Gallium nitride-based microwave high-power heterostructure field-effect transistors

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.

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

To meet the stringent requirements of 5G communication, we proposed a high-performance bulk acoustic wave (BAW) filter based on single crystal AlN piezoelectric films on a SiC substrate. The fabrication of the BAW filter is compatible with the GaN high electron mobility transistor (HEMT) process, enabling the implementation of the integration of the BAW device and high-performance monolithic microwave integrated circuit (MMIC). The single crystal AlN piezoelectric film with 650-nm thickness was epitaxially grown on the SiC substrate by Metal Organic Chemical Vapor Deposition (MOCVD). After wafer bonding and substrate removal, the single crystal AlN film with electrode layers was transferred to another SiC wafer to form an air gap type BAW. Testing results showed that the fabricated resonators have a maximum Q-factor up to 837 at 3.3 GHz resonant frequency and electromechanical coupling coefficient up to 7.2%. Ladder-type filters were developed to verify the capabilities of the BAW and process, which has a center frequency of 3.38 GHz with 160 MHz 3 dB bandwidth. The filter achieved a minimum 1.5 dB insertion loss and more than 31 dB out-of-band rejection. The high performance of the filters is attributed to the high crystallinity and low defects of epitaxial single crystal AlN films.

We report on investigation of self-heating effects in AlGaN/GaN HFETs (heterostructure field effect transistors) using numerical simulations and micro-Raman spectroscopy. In the numerical simulations, we used a temperature-dependent thermal conductivity for each constituent material. To reduce the size effect of the device, we added wide thermal diffusion regions to active device region. Both AlGaN/GaN HFETs on sapphire and SiC substrates were studied using both electro-thermal 2D (two-dimensional) simulations and also analytical 3D thermal simulations. Good agreements between the simulated and measured surface temperature distributions have been obtained, which supports the validity of simulation models. The simulated temperature distribution for HFETs on SiC substrates was found to have a much sharper peak than that on sapphire substrates. In addition, the region around the gate edge on the drain side usually showed a maximum temperature for the devices operating at drain voltages less than about 40 V, but this region shifted toward the drain side when the drain voltage was increased up to 50-80 V. These results show that micro-Raman spectroscopy can be used for high-resolution temperature measurements.

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.

Wide band gap semiconductors such as III-V nitride materials and SiC have higher breakdown fields, higher electron saturation velocities and higher thermal conductivity than those of Si and GaAs. These attractive material properties are suitable for high power devices operating at high supply voltages. GaN-based electron devices are especially promising for high frequency operation because modulation-doped AlGaN/GaN heterostructure with high electron mobility is available. Besides, the density of the two dimensional electron gases at the heterointerface is very high owing to the piezoelectric and spontaneous polarization effects. These heterostructures are grown on either sapphire or SiC substrates since no bulk GaN substrate is commercially available widely. For sapphire substrates, the thermal conductivity is relative low, which leads to the reduction of drain currents in the regions of high drain current and high drain voltages due to the self-heating effects. Thus the AlGaN/GaN Heterostructure Field Effect Transistors (HFETs) grown on SiC substrates with high thermal conductivity are more suitable for high power device applications. This paper describes the improvements of process technologies and fabrication of the devices. The fabricated 0.3μm gate HFETs showed a maximum drain current of 740mA/mm, a peak transconductance of 150mS/mm and a breakdown voltage higher than 50V. The cut-off frequency fT and maximum oscillation frequency fMAX were 22GHz and 85GHz, respectively. The fT and fMAX were almost constant with the increase of drain voltage up to 30V. The output power of 1W/mm at 1.9GHz was obtained. And then frequency dispersion was not observed with the high voltage operation. These results indicate that AlGaN/GaN HFETs grown on SiC substrates are quite suitable for high frequency microwave and millimeter-wave devices.

The monolithic microwave integrated circuit (MMIC) power amplifiers serve an essential and critical role in RF transmit/receive (T/R) modules of phased array radar systems, mobile communication systems and satellite systems. Over recent years, there has been an increasing requirement to develop wideband high-efficiency MMIC high power amplifiers (HPAs) to accommodate wideband operation and reduce power consumption. This paper presents a wideband high efficiency MMIC HPA for Sub-6-GHz applications using a 0.25-μm gate-length D-mode GaN/SiC high electron mobility transistor (HEMT) process. The amplifier consists of two stages with two HEMT cells for the driver stage and eight HEMT cells for the power stage. To obtain a flat gain while maintaining the wideband characteristic, a gain equalization technique is employed in the inter-stage matching circuit. Meanwhile, a low-loss output matching network is utilized to ensure high efficiency. The fabricated HPA occupies a compact chip area of 14.35 mm2 including testing pads. Over the frequency range of 2–6 GHz, measured results of this HPA show a saturated continuous wave (CW) output power of 44.4–45.2 dBm, a power added efficiency (PAE) of 35.8–51.3%, a small signal gain of 24–25.5 dB, and maximum input and output return losses of 14.5 and 10 dB, respectively.

