Multi-wafer Reactor Research Articles

Commercial production of QWIP wafers by molecular beam epitaxy

As the performance of quantum well infrared photodetectors (QWIPs) and QWIP-based imaging systems continues to improve, their demand will undoubtedly grow. This points to the importance of a reliable commercial supplier of semiconductor QWIP material on three inch and, in the near future, four-inch substrates. Molecular beam epitaxy (MBE) is the preferred technique for growing the demanding QWIP structure, as tight control is required over the material composition and layer thickness. We report the current status of MBE-grown GaAs-based QWIP structures in a commercial production environment at IQE. Uniformity data and run-to-run reproducibility on both three-inch and four-inch GaAs substrates are quantified using alloy composition and QW thickness. Initial results on growth technology transfer to a multi-wafer MBE reactor are also presented. High-resolution X-ray diffraction measurements demonstrate GaAs QW thickness variations and AlGaAs barrier compositions changes to be less than 4% and 1% Al, respectively, across four-inch QWIP wafers from both single- and multiple-wafer MBE platforms.

MOVPE of GaSb, (AlGa)Sb and (AlGa)(AsSb) in a multiwafer planetary reactor

Trimethylaluminum, tritertiarybutylaluminum (TTBAl) and dimethylethylamine alane (DMEAA) are compared as aluminum precursors for the growth of (AlGa)Sb and (AlGa)(AsSb) in an industrial size multiwafer planetary MOVPE reactor. The thermal activation behavior of the aluminum precursors is shown to be a critical factor as the growth is in the kinetically controlled regime. Both the efficiencies of TTBAl and DMEAA are shown to depend on the V/III-ratio which is attributed to prereactions with trimethylantimony. Oxygen and carbon concentrations are reported for various compositions of (AlGa)Sb. Compared to TTBAl, the use of DMEAA results in an oxygen level in (Al0.19Ga0.81)Sb which is reduced by approximately one order of magnitude.

Structure property relations of BST thin films

We report on the properties of thin BST films which were grown in a planetary multi-wafer MOCVD reactor which handles 5 six inch wafers simultaneously. The reactor is combined with a liquid delivery system which mixes the liquid precursors from three different sources: 0.35 molar solutions of Ba(thd)2 and Sr(thd)2 and a 0.4 molar solution of Ti(O-i-Pr)2(thd)2. Film growth on strongly (111)-textured Pt substrates is investigated within a wide parameter field. We focus on the properties of films with the composition Ba0.7Sr0.3TiO3 and thickness between 10 and 130 nm. Topics to be discussed include: the change of the texture of the BST films, which is (100) for deposition temperatures above 600°C and changes to a random orientation at lower temperature, and a comparison of planar films with films on structured substrates. The electrical properties, e.g., permittivity and leakage current, are discussed in relation to the microstructure.

BST thin films grown in a multiwafer MOCVD reactor

Abstract (Ba 0.7 Sr 0.3 )TiO 3 thin films with a typical thickness of 30 nm were deposited on platinized wafers in a planetary multi-wafer reactor combined with a liquid delivery system. As a direct consequence of the reactor design, we obtain high film uniformity over 6 inch wafers as well as efficiencies for the precursor incorporation as high as 40%. The composition and microstructure of the films were routinely investigated by X-ray diffraction and X-ray fluorescence analysis. Further details of the microstructure were investigated by scanning electron microscopy. The electrical properties are characterized in terms of permittivity, loss angle, leakage current and the response to DRAM pulses and characteristic values obtained for depositions at 625°C include a specific capacitance of 60 fF/μm 2 , a tan δ of 0.002 and leakage currents in the order of 10 −10 A/cm 2 at 1 V. The electrical properties are discussed in relation to the microstructure and stoichiometry within a wide range of parameters.

