Systematic study of epitaxy growth uniformity in a specific MOCVD reactor

Gallium nitride (GaN) is a direct bandgap semiconductor widely used in bright light‐emitting diodes (LEDs). Thin‐film GaN is grown by metal‐organic chemical vapour deposition (MOCVD) technique. Reliability, efficiency and durability of LEDs are influenced critically by the quality of GaN films. In this report, a systematic study has been performed to investigate and optimize the growth process. Fluid flow, heat transfer and chemical reactions are calculated for a specific close‐coupled showerhead (CCS) MOCVD reactor. Influences of reactor dimensions and growth parameters have been examined after introducing the new conceptions of growth uniformity and growth efficiency. It is found that GaN growth rate is mainly affected by the concentration of (CH3)3Ga:NH3 on the susceptor, while growth uniformity is mainly influenced by the recirculating flows above the susceptor caused by natural convection. Effect of gas inlet temperature and the susceptor temperature over the growth rate can be explained by two competing mechanisms. High growth efficiency can be achieved by optimizing the reactor design.     

Numerical analysis for the growth of GaN layer in MOCVD reactor

The axi-symmetric vertical reactor is a classical reactor configuration for the growth of compound semiconductors by MOCVD. In the present study, the modified reactor is developed to produce uniform and large-volume epitaxial deposition of gallium nitride (GaN). A comprehensive knowledge of the flow, thermal and concentration fields, as well as gas surface reaction, is necessary to develop a CVD reactor. The full elliptic governing equations for continuity, momentum, energy and chemical species are solved numerically. It is investigated how thermal characteristics, reactor geometry, and the operating parameters affect flow fields, mass fraction of each reactant, and deposition rate uniformity. As results, inlet flow rate, inclination angle of wall and inlet design are proposed for optimum operational conditions.     

一种多喷淋头式MOCVD反应器的设计与数值模拟

提出了一种多喷淋头式MOCVD反应器.针对新型反应器,对GaN生长的MOCVD过程进行了数值模拟,模拟考虑了热辐射和化学反应,计算了反应器内流场、温场和浓度场,导流(筒)壁面的寄生沉积以及GaN生长速率,并分析了反应室几何因素对生长均匀性的影响.模拟结果显示,衬底表面大部分区域具有均匀的温场和良好的滞止流.通过对浓度场和GaN生长速率的分析,得出MMGa是薄膜生长的主要反应前体.通过对反应器高度H、导流筒与托盘间距h、导流筒半径R等参数的优化,给出了提高薄膜生长速率和均匀性的条件.     

Modelling of Cu thin film growth by MOCVD process in a vertical reactor

Transport phenomena in a vertical reactor for metalorganic chemical vapor deposition (MOCVD) of copper thin films have been analyzed by numerical simulation of the process. The equations of the mathematical model were solved numerically using the Galerkin finite element method, Newton-Raphson iterations and the frontal algorithm for the gas flow structure, temperature distribution and concentration distribution of the reacting species. Deposition rates of copper thin films using Cu(hfac)VTMS as a precursor were estimated from numerical solutions. Standard process conditions were selected as: a reactor pressure of 1 Torr, a substrate temperature and inlet gas temperature of 200°C and 70°C, respectively, and an inlet gas flow rate of 50 sccm. Under standard conditions, the deposition rates of copper were in the range of 160–230 Å/min. The effects of the process conditions, reactor geometry and shower head structure on the deposition rate and thickness uniformity were examined. It has been demonstrated that numerical simulation can be used for improving the film thickness uniformity and the utilization of source gas.     

Measurement and control of reagent concentrations in MOCVD reactor using ultrasonics

An ultrasonic cell for monitoring gas mixtures on MOCVD reactors has been developed from being a laboratory instrument to a standard, rack-mounted, system which interacts with the reactor's computer, and is marketed under the trade name of EPISON. A feedback circuit has been implemented to stabilize the TMI mole fraction in the reactor. The effectiveness of the feedback circuit has been tested by growing InGaAs layers, during which the TMI concentration was deliberately perturbed by changing the pressure of the TMI line. Using feedback correction an X-ray rocking-curve peak with a FWHM of 25 arc sec was obtained for a layer grown with the same perturbations which had previously led to a 400 arc sec FWHM without feedback.     

