Environmental safety issues for molecular beam epitaxy platform growth technology

Differences in epitaxial-layer/substrate interface properties of hetero-junction field effect transistors fabricated by molecular beam epitaxy and metal organic chemical vapor deposition

MBE and MOCVD growth and properties of self-assembling quantum dot arrays in III-V semiconductor structures

Far-infrared photothermal ionisation spectroscopy is used to investigate the nature of the donors present in high-purity epitaxial films of GaAs grown by metal-organic chemical vapour deposition (MOCVD), metal-organic gas source molecular beam epitaxy (MOMBE) and solid source molecular beam epitaxy (MBE). Except for one lower mobility sample, the MOCVD samples showed the donor line X1 to be dominant with >50% relative intensity. The higher mobility samples showed the presence of two relatively closely spaced components to X1. Hydrostatic pressures up to 20 kbar were used to increase the chemical shifts and to vary the absorption strengths of the lines. By this means, and by the simultaneous observation of the same components on the 1s-2p and 1s-3p transitions, it was possible to show that these components were not artefacts of over-absorption but arose from two different donor species of very similar chemical shifts. Consequently, any identification of donor X1 as being solely due to silicon must be treated with caution. Germanium and sulphur accounted for the remaining donor contamination in the MOCVD material, with germanium usually being present at higher levels than sulphur. In contrast, earlier far-infrared and photoluminescence data with samples of different origin have shown germanium to be dominant in MOCVD material. MOMBE growth produced very similar proportions of X1, germanium and sulphur, indicating that the gas sources are responsible for the contamination. The MBE samples grown in reactors of different manufacture showed levels of sulphur donors comparable with the background carbon acceptors as well as traces of germanium, in addition to the silicon deliberately introduced. The results for the highest mobility MBE sample with deliberate silicon doping suggest that the lower energy component of the 'X1' doublet is due to silicon. The most likely candidate for the higher energy component is selenium. An even higher mobility MBE sample which was n-type without deliberate silicon doping showed the higher energy component attributed to selenium.

We describe simple interferometric reflectometry systems that have been applied to real-time monitoring of epitaxial growth in a variety of crystal growth systems. These include molecular beam epitaxy (MBE) systems from Vacuum Generators and Varian, and metalorganic chemical vapor deposition reactors from Aixtron and Emcore. The reflectometry systems are composed entirely of off-the-shelf hardware, including charge-coupled-device-based spectrometers and light bulb white light sources. These fixed modules are connected by simple fiber optic links. Input and output lenses are matched to the crystal growth systems such that unmodified viewports and rotating sample stages/platens are fully accommodated. The systems are driven by a WindowsTM-based program operating on a conventional PC. This program permits display of the time-dependent spectra as either line graphs or grayscale converted ‘‘fingerprints’’ that may be overlaid, in real time, on data from reference growth runs. It also includes provision for full simulation of complex layered structures in the GeSi, AlGaAs, and InGaAsP materials systems, along with corrections for elevated growth temperature. For the MBE-based GeSi system, vertical resonant cavity photodiode structures were monitored. The reflectometry system easily revealed growth rate deviations of less than 1% and permitted termination of growth at precise resonance conditions.

Chapter 6 - Chemical Beam Epitaxy

Ultrawide bandgap (UWBG) gallium oxide (Ga2O3) represents an emerging semiconductor material with excellent chemical and thermal stability. It has a band gap of 4.5-4.9 eV, much higher than that of the GaN (3.4 eV) and 4H-SiC (3.2 eV). The monoclinic beta-phase Ga2O3 represents the thermodynamically stable crystal among the known five phases . The breakdown field of beta-Ga2O3 is estimated to be 6-8 MV/cm, which is much larger than that of the 4H-SiC and GaN. These unique properties make beta-Ga2O3 a promising candidate for high power electronic device and solar blind photodetector applications. More advantageously, single crystal beta-Ga2O3 substrates can be synthesized by scalable and low cost melting based growth techniques. Different from the molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) growth techniques, we have developed a low pressure chemical vapor deposition (LPCVD) method to grow high quality beta-Ga2O3 thin films on both native Ga2O3 and c-sapphire substrates with controllable doping and fast growth rates up to 10 um/hr. In this talk, we present the growth, material characterization and device demonstration of beta-Ga2O3 thin films grown via LPCVD. The beta-Ga2O3 thin films were grown on native beta-Ga2O3 (010), (001) and (-201) substrates and sapphire substrates using high purity gallium and oxygen as the precursors, and argon (Ar) as the carrier gas. The growth temperature ranged between 850 ˚C and 950 ˚C. Fundamental material properties including temperature dependent Hall measurements and device demonstration based on vertical Schottky barrier diodes will be discussed.

