Beta-Ga2O3: materials synthesis and device demonstration (Conference Presentation)

Ultrawide bandgap (UWBG) gallium oxide (Ga2O3) represents an emerging semiconductor material with excellent chemical and thermal stability up to 1400 oC. 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). It exhibits high transparency in the deep ultraviolet (DUV) and visible wavelength region due to its very large bandgap. The monoclinic b-phase Ga2O3 represents the thermodynamically stable crystal among the known five phases (α, β, γ, δ, ε). The breakdown field of β-Ga2O3 is estimated to be 8 MV/cm, which is about three times larger than that of 4H-SiC and GaN. These unique properties make β-Ga2O3 a promising candidate for high power electronic device and solar blind photodetector applications. More advantageously, single crystal β-Ga2O3 substrates can be synthesized by scalable and low cost melting based growth techniques such as edge-defined film-fed growth (EFG), floating zone (FZ) and czochralski methods. For β-Ga2O3 thin film synthesis, both molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) have been demonstrated to produce high quality and controllable doping films but with slow growth rates (2-10 nm/min). Halide vapor phase epitaxy (HVPE) using chloride precursors was demonstrated to grow β-Ga2O3 films with fast growth rates (>5 μm/hr). Recently, we have developed a low pressure chemical vapor deposition (LPCVD) method to grow high quality β-Ga2O3 thin films on both native Ga2O3and c-sapphire substrates with controllable doping and fast growth rates up to 10 μm/hr. In this talk, we present a study on the defects in β-Ga2O3 thin films grown via LPCVD. The β-Ga2O3 thin films were grown on native β-Ga2O3 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. The β-Ga2O3 thin films were characterized by using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). FESEM images were taken with Helios 650. High resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction (SAED) were taken using a FEI Tecnai F30 at 300 kV. Bright field, dark field, combined with two-beam condition TEM were used to characterize the defects in the as-grown films. From our studies, β-Ga2O3 thin films grown on (010), (001) and (-201) β-Ga2O3 substrates have shown different growth rates as well as different properties of interfacial defects. Improved LPCVD growths to suppress defects in β-Ga2O3thin films grown on different substrates will be discussed. Room temperature Hall measurements will be performed to understand the dependence of carrier transport on defects. In summary, the fundamental defect studies are performed on β-Ga2O3 thin films grown via LPCVD on Ga2O3 and sapphire substrates. The results from this study are important knowledge for power device applications.

In this talk, metalorganic chemical vapor deposition (MOCVD) development of GaN-on-GaN targeting for vertical power devices will be presented. To achieve vertical GaN PN diodes with high breakdown voltage (Vbr) and low on resistance (Ron), it is critical to develop thick GaN drift layer with low controllable doping (Nd-Na) and high electron mobility. Key impurities in MOCVD GaN serving as charge compensation centers were identified. For example, mechanisms of Fe incorporation in MOCVD GaN were identified and addressed [1]. Source of C incorporation is mainly from metalorganic precursor (TMGa), and the C concentration in GaN is highly dependent on MOCVD growth condition, particularly the growth rate. Typically, as GaN growth rate increases, C incorporation increases undesirably. To address this key challenge for achieving high quality thick GaN drift layer, we developed a laser-assisted MOCVD (LA-MOCVD) growth technique to enable fast GaN epitaxy while suppressing C incorporation. The effects of LA-MOCVD GaN growth parameters on GaN growth rate, impurity incorporation and charge transport properties will be discussed. GaN PN diodes with breakdown voltage ~ 3kV are demonstrated.This talk will also discuss our recent developments of ultrawide bandgap (UWBG) Ga2O3, (AlxGa1-x)2O3, and heterostructures [2-5]. Challenges and opportunities for developing (AlxGa1-x)2O3 on Ga2O3 substrates with different crystal orientations ((010), (100), (-201)) be discussed. Band offsets between AlGaO/GaO heterostructures were determined based on high quality epitaxy of AlGaO/GaO heterostructures. Experimental results agree very well with the values predicted from theoretical density functional theory (DFT). Growth techniques based on MOCVD and low pressure chemical vapor deposition (LPCVD) will be compared and discussed. Acknowledgement: The authors acknowledge the funding support from Advanced Research Projects Agency-Energy (ARPA-E) DE-AR0001036, U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) under the Advanced Manufacturing Office, FY18/FY19 Lab Call, the Air Force Office of Scientific Research FA9550-18-1-0479 (AFOSR, Dr. Ali Sayir) and the National Science Foundation (1810041, 1755479, 2019753). [1] Y. Zhang, Z. Chen, W. Li, H.-S. Lee, M. R. Karim, A. R. Arehart, S. A. Ringel, S. Rajan, H. Zhao, J. Appl. Phys., 127, 215707, 2020.[2] Z. Feng, A F M A. U. Bhuiyan, M. R. Karim, H. Zhao, Appl. Phys. Letts., 114, 250601, 2019.[3] A F M A. U. Bhuiyan, Z. Feng, J. M. Johnson, Z. Chen, H. -L. Huang, J. Hwang, H. Zhao, Appl. Phys. Lett., 115, 120602, 2019.[4] A F M A. U. Bhuiyan, Z. Feng, J. M. Johnson, H.-L. Huang, J. Sarker, M. Zhu, M. R. Karim, B. Mazumder, J. Hwang, and H. Zhao, APL Materials, 8, 031104, 2020.[5] A F M A. U. Bhuiyan, Z. Feng, J. M. Johnson, H.-L. Huang, J. Hwang, and H. Zhao, Appl. Phys. Letts, 117, 252105, 2020.