多片式热壁 MOCVD 反应器的设计与数值模拟分析

本文提出一种多反应腔并联的水平热壁 MOCVD 反应器,反应器上下(左右)壁面都采用高温,减少了热泳力的排斥作用,提高了衬底上方的 TMG 浓度.由于取消了传统反应器的冷壁,减少了寄生产物的凝结,提高了反应前体的利用率和 GaN 的生长速率.可以多个反应腔并联生长,从而实现反应器的扩容.针对这种热壁式反应器,结合 GaN 的 MOCVD 生长进行了二维数值模拟,计算了不同流速、高度、长度和压力时反应器内流场、温场、浓度场分布以及生长速率,发现存在一个最佳的气体流速、反应器高度和长度条件,在此条件下,反应前体的产生与沉积达到平衡,从而有效抵消反应前体的沿程损耗,实现均匀的 GaN 生长.     

Heat and mass transfer in a gas-transport reactor for multicomponent solid phase deposition

Gas temperature distribution in an epitaxial horizontal flow reactor was measured by means of a thin thermoprobe and measurement error was calculated. For a turbulent gas flow, the thickness of an equilibrium layer was estimated, the results obtained being coincident with the boundary layer thickness determined from the relationships for gas temperature distribution in the reactor. The distribution pattern of the rates of mass-transfer processes is discussed with reference to multicomponent solid phase deposition lengthwise the reactor. That the layer of nearly uniform composition can be deposited under turbulent flow conditions only is exemplified by the deposition of solid solutions of aluminium and gallium nitrides.     

Transport phenomena analysis for boron CVD in a horizontal cold-wall reactor

The transport phenomena in a horizontal cold-walled semicircular reactor are analyzed for the CVD of boron from BCl₃ and H₂. The mixed problem of energy, momentum, and mass conservations is solved by a simple finite difference method. The concentration of the B-reactant on the deposition surface is substituted by the sum of equilibrium mole fractions of the B-containing gas species. The profiles of temperature, velocity, and reactant concentrations in the CVD reactor are illustrated, and the boron deposition rate profile along the substrate is predicted. The effect of the reactant input composition on the deposition rate is calculated, and compared with the experimental data.     

垂直式MOCVD反应器中热泳力对浓度分布的影响分析

从分子动力学理论出发,推导出垂直式MOCVD反应器中热泳力和热泳速度与温度、温度梯度、压强、粒子直径的关系式,以及热泳速度与扩散速度、动量速度平衡时的关系式.在典型的生长条件下,计算得到在温度T=605K时,热泳速度与扩散速度、动量速度动量平衡,TMGa浓度达到最大.然后在不考虑化学反应和考虑化学反应两种情况下,针对垂直式MOCVD反应器内的热泳力对粒子浓度分布和沉积的影响进行数值模拟,模拟给出反应粒子在反应器不同进口温度、衬底温度时的温度分布、浓度分布和反应速率.并与文献中的实验值进行对比,模拟结果与实验值有很好的吻合.     

Modeling particle growth and deposition in a tubular CVD reactor

A computational model is developed to simulate the particle formation, growth, and deposition in a tubular CVD reactor. The model takes into account the momentum, heat, and mass transfers, chemical reaction, Brownian coagulation, Brownian diffusion, thermal diffusion, and thermophoresis that occur within the reactor. The chemical system is illustrated by the formation of TiO₂ particles through oxidation of TiCl₄. A set of coupled partial differential equations is solved with the finite volume method to give spatial distributions of velocities, pressure, temperature, TiCl₄ concentration, and sizes of TiO₂ particles, from which the deposition mass fluxes and size distributions of exit TiO₂ particles can be readily evaluated. In the model, the growth of particles is simulated by the discrete-sectional model, which can cover particle growth from monomers up to the submicron size. Among other things, the simulation predicts a maximum deposition flux near the reactor entrance and a rebound in deposition flux near the reactor exit, which have been observed in several experimental studies. The model also reveals a delay in evolution of size distribution near the reactor wall as compared to that of the bulk. This delay signifies that the size distribution of depositing particles at the wall is in general different from that of the bulk, which normally is the measured one. The effects of important operating parameters such as temperature, pressure, inlet TiCl₄ concentration, and inlet stream velocity on various system performance indicators are also investigated.     

