We have grown AlGaN/GaN high electron mobility transistor (HEMT) structures by plasma-assisted molecular beam epitaxy on free-standing n -GaN substrates grown by hydride vapor phase epitaxy. Reflection high energy electron diffraction patterns of the as-loaded wafers exhibit narrow streaks which persist throughout the growth. Atomic force microscopy shows smooth AlGaN surfaces with root-mean-square roughness of 10 Å over a 20×20 μ m 2 area. High resolution x-ray diffractometry indicates that the AlGaN peak is ∼20 % narrower than for similar structures grown on SiC. Hall mobilities, electron sheet densities, and sheet resistances were measured on ten 60×60 μ m 2 Hall test patterns defined photolithographically across the surface of the 10×10 mm 2 sample. Buffer leakage measurements demonstrate that a Be:GaN layer effectively isolates the channel from the conductive substrate. Average sheet resistances and sheet densities were 380 Ω/ ◻ and 0.94×1013 cm -2 , respectively. These HEMT structures exhibit room-temperature Hall mobilities in excess of 1900 cm 2/ V s . In addition, devices on these structures exhibit excellent pinch-off, low gate leakage, and saturated drain current densities of almost 700 mA/mm. Further details regarding the structural and electrical properties will be described along with device testing.
[Show abstract][Hide abstract] ABSTRACT: AlGaN/GaN high electron mobility transistors (HEMTs) by plasma-assisted molecular beam epitaxy on free-standing GaN substrates grown by hydride vapour phase epitaxy (HVPE) have been fabricated. Hall measurements yielded typical electron mobilities of 1750 cm<sup>2</sup>/Vs with sheet densities of 1.1×10<sup>13</sup> cm<sup>-2</sup>. Off-state breakdown voltages as high as 200 V were measured on unpassivated devices. Output power density at 4 GHz was measured to be 5.1 W/mm at a power-added efficiency of 46% and an associated gain of 13.4 dB. This constitutes significant improvement of RF performance by MBE-grown AlGaN/GaN HEMTs on free-standing HVPE GaN.
[Show abstract][Hide abstract] ABSTRACT: The intensity of the flux of activated nitrogen from an RF inductively coupled discharge source for the plasma-assisted molecular
beam epitaxy (PAMBE) of group III nitrides (A3N) can be linearly controlled using a modified output diaphragm design and increased nitrogen supply (∼5 sccm). This source
provides a linear variation of the maximum A3N growth rate from 0.2 to 0.8 μm/h for the RF power controlled between 110 and 200 W, respectively. The use of excited nitrogen
molecules favorably influences the growth of GaN and InN epilayers, which are characterized by a perfect structure and high
[Show abstract][Hide abstract] ABSTRACT: We investigate the role of substrate temperature and gallium flux on the DC and microwave properties of AlGaN/GaN high electron mobility transistors grown by molecular-beam epitaxy on free standing, hydride vapor phase epitaxy grown GaN substrates. The free-standing substrates have threading dislocation densities below 107 cm−2. We find that AlGaN/GaN heterostructures with excellent properties may be grown within a wide range of substrate temperatures and fluxes. Electron Hall mobilities above 1700 cm2/V s and sheet resistances below 370 Ω/□ are typical. We are able to obtain high saturated drain currents with low gate leakage. Off-state breakdown voltages as high as 200 V with low drain and gate leakage currents have been measured. Further, we have measured microwave output power densities above 5 W/mm at 4 GHz with a power-added efficiency of 46% and an associated gain of 13.4 dB. We attribute improved electrical properties in these devices to the reduced threading dislocation density compared to those grown on non-native substrates.
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