Herein, metal–organic chemical vapor deposition (MOCVD) of GaN layers doped with Mg atoms to the recognized optimum level of [Mg] ≈2 × 1019 cm−3 is performed. In a sequence of MOCVD runs, operational conditions, including temperature and flow rate of precursors, are maintained except for intentionally larger flows of hydrogen carrier gas fed into the reactor. By employing the largest hydrogen flow of 25 slm in this study, the performance of the as‐grown Mg‐doped GaN layers is certified by a room‐temperature hole concentration of p ≈2 × 1017 cm−3 in the absence of any thermal activation treatment. Experimental evidence is delivered that the large amounts of hydrogen during the MOCVD growth can regulate the incorporation of the Mg atoms into GaN in a significant way so that MgH complex can coexist with a dominant and evidently electrically active isolated MgGa acceptor.

We investigate the growth of Mg doped Al x Ga 1-x N films grown by metal organic chemical vapor deposition (MOCVD). The high temperature AlN interlayer can improve the quality of films, reducing the density of dislocations. With the increase of Al composition, the quality of films dramatically decreases, and a large number of island-shaped crystal nuclei can be observed, for three-dimensional growth mode of Mg-doped Al x Ga 1-x N can not be transformed into two-dimensional mode easily. Both the increase of Al composition and Cp 2 Mg flux can increase the density of screw dislocations. The increase of Cp 2 Mg flux can considerably increase the density of edge dislocations, while the increasing Al composition has little impact on the density of edge dislocations. Finally, annealing at 900°C for 15 minutes after growth is the ideal annealing condition for obtaining p-type Al 0.2 Ga 0.8 N, with hole concentration of 1.0×10 17 cm -3 .

High electron mobility was achieved in Al1−xInxN∕AlN∕GaN (x=0.20–0.12) heterostructure field effect transistors (HFETs) grown by metal-organic chemical vapor deposition. Reduction of In composition from 20% to 12% increased the room temperature equivalent two-dimensional-electron-gas density from 0.90×1013to1.64×1013cm−2 with corresponding electron mobilities of 1600 and 1410cm2∕Vs, respectively. The 10K mobility reached 17600cm2∕Vs for the nearly lattice-matched Al0.82In0.18N∕AlN∕GaN heterostructure with a sheet carrier density of 9.6×1012cm−2. For comparison, the AlInN∕GaN heterostructure without the AlN spacer exhibited a high sheet carrier density (2.42×1013cm−2) with low mobility (120cm2∕Vs) at room temperature. The high mobility in our samples is in part attributed to ∼1nm AlN spacer which significantly reduces the alloy scattering as well as provides a smooth interface. The HFETs having gate dimensions of 1.5×40μm2 and a 5μm source-drain separation exhibited a maximum transconductance of ∼200mS∕mm with good pinch-off characteristics and over 10GHz current gain cutoff frequency.

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

A novel method of the selective epitaxy of InP was developed by employing a two-step growth in atmospheric pressure metalorganic chemical vapor deposition. By this method, selective epitaxy, without ridge and/or lateral growth, and with uniform deposition of the polycrystalline layer on the SiO2 mask, could be achieved. First, a thin uniform InP polycrystalline layer was deposited on a SiO2/InP substrate and a stripe pattern was formed by photolithography, and then selective epitaxial growth was performed. Selective epitaxial layers thus obtained were found to have a photoluminescence half width of ∼40 meV at room temperature, which was comparable to the one reported by other workers in homoepitaxial metalorganic chemical vapor deposition growth of InP. The resistivity of the polycrystalline layer was ∼700 Ω cm at room temperature, which was four orders of magnitude larger than that of the selective epitaxial layer. Therefore, electrical isolation may be sufficient for fabrication of InP planar devices.

Environmental safety issues for molecular beam epitaxy platform growth technology

A coupled δ-doped In0.34Al0.66As0.85Sb0.15/InP heterostructure field-effect transistor (HFET) has been successfully grown by metalorganic chemical vapor deposition. Electron mobilities can be enhanced without sacrificing the carrier densities. An extremely low gate reverse leakage current of 111 µA/mm at Vgs=-40 V is achieved. The three-terminal on- and off-state breakdown voltages are as high as 40.8 V and 16.1 V, respectively. These characteristics are attributed to the use of the coupled δ-doped structure, InP channel, In0.34Al0.66As0.85Sb0.15 Schottky layer, and to the large conduction-band discontinuity (ΔEC) at the InAlAsSb/InP heterojunction. In addition, on- and off-state breakdown mechanisms in the InAlAsSb/InP HFET are also studied.

Al0.25Ga0.75N/GaN heterostructure field‐effect transistor (HFET) structures are grown on semi‐insulating GaN substrates by metalorganic chemical vapor deposition with Fe‐doped GaN buffer layer and various surface treatments prior to the epitaxial growth of the structure. The surface treatments are intended to remove suspected surface‐charge‐containing layer and they include wet chemical etching, in‐situ thermal etching, plasma dry etching, and photo‐enhanced chemical etching. Photo‐enhanced chemical (PEC) etching was effective in achieving microscopically smooth surface and in eliminating surface charge layer on the surface of semi‐insulating GaN substrates. RMS surface roughness of PEC etched surface of the semi‐insulating GaN substrate is 0.1∼0.2 nm for 1×1 μm2 scan. C ‐V measurement shows well‐defined 2‐dimensional‐gas‐related charge without any growth‐interface‐related charge for a HFET grown on a semi‐insulating GaN substrate with optimized photo‐enhanced chemical etching of the surface. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)