This study examines the impact of curtain gas flow on metal-organic chemical vapor deposition (MOCVD) growth of two-dimensional (2D) transition metal dichalcogenides using finite element method simulations and growth experiments. The simulation results demonstrate that the curtain gas changes precursor transfer dynamics, concentrates the flow toward the substrate, and potentially lowers contamination from chamber walls. The simulation findings are supported by experimental validation using tungsten and sulfur sources, which confirms that curtain gas flow is critical in enhancing the reproducibility of 2D WS2 growth. The research highlights the need to optimize gas flow dynamics in MOCVD processes to unlock the full potential of 2D materials in future electronic devices.

In recent years, wide band-gap semiconductor materials have attracted a great deal of attention for use in blue light-emitting and short-wavelength diodes. Additionally, due to its high conductance, chemical and thermal stability, and high piezoelectric coupling coefficient, ZnO is also used for piezoelectric devices, such as surface acoustic wave (SAW) devices [1] and bulk acoustic devices [2]. Various deposition techniques, including sputtering [3, 4], pulsed laser deposition (PLD) [5, 6], ion beam deposition [7], chemical vapor deposition (CVD) [8, 9], atomic layer deposition (ALD) [10], metal-organic chemical vapor deposition (MOCVD) [11, 12], and molecular beam epitaxy (MBE) [13] have been employed for the growth of ZnO films. However, MOCVD has an advantage in achieving devices in commercial level since high deposition rate and highquality film is attainable, especially at low pressure. High quality ZnO films grown on Si substrate pave the way from integration of devices with Si IC technology. Also, amorphous substrates such as SiO2 and glass substrate have obvious technological advantages and potential applications [14]. Although most researchers have grown ZnO films on sapphire substrates, there are not many reports on growing ZnO thin films on Si or Sibased materials by MOCVD technique. Furthermore, there are rare reports on the MOCVD growth of ZnO at low temperatures below 300 ◦C. In this work, we investigate the growth of ZnO thin films on silicon substrate at the temperature range of 100–200 ◦C by employing the MOCVD technique. Since the material properties strongly affect the performance of a device, the influence of substrate temperature on the structural properties of the samples are presented. The ZnO films were deposited on p-type Si(100) substrates by MOCVD system using Zn(C2H5)2 (99.9999% purity DEZn (diethylzinc)) and O2 (99.999% purity). Fig. 1 shows a schematic diagram of the MOCVD reactor used in our experiments. Before loading into the reactor, the substrate was cleaned in acetone for 10 min, HF (20:1) for 1 min and then rinsed by deionized water for 1 min. High-purity Ar was passed through the DEZn bubbler and saturated with DEZn vapor to the reactor. For MOCVD growth of ZnO films, the gas phase reaction will result in particle formation, which will degrade the ZnO film quality. In order to minimize the gas phase reaction, Zn(C2H5)2

Epitaxial film growth and characterization

Selective metalorganic chemical vapor deposition growth of GaAs on AlGaAs combined with in situ HCl gas etching

Single junction AlGaAs/GaAs and InGaP/GaAs solar cells and test structures have been grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD), respectively, on Si wafers coated with compositionally-graded GeSi buffers. The combination of controlled strain relaxation within the GeSi buffer and monolayer-scale control of the Ill-V layer nucleation is shown to reproducibly generate minority carrier lifetimes exceeding 10 nanoseconds within GaAs overlayers. The III-V layers are free of long-range antiphase domain disorder, with threading dislocation densities in the high-10/sup 5/ cm/sup -2/ range, consistent with the low residual dislocation density in the Ge cap of the graded buffer structure. Single junction GaAs cells grown by both MBE and MOCVD on the Ge/GeSi/Si substrates demonstrated high V/sub oc/ values for GaAs cells grown on Si. Record V/sub oc/ values for MOCVD-grown single junction InGaP/GaAs cells exceeded 980 mV (AMO) with fill factors of 0.79. Additionally, external quantum efficiency data indicates no degradation in carrier collection from GaAs homoepitaxial cells for current single-junction cell designs grown by MBE. Based on these results, cell efficiencies in excess of 18.5% under AM0 conditions should be attainable with cell designs demonstrating state of the art J/sub sc/ values. Such cell performance demonstrates the potential and viability of graded GeSi buffers for the development of Ill-V cells on Si wafers.

The GaN-rich side of GaP/sub x/N/sub 1-x/ alloy exhibits a potentially large bandgap bowing with only a small amount of P incorporation. As a result, there could be interesting device applications. GaN:P layers were grown by molecular beam epitaxy (MBE) on GaN template layers grown by metalorganic chemical vapor deposition. GaP was used as the phosphorus source, P/sub 2/, and the N/sub 2/ plasma as the nitrogen source.