In situ mesa etching and immediate regrowth in a HVPE reactor for buried heterostructure device fabrication

Mesa etching in a hydride vapour-phase epitaxy (HVPE) reactor has been studied. Etched depth, underetching and shape of the mesas have been analysed as a function of partial pressures of active gases (HCl, PH₃ and InCl), stripe orientation and etching temperature. The experimental results show that the depth and undercut can be etched independently. We propose qualitative mechanisms for etching each of the emerging crystallographic planes ((0 0 1), (1 1 0) and {1 1 1}). In situ mesa etching with immediate regrowth was applied to the fabrication of buried heterostructure Fabry–Perot lasers. No surface contamination due to exposure to ambient and low process time are advantages of this technique.     

Replacement of hydrides by TBAs and TBP for the growth of various III–V materials in production scale MOVPE reactors

Besides the standard group V precursors AsH₃ and PH₃, so-called alternative precursors like TBAs and TBP (tertiary-butyl-arsine and tertiary-butyl-phosphine) are more and more important in today's MOVPE processes. A lot of publications have demonstrated that these precursors can be successfully used for the growth of different III–V materials. In this study we want to demonstrate that TBAs and TBP can be used as the group V precursor in a complete family of production scale reactors. It is shown that these precursors can be used for the growth of InP-based as well as for GaAs-based materials. The reactors that have been employed are medium scale reactors (AIX 200/4; 1 × 2 inch, 3 or 4 inch or 3 × 2 inch capability) and large scale Planetary Reactors®, in particular the AIX 2400 system (15 × 2 inch or 5 × 4 inch). Materials that have been grown are (Al)GaInP on GaAs and GaInAsP on InP. The lower cracking energy of these precursors compared to PH₃ and AsH₃ allows one to use lower growth temperatures and lower V/III ratios, particularly in combination with the high cracking efficiencies of the used reactors. For the growth of GaInAsP on InP, the consumption of TBP and TBAs is up to 8 times lower than using PH₃ and AsH₃. GaInP on GaAs could be grown with a V/III ratio as low as 25 in a Planetary Reactor®. Good crystalline quality is demonstrated by DCXD (e.g. for GaInP: FWHM = 35 arcsec, substrate 32 arcsec). PL intensity and growth rate are not affected by using the alternative precursors. The compositional uniformity is similar to layers grown with arsine and phosphine (e.g. 1.5 nm uniformity for GaInAsP (λ = 1.5 μm) on 2 inch; approximately 1 nm uniformity for GaInP) [1,2]. The purity of the grown layers depends mainly on the quality of the TBP and TBAs. Using high purity TBP, InP revealed background carrier concentration in the mid 10¹⁴ cm⁻³ regime. Our investigation shows that TBP and TBAs can replace phosphine and arsine in state of the art MOVPE reactors. Both for single and multi-wafer production MOVPE reactors these compounds can be used successfully for the growth of the entire material spectrum in the Al---Ga---In---As---P system.     

Soft hydrogels for dual use: Template for metal nanoparticle synthesis and a reactor in the reduction of nitrophenols

A novel hydrogel based on poly(sulfopropylmethacrylate) (p(SPM) with different crosslinking degrees was synthesized and characterized. The prepared hydrogels were for the first time, utilized for in situ metal nanoparticle preparation such as Ni, Co, and Cu and employed as a reaction media in catalytic reduction of 4-nitrophenol (4-NP), and 2-nitrophenol (2-NP) to 4-aminophenol and 2-aminophenol, respectively. The experimental parameters that effect reduction rates such as temperature and the amount of catalyst were investigated. The kinetics of the reduction reaction of nitro compounds under different reaction conditions were investigated to determine the activation parameter. Activation energies were found as 33.86 kJ mol^? 1 and 24.96 kJ mol^? 1 for 4-NP and 2-NP, respectively. It was found that hydrogel–Cu composites can provide 98% activity even at the end of the 7th repetitive usage.     