High-efficiency (AlGa)As/GaAs solar cells grown by MOVPE using TBAs at low-temperatures and low V/III-ratios

The alternative As-precursor tertiarybutylarsine (TBAs) is used to grow (AlGa)As–GaAs heteroface solar cell structures. In a horizontal reactor (AIX200) a low growth temperature of 625°C and a low V/III-ratio of 10 was used. Solar cell structures using a arsine (AsH 3)-based MOVPE process were grown in a multi-wafer reactor (AIX2600G3) using growth temperatures of 700°C and a V/III-ratio of 31. The solar cell efficiencies of both structures are 24.2% (AM 1.5 g). The obtained efficiency values are state of the art and demonstrate that TBAs can be an alternative to the more commonly used arsine with no decrease in the solar cell performance.

Advanced III–V solar cell structures grown by MOVPE

The paper reports on the progress of MOVPE-grown III–V epitaxial structures and tandem solar cell development. A multi-wafer reactor with a capacity of five 4-in wafers is used to grow high-efficiency, large-area tandem solar cells. The benefits of advanced tandem structures for concentrator applications based on Ga 0.35In 0.65P/Ga 0.83In 0.17As grown lattice mismatched to GaAs are discussed. In order to achieve super-high efficiencies a mechanical stack of monolithic tandem cells made from Ga 0.51In 0.49P/GaAs and Al 0.25Ga 0.75As 0.02Sb 0.98/GaSb is suggested.

Deposition of thin BST films in a multi-wafer planetary reactor

We report on the performance of a planetary multi-wafer reactor offering extremely high throughput due to the batch mode processing and a low cost of ownership. This reactor is combined with a liquid delivery system which mixes the liquid precursors from three different sources: 0.35 molar solutions of Ba(thd)2 and Sr(thd)2 and a 0.4 molar solution of Ti(O-iPr)2(thd)2. The composition and microstructure of the films were routinely investigated by X-ray diffraction and X-ray fluorescence analysis. As a direct consequence of the reactor design we obtain high efficiencies for the precursor incorporation in the order of 40%. Furthermore, we obtain a high uniformity of the films over 6 inch wafers. Details of the chemistry and microstructure were investigated by secondary neutral mass spectroscopy and scanning electron microscopy. The electrical film properties are discussed in relation to microstructural properties.

Growth of antimony-based materials in a multiwafer planetary MOVPE-reactor

Four-junction photovoltaic solar cells are expected to reach conversion efficiencies around 40%. Mechanical stacking of two monolithic tandem cells is suggested. As a top cell the highly developed GaInP:GaAs tandem cell is considered. Device modelling is presented for the bottom cell. An infrared monolithic tandem solar cell is proposed which is a combination of an (AlGa)(AsSb) top cell (E/sub gap/=1.03 eV) and a GaSb bottom cell based on GaSb as substrate material. The MOVPE growth of GaSb, GaAsSb and AlGaSb in a multiwafer planetary reactor that is suited to large-scale industrial production is reported. Influences of the substrate preparation, orientation of the growth surface, growth temperature and the V-III ratio on material quality are presented. Both hydrogen and nitrogen were employed as the carrier gas in different experiments. The best layers were obtained on [100] substrates oriented 2/spl deg/ toward [111]A, using hydrogen as carrier.

Growth of (Ba,Sr)TiO3 Thin Films in a Multi-wafer MOCVD Reactor

AbstractWe report on the performance of a planetary multi-wafer MOCVD reactor which handles 5 six inch wafers simultaneously. The reactor is combined with a liquid delivery system which mixes the liquid precursors from three different sources: 0.35 molar solutions of Ba(thd)2 and Sr(thd)2 and a 0.4 molar solution of Ti(O-i-Pr)2(thd)2. The microstructure and the film stress were investigated by X-ray diffraction and the composition of the films was determined by X-ray fluorescence analysis. As a direct consequence of the reactor design we obtain a high uniformity of the films over 6 inch wafers, as well as high efficiencies for the precursor incorporation. Film growth is discussed within a wide parameter field and the finally achieved electrical properties, e.g., permittivity, loss tangent, leakage current, are discussed in relation to the microstructural properties.