Because of several potential applications and advantages afforded by the heteroepitaxial GaAs-on-Silicon material system, several groups world-wide are attempting to grow device-quality GaAs on Si substrates.eg.1 Both metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) growth techniques have been widely utilized to achieve heteroepitaxial growth. However, certain fundamental materials and growth problems have thus far prevented any group from achieving heteroepitaxial GaAs of a quality similar to that obtainable from bulk GaAs crystals. A high density of threading dislocations, microtwins/stacking faults, antiphase domain boundaries (APBs) and microfissures can form under non-ideal conditions. These defects result, in part, from stresses generated due to the ∼4% lattice mismatch and the different coefficients of thermal expansion between GaAs and Si.2 Ex-situ characterization of this materials system is essential to assess the material quality and to provide direction for future growth experiments. This contribution describes the TEM characterization methodology that we employ to analyze our GaAs grown on Si substrates by MBE.

In this work, a novel device concept for improving the efficiency and field operation of conventional InAs/GaAs quantum dot (QD) based intermediate band solar cell (IBSC) with the feature of current constraint (CC-QD-IBSC) was proposed and evaluated from both theoretical simulations and experimental demonstrations. The advantages of the proposed device concept including increasing efficiency, obtaining higher suitability for operation under concentrated photovoltaic (CPV) condition and more robust efficiency stability against intermediate band fluctuation were rigorously identified using the detailed balance theorem by comparing with conventional InAs/GaAs QD-IBSC. Furthermore, a hybrid molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) shuttle growth of the proposed CC-QD-IBSC were adopted in this work for better preserving QD induced intermediate band feature and overcoming practical growth difficulties. By introducing a InGaP top cell for constraining the current of InAs/GaAs QD-IBSC, the short circuit current density of CC-QD-IBSC decreased to 13.0mA/cm2 under 1 sun illumination, a near 50 % reduction compared to the conventional InAs/GaAs QD-IBSC. On the contrary, by well adjusting the regrowth interface between MBE and MOCVD, we managed to reach the highest conversion efficiency of 22.8 % measured at an UL certified laboratory under light concentration ratio of 93 suns for the proposed CC-QD-IBSC with well-functioning QD and IB feature. It is our belief that our results pave the road for developing techniques geared towards higher efficiency and more practical field applications.

InP/InGaAsP/InGaAs avalanche photodiodes with sepa- rate absorption, grading, and multiplication regions (SAGM-APD's) have been fabricated from wafers grown by chemical beam epitaxy (CBE). These APD's exhibit low dark current ( 500-Mbit/s) lightwave sys- tems experiments have shown that avalanche photodiodes (APD's) can provide several decibels of improvement in receiver sensitivity compared to p-i-n photodiodes ( 11-(5). In long-wavelength (= 1.3- or 1.5-pm) lightwave receivers the highest sensitivities have been achieved with APD's having an InP multiplication region and an 1~,~~Ga,-,.~~As (subscripts deleted in following text) absorption region separated by a transition region consisting of one or more lattice-matched intermediate-bandgap In,Gal -,AsL.P1 -y layers, the separate absorption, grading, and multiplication regions (SAGM)- structure APD ( 11. To operate properly, the carrier concentra- tions and thicknesses of the different layers which comprise this structure must be controlled within very narrow tolerances (6). Although most of the SAGM-APD's that have been reported to date have been grown by liquid-phase epitaxy (LPE), the degree of precision and uniformity necessary to maintain the wafer parameters within the constraints required by this structure are difficult to achieve by this growth technique. Also, LPE is more difficult and more expensive to implement in large-scale production than vapor-phase tech- niques such as hydride vapor-phase epitaxy (VPE), metal- organic chemical vapor deposition (MOCVD), or molecular beam epitaxy (MBE). In the past, however, InP/In,- Gal-,As,P1-, photodetectors grown by MBE and MOCVD have not achieved performance characteristics as good as those grown by LPE due to the high background carrier concentra- tions of the In,Gal-,As,PI-, layers and the difficulty in achieving good InP/In,Gal -,As,P1 ->, interfaces. Recently, InPIInGaAsPiInGaAs SAGM-APD's grown by MOCVD (7) and VPE (8), (9) have been reported. In this paper we report on InP/InGaAsP/InGaAs SAGM-APD's grown by chemical beam epitaxy (CBE) (lo), a crystal growth technique that combines many of the advantages of MOCVD and MBE. A schematic cross section of the back-illuminated InP/