Quartz crystal microbalance as a tool for scale deposition monitoring in Bayer process conditions

This paper describes the study of bayerite or gibbsite scale formation and growth from supersaturated sodium aluminate solutions using a quartz crystal microbalance. Analysis of frequency vs time curves and scanning electron microscopy images allowed us to propose a surface nucleation mechanism that leads to cementation of particles produced by catastrophic secondary nucleation. We also highlighted the influence of the filtration of the supersaturated sodium aluminate solution step on the scale nucleation kinetics. It was observed that use of filter with porosity below 2.5 μm delays the formation of scale.     

Heat transfer and mass transport in a multiwafer MOVPE reactor: modelling and experimental studies

An improved detailed model for the calculation of the temperature distribution in a multiwafer Planetary Reactor™ has been developed. The temperature field of the reactor has been calculated in dependence of the reactor parameters for (Al,Ga)As growth as well as on the kind and the thickness of the wall and susceptor deposits. The amount of parasitic wall deposits can be minimized by a proper tuning of the reactor temperature distribution. Calculated GaAs growth rate profiles on 3 inch wafers show a strong dependence on the temperature field in the reactor and the amount of parasitic deposits. These predicted relationships have been used to optimize the reactor temperature distribution in order to minimize parasitic wall depositions. By this procedure a growth rate uniformity of < 1% on 3 inch wafers can be reproducibly achieved.     

Thermal decomposition of di-tertiarybutyl selenide and dimethylzinc in a metalorganic vapour phase epitaxy reactor

The thermal decomposition of di-tertiarybutyl selenide (DtBSe), both alone and in the presence of dimethylzinc (DMZn), was investigated using “ex-situ” Fourier transform infrared (FTIR) absorption spectroscopy in a low-pressure metalorganic vapour phase epitaxy (LP-MOVPE) reactor. The decomposition of DtBSe alone, yields isobutene as the major product, with a much smaller proportion of isobutane detected. Pyrolysis of DMZn in dihydrogen in the presence of DtBSe is very similar to pyrolysis of DMZn alone in dihydrogen with methane the exclusive product. This indicates that co-pyrolysis of the DMZn/DtBSe mixture occurs via radical attack by H on DMZn and largely independent pyrolysis of DtBSe via a β-hydrogen elimination reaction. Traces of the intermediate tertiarybutyl selenol (^tBuSeH) were also detected. The small difference observed in the decomposition behaviour of the DtBSe-DMZn mixture in a dihydrogen compared to a helium ambient further indicate that the pyrolysis processes are independent. These conclusions are supported by PM3 semi-empirical molecular orbital calculations, which indicate that the most likely pathway for unimolecular dissociation of DtBSe is via β-hydrogen elimination with C---Se bond homolysis only likely to be an effective competing mechanism at higher growth temperatures and reactor pressures.     

Microstructure and growth rate of chemical vapour deposited TiN films in a vertical cold wall reactor

TiN films were grown on SUS304 substrates heated by an induction furnace in a vertical cold wall reactor. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction were used to characterize the microstructures of films obtained at different deposition conditions (temperature, gas flow rate and gas composition). Film structures obtained in the present vertical reactor had the following features compared with those in the tubular reactor: (1) Abnormally grown “star-shaped” crystals were observed on the surfaces of films deposited in the following ranges of total gas flow rate (Q_T), temperature (T) and partial pressures (P): 9.0×10⁻⁶ ≤ Q_T ≤ 1.6×10⁻⁵ m³ s⁻¹, 1223 ≤ T ≤ 1273 K, 0.92 ≤ P_TiCl4 ≤ 6.18 kPa, P_H2 = P_N2. The matrix grains were responsible for (211) preferred orientation. (2) Surface morphologies did not vary so much with P_TiCl4. On the other hand, a drastic change was brought about by adding HCl to the source gas, i.e., plate-shaped crystals dominated and the large “star-shaped” crystals were no longer present. (3) The apparent activation energy for deposition reaction was 230 kJ/mol (1173 ≤ T ≤ 1273 K) and 76.5 kJ/mol (1273 ≤ T ≤ 1373 K) at P_TiCl4 = 2.43 kPa and P_H2 = P_N2 = 49.45 kPa.     