High temperature CVD systems to grow GaN or SiC based structures

Silicon Carbide is presently gaining much attention as a material for high temperature, high speed and high power devices. However, fabricating epitaxial SiC or GaN films is still a challenge since very high growth temperatures (up to 1600°C) must be used. This requires a carefully adapted design of reactors to ensure laminar flow conditions and a controlled depletion of the reactants inside the reactor. A second class of materials that is also playing a more and more important role today are the III nitrides (AlN, GaN, InN and alloys consisting of these). These materials are also grown at high deposition temperatures (up to 1300°C). Furthermore, these materials are grown on special buffer layers grown at much lower temperatures and are typically part of complex heterostructures which require abrupt changes of the growth temperature to realize sophisticated optoelectronic and electronic devices. As an example our high quality InGaN layers were grown at about 700°C and the AlGaN cladding layer of a laser or a LED require temperatures higher than 1100°C. In general, both nitrides and SiC are similar in their challenges to the growth equipment. This study uses a family of high temperature reactors to grow SiC and Nitrides. We describe the use of high temperature reactors to grow SiC and Nitrides. The load capacity ranges from single wafer machines to multiple wafer mass production reactors. All these reactors have a two flow injection system allowing a separated inlet of the various reactants. To achieve maximum uniformity of the growth, the Gas Foil Rotation ® Principle is applied. The multiwafer reactors are Planetary Reactors with double rotation of substrates. Extensive modeling has been used in order to find the optimum reactor geometries. Gas flow dynamics, temperature management and the physical and chemical properties of the precursors were considered in these models. Thus an optimization of uniformity and efficiency and a minimization of undesired parasitic reactions has been obtained. This contribution will work out the differences and the similarities of the high temperature growth of both material systems in AIXTRON reactors already established in production. Results on the growth of SiC and GaN will be presented.

Epitaxial growth of SiC in a new multi-wafer VPE reactor

SiC epitaxial layers have been grown in a commercial multi-wafer reactor. Results from the initial growth runs are presented. The reactor is vertical and has a high speed rotating susceptor that can support up to six 50 mm diameter wafers. The surface morphology of the grown layers are specular and show no indication of step-bunching. The unintentional background doping is p-type in the low 10 15 cm −3 range, consisting mainly of Al. Both N and Al have been used for doped layers showing wide doping range capability and sharp transients. The best uniformity in thickness and doping achieved so far on the same 35 mm wafer are ±7% and ±10%, respectively.

SiC epitaxial layer growth in a novel multi-wafer vapor-phase epitaxial (VPE) reactor

Experimental results are presented for SiC epitaxial layer growths employing a unique planetary SiC-VPE reactor. The high-throughput, multi-wafer (7×2″) reactor, was designed for atmospheric and reduced pressure operation at temperatures up to and exceeding 1600°C. Specular epitaxial layers have been grown in the reactor at growth rates ranging from 3–5 μm/h. The thickest layer grown to date is 42 μm thick. The layers exhibit minimum unintentional n-type doping of ∼1×10 15 cm −3, and room temperature mobilities of ∼1000 cm 2/V s. Intentional n-type doping from ∼5×10 15 cm −3 to >1×10 19 cm −3 has been achieved. Intrawafer layer thickness and doping uniformities (standard deviation/mean at 1×10 16 cm −3) are typically 4 and 7%, respectively, on 35 mm diameter substrates. Moderately doped, ∼4×10 17 cm −3, layers, exhibit ∼3% doping uniformity. Recently, 3% thickness and 10% doping uniformity (at 1×10 16 cm −3) has been demonstrated on 50 mm substrates. Within a run, wafer-to-wafer thickness deviation averages ∼9%. Doping variation, initially ranging as much as a factor of two from the highest to the lowest doped wafer, has been reduced to ∼13% at 1×10 16 cm −3, by reducing susceptor temperature nonuniformity and eliminating exposed susceptor graphite. Ongoing developments intended to further improve layer uniformity and run-to-run reproducibility, are also presented.