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

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

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.

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

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.

Room temperature excitonic nonlinear absorption in multiple quantum wells (MQW) has great potential for making low-power high-speed optical switching devices. The required ultrathin multilayers have been grown by both molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). Efficient, narrow-linewidth luminescence and low-threshold laser operation have been achieved in MQW’s with both techniques. However, virtually all of the reported work to date dealing with nonlinear optical properties has employed MBE-grown materials CHEMLA /1/, CHEMLA /2/, MILLER /3/. The implementation of arrays of photonic switches in optical computing and processing systems will require large area devices with uniformity that exceeds the capability of current technology. The potential advantages of MOCVD for large area, multiple wafer growth of these complex structures provides motivation to explore this technique for their fabrication. In this paper, the growth by MOCVD of GaAs/AlGaAs MQW structures that exhibit narrow-line, room temperature excitonic absorption are reported. We further describe the linear and nonlinear optical properties of these structures and discuss the implications of this work for the construction of large area arrays of switches.

Degradation prediction of AlGaN/GaN MODFET is explored based on characterization of gate and drain low- frequency noise. Heterostructures grown by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) are used for this purpose. Devices from the former category were unpassivated while those of the latter were passivated. Despite the highly variable gate noise current characteristics among unpassivated MBE devices and between MBE and MOCVD-based devices, it is demonstrated that the drain noise current characteristics of the two groups of devices have considerable resemblance. Moreover, it is shown that the drain noise current level can be used as a means for gate degradation prediction

Chapter 6 - Chemical Beam Epitaxy

The CdS/CdTe/ZnTe n-i-p solar cell and its ternary relatives have the potential to meet Department of Energy cost, efficiency, and stability goals. This report describes results of a continuing program to achieve these goals. A record-breaking efficiency of 11% has been demonstrated and verified at the Solar Energy Research Institute (SERI). Stability testing for 3000 hours indicates that the n-i-p structure is stable. Improving the short-circuit current by substituting Cd/sub x/Zn/sub 1-x/S for CdS has been successful and has produced 8+% efficient cells with 2.6 eV windows using improved pyrolysis equipment. Transparent n-i-p devices have been produced with a SERI-verified efficiency of 9.4%. Collaborations with researchers at the Georgia Institute of Technology, the Institute of Energy Conversion, and Jet Propulsion Laboratory have resulted in jointly produced n-i-p cells. Cells produced by molecular beam epitaxy and metal organic chemical vapor deposition had efficiencies greater than 9%; cells produced by thermal vacuum evaporation had efficiencies greater than 7%. 18 refs., 35 figs., 13 tabs.

Application of spectrally resolved scanning photoluminescence to assess relaxation processes of InGaAs and InAlAs layers strained in compression and tension

Surface morphology of cubic and wurtzite GaN films

Rutile GeO2 is a promising ultra-wide bandgap semiconductor for future power electronic devices whose alloy with the wide bandgap semiconductor rutile-SnO2 enables bandgap engineering and the formation of heterostructure devices. The (SnxGe1−x)O2 alloy system is in its infancy, and molecular beam epitaxy (MBE) is a well-suited technique for its thin-film growth, yet it presents challenges in controlling the alloy composition and growth rate. To understand and mitigate this challenge, the present study comprehensively investigates the kinetics and thermodynamics of suboxide incorporation into GeO2, SnO2, and (SnxGe1−x)O2 during suboxide MBE (S-MBE), the latest development in oxide MBE using suboxide sources. We find S-MBE to simplify the growth kinetics, offering better control over growth rates than conventional MBE but without supporting cation-driven oxide layer etching. During binary growth, SnO incorporation is kinetically favored due to its higher oxidation efficiency and lower vapor pressure (limiting its loss by desorption) compared to those of GeO. In (SnxGe1−x)O2 growth, however, the GeO incorporation is preferred and the SnO incorporation is suppressed, indicating a catalytic effect, where SnO promotes GeO incorporation. The origin of this catalytic effect cannot be understood by comparing the binary kinetics or thermodynamics (cation–oxygen bond strengths), thus calling for further theoretical studies. Our experimental study provides guidance for controlling the growth rate and alloy composition of (SnxGe1−x)O2 in S-MBE, highlighting the impact of the substrate temperature and active oxygen flux besides that of the mere SnO:GeO flux stoichiometry. The results are likely transferable to further physical and chemical vapor deposition methods, such as conventional and hybrid MBE, pulsed laser deposition, mist-, or metalorganic chemical vapor deposition.