Numerical simulation of thermoconvective flows and more uniform depositions in a cold wall rectangular APCVD reactor

At atmospheric pressure, the usual flow conditions in the cold wall horizontal rectangular thermal CVD reactors correspond to steady longitudinal thermoconvective rolls that make non-uniform vapour depositions, in shape of longitudinal parallel ridges. In order to get more uniform depositions, the pressure is generally lowered under the atmospheric pressure to promote forced convection flows, instead of mixed convection ones. In the present paper, using three-dimensional direct numerical simulations, we propose and analyse a method to get uniform deposition without lowering the pressure in the reactor. It consists in adequately exciting the parallel thermoconvective rolls at channel inlet to make them unsteady, periodic and sinuous in order to get a uniform time average of the deposition. This method is shown to be adapted for the horizontal and rectangular APCVD reactors with large longitudinal and transversal aspect ratios, when the Reynolds number of the gas flow is O(100), whatever the value of the surface Damköhler number. This situation is encountered in the online or scrolling APCVD reactors used to deposit coatings on float glass in the flat glass industry for instance. The simulations are based on simplified models for the transport equations (Boussinesq model) and the kinetics of the heterogeneous reactions (deposition model of silicon from hydrogen and silane: SiH₄→Si+2H₂).     

Highly uniform AlGaAs/GaAs and InGaAs(P)/InP structures grown in a multiwafer vertical rotating susceptor MOVPE reactor

Highly uniform AlGaAs/GaAs and InGaAs(P)/InP epitaxial layers have been grown in a vertical rotating susceptor MOVPE reactor capable of accommodating three 2′ wafers. The unique water-cooled “showerhead”-type injection distributor which is located 1.5 cm above the substrates ensures a uniform reactant distribution, resulting in uniform growth over a wide range of growth conditions. Periodic multilayer and single layer structures have been used to investigate the thickness and compositional uniformities. The thickness variations over a radial distance of 48 mm for three wafers grown in the same run are within ± 2% for both AlGaAs and InGaAs layers, resulting in a standard deviation of only 0.9%. The gallium concentration of an InGaAs layer varies from 46.88% to 47.01% over the same radial distance with the standard deviation of 0.043%. Measurements of InGaAsP layers grown onto 2′ InP wafers with different alloy compositions show good compositional uniformity yielding standard deviations within 4.4 nm in PL wavelength and 135 ppm in lattice mismatch over a 46 mm radial distance.     

Crystallization of potassium sulfate by cooling and salting‐out using 1‐propanol in a calorimetric reactor

A study was made on a isothermal process for the crystallization of potassium sulfate as an alternative to the cooling process. The process employs addition of 1‐propanol to aqueous salt solutions to achieve the “saltingout” of the K₂SO₄. This work was carried out using an automated Mettler Toledo model RC1 reactorcrystallizer with 800 ml capacity, and controlled isothermally at 25 °C to test the crystallization of K₂SO₄ by addition of the alcohol, and from 50 to 10 °C for the cooling crystallization. In both systems, the line of nucleation points was shown to be approximately parallel to the saturation curve, with an average width of 13°C or 3 % mass for crystallization by cooling, compared with 0.2 to 1 % by salting‐out. In experiments on crystallization by cooling, the K₂SO₄ crystals were 0.27 mm in mean size, showed 7 % agglomeration, and contained 8.5 % moisture. Crystals obtained by salting‐out had a mean size of 0.79 mm, 28 % agglomeration, and 9‐10 % moisture content. A crystal shape factor of approximately of 0.7 was obtained in both systems, apart from the agglomeration.