Contact Resistance of InGaN/GaN Light Emitting Diodes Grown on the Production Model Multi-Wafer MOVPE Reactor

We report both the device fabrication and characterization of InGaN/GaN single quantum well LEDs grown on sapphire substrates using multi-wafer MOVPE reactor. To improve current spreading of the LEDs, a self-aligned process is developed to define LED mesa that is coated with a thin, semi-transparent Ni/Au (40 Å/40 Å) layer. A detailed study on the ohmic contact resistance of Ni/Cr/Au on p-GaN versus annealing temperatures is carried out on transmission line test structures. It was found that the annealing temperatures between 300 to 500 °C yield the lowest specific contact resistance rc ( 0.016 Ω-cm2 at a current density of 66.7 mA/cm). Based on the extracted rc from the transmission line measurement, we estimate that the contact resistance of the p-type GaN accounts for ∼ 88% of the total series resistance of the LED.

High quality III-nitride material grown in mass production MOCVD systems

We present some of the latest results obtained from single wafer horizontal tube reactors (AIX 200RF) and multiwafer planetary reactors ® (AIX 2000HT) as used for MOVPE of blue LEDs. The AIX 2000HT was set up in a configuration of 7×2″ which provides unique uniformity capabilities due to the two fold rotation of the substrates. Composition uniformity of ternary material determined by 300 K PL-mappings show peak wavelength variations typically about 1 nm for AlGaN and GaInN. Our latest results are based on studies of various heterointerfaces in the GaInN/GaN system. GaInN single/hetero layers were investigated to optimise photoluminescence properties. Several approaches of capping GaInN/GaN with another GaN layer to develop high quality DH (doublehetero) structures are presented. Using an optimised interfacing technique we obtained device quality DH structures with state of the art composition uniformity across 2″ wafers. The information obtained in the investigation of these DH structures was used to produce high quality SQW (single quantum well) and MQW (multi quantum well) structures.

Multiwafer molecular beam epitaxy for high volume production of GaAs/AlGaAs heterojunction bipolar transistor wafers

Circuits incorporating GaAs/AlGaAs heterojunction bipolar transistors (HBTs) are being increasingly used in commercial applications with volume requirements that can only be met using multiwafer reactors. HBT wafers grown in single and multiwafer molecular beam epitaxy (MBE) reactors are found to be very similar in terms of both material characteristics and resultant device performance. Postgrowth defect levels are reduced by an order of magnitude with the multiwafer system compared to single wafer systems and are similar to those obtained on commercially purchased metal organic chemical vapor deposition wafers. Multiwafer MBE systems are thus shown to be a viable means for the high volume production of HBT wafers.

Production scale MOCVD growth of II–VI semiconductors

The growth of II–VI semiconductors is performed in MOCVD systems (AIX 200 and AIX 200/4) with a capacity of one 2 in and one 4 in wafer or equivalent, respectively, on 2 in substrates. The growth of ZnSe, ZnSSe and ZnMgSSe has been carried out with several different precursor materials. The growth rate and compositon homogeneities across a 2 in wafer for ZnSe and ZnSSe, with and without rotation of the susceptor, were investigated. The growth rate homogeneity for ZnSe was calculated to be ±2%. The fabrication technology can be easily transferred to multiwafer reactors for the production of devices. Homogeneities in the range of ±1% are expected, due to the unique design with double rotation of the substrates.

Contact Resistance of InGaN/GaN Light Emitting Diodes Grown on the Production Model Multi-Wafer Movpe Reactor

AbstractWe report both the device fabrication and characterization of InGaN/GaN single quantum well LEDs grown on sapphire substrates using multi-wafer MOVPE reactor. To improve current spreading of the LEDs, a self-aligned process is developed to define LED mesa that is coated with a thin, semi-transparent Ni/Au (40 Å/40 Å) layer. A detailed study on the ohmic contact resistance of Ni/Cr/Au on p-GaN versus annealing temperatures is carried out on transmission line test structures. It was found that the annealing temperatures between 300 to 500°C yield the lowest specific contact resistance rc (0.016 Ω-cm2 at a current density of 66.7 mA/cm). Based on the extracted rc from the transmission line measurement, we estimate that the contact resistance of the p-type GaN accounts for ∼ 88% of the total series resistance of the LED.

