology. A new 3 inch production molecular beam epitaxy (MBE) system, Riber Model 32P, was installed at Rockwell in 1994. The growth technology developed over the years at Rockwell using the Riber 2300 R&D system was transferred to the 32P system in less than six months. This short period of technology transfer attests to our understanding of the MBE HgCdTe growth dynamics and the key growth parameters. Device quality material is being grown routinely in this new system. Further advances have been made to achieve better growth control. One of the biggest challenges in the growth of MBE HgCdTe is the day-to-day control of the substrate surface temperature at nucleation and during growth. This paper describes techniques that have led to growth temperature reproducibility within + - 1°C, and a variation in temperature during substrate rotation within 0.5°C. The rotation of the substrate during growth has improved the uniformity of the grown layers. The measured uniformity data on composition for a typical 3 cm × 3 cm MBE HgCdTe/CdZnTe shows the average and standard deviation values of 0.229 and 0.0006, respectively. Similarly, the average and standard deviation for the layer thickness are 7.5 and 0.06 μm, respectively. P-on-n LWIR test structure photodiodes fabricated using material grown by the new system and using rotation during growth have resulted in high-performance (R₀)A, quantum efficiency) devices at 77 and 40K. In addition, 128 × 28 focal plane arrays with excellent performance and operability have been demonstrated.     

Plasma characteristics and the growth of group III-nitrides by metalorganic molecular beam epitaxy

Improved quality and controllability of growth processes are key issues for the maturation of III-N technologies. One of the most important concerns for the growth of III-N materials in ultra high vacuum is the ability to provide an effective nitrogen flux to the growth surface. This work has sought to correlate radio frequency (rf) plasma parameters and their impact on the growth of GaN by plasma-assisted metalorganic molecular beam epitaxy. Utilizing optical emission spectrometry, the atomic nitrogen production has been optimized as a function of rf power and N₂ flow rate. Growth experiments indicate that the abundance of atomic nitrogen alone does not control growth. Increasing energy per molecule in the rf source, with a constant level of atomic nitrogen, dramatically decreases the GaN growth rate. High levels of atomic nitrogen with a low energy per molecule resulted in restoration of the growth rate to ∼0.5 μm/h.     

利用全固态分子束外延方法在Ge(100)衬底上异质外延GaAs薄膜及相关特性表征

利用全固态分子束外延(MBE)方法在Ge(100)衬底上异质外延GaAs薄膜,并通过高能电子衍射(RHEED)、高分辨X射线衍射(XRD),原子力显微镜等手段研究了不同生长参数对外延层的影响.RHEED显示在较高的生长温度或较低的生长速率下,低温GaAs成核层呈现层状生长模式.同时降低生长温度和生长速率会使GaAs薄膜的XRD摇摆曲线半高宽(FWHM)减小,并降低外延层表面的粗糙度,这主要是由于衬底和外延薄膜之间的晶格失配度减小的结果.     

Molecular beam epitaxial growth of strained AIGalnAs multi-quantum well lasers on InP

State of the art transparency currents as low as 41 A/cm² per well have been achieved in strained AIGalnAs multi-quantum well (MQW) 1.5 urn lasers. Grown by solid source molecular beam epitaxy, broad area lasers with seven quantum wells exhibit threshold current densities of less than 900 A/cm² for a 300 μm device length, comparable to the best results in this material system by any growth technology. The key to this threshold current density reduction is the optimization of the quantum well width. Experimentally, we found that thresh-old current densities can be reduced by a factor of two by using MQW active regions with wider wells which we attribute to a reduction in the nonradiative recombination and improved electron-hole overlap. High resolution x-ray diffraction, photoluminescence, and broad area lasers were used to characterize the MQW active regions.     

MBE growth and properties of ZnO on sapphire and SiC substrates

Molecular beam epitaxy (MBE) of ZnO on both sapphire and SiC substrates has been demonstrated. ZnO was used as a buffer layer for the epitaxial growth of GaN. ZnO is a würtzite crystal with a close lattice match (<2% mismatch) to GaN, an energy gap of 3.3 eV at room temperature, a low predicted conduction band offset to both GaN and SiC, and high electron conductivity. ZnO is relatively soft compared to the nitride semiconductors and is expected to act as a compliant buffer layer. Inductively coupled radio frequency plasma sources were used to generate active beams of nitrogen and oxygen for MBE growth. Characterization of the oxygen plasma by optical emission spectroscopy clearly indicated significant dissociation of O₂ into atomic oxygen. Reflected high energy electron diffraction (RHEED) of the ZnO growth surface showed a two-dimensional growth. ZnO layers had n-type carrier concentration of 9 × 10¹⁸ cm⁻³ with an electron mobility of 260 cm²/V-s. Initial I-V measurements displayed ohmic behavior across the SiC/ZnO and the ZnO/GaN heterointerfaces. RHEED of GaN growth by MBE on the ZnO buffer layers also exhibited a two-dimensional growth. We have demonstrated the viability of using ZnO as a buffer layer for the MBE growth of GaN.     

Influence of growth conditions and surface reaction byproducts on GaN grown via metal organic molecular beam epitaxy: Toward an understanding of surface reaction chemistry

The surface reaction byproducts during the growth of GaN films via metal organic molecular beam epitaxy (MOMBE) were investigated as a means to optimize material properties. Ethylene and ethane were identified as the dominant surface reaction hydrocarbon byproducts, averaging 27.63% and 7.15% of the total gas content present during growth. Intense ultraviolet (UV) photoexcitation during growth was found to significantly increase the abundance of ethylene and ethane while reducing the presence of H₂ and N₂. At 920°C, UV excitation was shown to enhance growth rate and crystalline quality while reducing carbon incorporation. Over a limited growth condition range, a 4.5×10¹⁹−3.4×10²⁰ cm⁻³ variation in carbon incorporation was achieved at constant high vacuum. Coupled with growth rate gains, UV excitation yielded films with ∼58% less integrated carbon content. Structural material property variations are reported for various ammonia flows and growth temperatures. The results suggest that high carbon incorporation can be achieved and regulated during MOMBE growth and that in-situ optimization through hydrocarbon analysis may provide further enhancement in the allowable carbon concentration range.     

Formation and control of defects during molecular beam epitaxial growth of HgCdTe

Void defects were demonstrated to form away from the substrate-epifilm interface during the molecular beam epitaxial growth of mercury cadmium telluride on cadmium zinc telluride substrates. These were smaller in size compared to voids which nucleated at the substrate-epifilm interface, which were also observed. Observations of void nucleation away from the substrate-epifilm interface were related to the respective growth regimes active at the time of the void nucleation. Once nucleated, voids replicated all the way to the surface even if the flux ratios were modified to prevent additional nucleation of voids. For a significant number of films, void defects were observed co-located with hillocks. These voids were usually smaller than 1 μm and appeared almost indistinguishable from unaccompanied simple voids. However, these void-hillock complexes displayed a nest of dislocation etch pits around these defects upon dislocation etching, whereas unaccompanied voids did not. The nests could extend as much as 25 μm from the individual void-hillock complex. The density of dislocations within the nest exceeded 5×10⁶ cm⁻², whereas the dislocation density outside of the nest could decrease to <2×10⁵ cm⁻². The void-hillock complexes formed due to fluctuations in growth parameters. Elimination of these fluctuations drastically decreased the concentrations of these defects.