Efficient and uniform production of III-nitride films by multiwafer MOVPE

We present results on the growth of Ga–In–Al–N films in multiwafer Planetary Reactors® with 7×2 in. wafer capacity. The unique design of these reactors allows us to combine high efficiency (Ga efficiency in GaN around 30%) and excellent uniformity. Thickness uniformities better than ±5% on a 2 in. wafer are routinely obtained. Compositional uniformity of GaInN is better than 2 nm. Wafer-to-wafer and run-to-run reproducibility concerning thickness and composition is within the 1% range. Besides this, the material grown also has a high purity. Hall mobilities higher than 500 cm 2 V −1 s −1 are obtained for GaN. Furthermore, the design of the reactors is dedicated to the growth of complex nitride heterostructures. Fast switching manifolds for various group III precursors and different carrier gases allow a flexible design of the growth runs including nucleation, bulk layers (doped/undoped) and quantum wells. The extremely low thermal mass of the reactor permits very rapid changes of the deposition temperature between room temperature and 1300°C (optional: 1600°C). Of course p- and n-type doping is also possible. Doping uniformities are below 1% indicating extremely uniform wafer temperatures. The results demonstrate that Planetary Reactors® are reliable and efficient tools for the mass production of blue-green optoelectronic devices.

Structural properties of GaN grown by MOVPE turbodisc mass-production reactor

The structural properties of GaN films grown on sapphire in a turbodisc multiwafer reactor have been investigated using high-resolution X-ray diffraction and transmission electron microscopy. We found that the line shape of the (0 0 2) rocking curve can be fitted with two peaks. Therefore, we suggest that the GaN films consist of two layers: one related to extended structural defects such as dislocations originating from the film-buffer layer interface and one that is of high quality. The crystalline quality of the second layer continues to improve with increasing layer thickness. We also show that the relatively large width of the (0 0 2) peaks (270 arcsec for a 3 μm film) has no causal link with the GaN film quality. In contrast, we demonstrate that there is a correlation between the in-plane structural features and optical and electrical properties. Using the (1 0 2) asymmetric reflection, 2″ wafer mapping of the (1 0 2) peak width closely mimics the resistivity mapping and photoluminescence intensity mapping. The (1 0 2) peak width is also sensitive to growth conditions of the buffer layer. By optimization of the buffer layer, the (1 0 2) peak width can be reduced to less than 600 s for a 3 μm GaN film. A uniformity better than 2% can be achieved in both the (1 0 2) width and the resistivity mapping over 2″ wafers for all six wafers in the same growth run.

High Quality Al-Ga-In-N Heterostructures Fabricated by MOVPE Growth in Multiwafer Reactors

We present results on the growth of Al-Ga-In-N films in multiwafer reactors with 7×2″ wafer capacity. The design of these reactors allows the combination of high efficiency (TMGa efficiency for GaN around 30%) and excellent uniformity. Results on the growth of all materials from the Al-Ga-In-Nitride family are presented in detail. GaN is grown with an excellent optical quality and very good thickness uniformity below 2% across 2″ wafers. The material quality is shown by electron mobility of more than 500 cm2/Vs at an intentional Si-doping of approximately 1×1017 cm−3. Controlled acceptor doping with Mg yields carrier concentrations between 5×1016 and 1018 cm−3. The layer thickness uniformity of the films are better than 2% over a 2″ wafer area. GaInN is grown with PL emission wavelengths in the visible blue region showing a uniformity better than 1.5 nm standard deviation. The film thickness uniformity represents the same figures as obtained for the binary. The compositional uniformity of AlGaN is in the sub 1% range corresponding to a wavelength variation below 1 nm.The fabrication of heterostructures from these binary and ternary materials is described as well as results from the characterization of these structures. The results show that reliable and efficient production of Al-Ga-In-Nitride based optoelectronic devices can be performed in multiwafer reactors.