To realize the full spectrum of advantages that the III-nitride materials system offers, the demonstration of p-channel III-nitride based devices is valuable. Authors report the first ptype field effect transistor (pFET) based on an AlGaN/GaN superlattice (SL), grown using MOCVD. Magnesium was used as the p-type dopant. A sheet resistance of 11.6 kΩ/■, and a contact resistance of 14.9 Ω.mm was determined using transmission line measurements (TLM) for a Mg doping of 1.5 × 10 19 cm -3 of Mg. Mobilities in the range of 7−10 cm 2 /Vs and a total sheet charge density in the range of 1 × 10 13 -6 × 10 13 cm -2 were measured using room temperature Hall effect measurements. Without Tetramethylammonium hydroxide (TMAH) treatment, the fabricated pFETs had a maximum drain-source current (IDS) of 3mA/mm and an On-Resistance (RON) of 3.48 kΩ.mm, and did not turn-off completely. With TMAH treatment during fabrication, a maximum IDS of 4.5mA/mm, RON of 2.2kΩ.mm, and five orders of current modulation was demonstrated, which is the highest achieved for a p-type transistor based on (Al,Ga)N. AlGaN/GaN superlattice (SL). MOCVD growth technique was used magnesium was used as the p-type dopant. With Tetramethylammonium hydroxide (TMAH) treatment during fabrication, a maximum IDS of 4.5mA/mm, RON of 2.2kΩ.mm, and five orders of current modulation was demonstrated.
We propose the existence of an acceptor-like trap at positive polarization interfaces in p-type III-nitride semiconductor heterostructures, using N-polar p-type GaN/AlN/AlGaN superlattices as a demonstration platform. Metal Organic Vapor Phase Epitaxy was used to grow all samples, with a p-type modulation doping scheme using Mg as the dopant. The samples were characterized using x-ray diffraction and room-temperature Hall measurements, and energy band-diagram simulations were carried out using STR FETIS® and Silvaco packages. For higher doped samples (Mg > 1.5 × 1019 cm−3) with thinner AlN interlayers (≤0.7 nm), the total sheet charge measured using Hall measurements agreed with the value observed in standard simulations without invoking any traps, whereas for lower doped samples (Mg < 1.5 × 1019 cm−3) and those with thicker AlN interlayers (≥ 0.7 nm), the measured charge was very high compared to the value obtained from simulations and higher than the Mg doping in the films. The higher charge was attributed to the existence of an acceptor trap at positive polarization interfaces, which became ionized at lower doping and/or at higher AlN thicknesses. A consistent ionization energy of the trap was obtained by comparing the energy band diagram with and without acceptor traps with the experimental results. This work also elucidates the source of charge balance in p-type samples with insufficient or no Mg doping.
In this study the MOCVD growth and electrical properties of N-polar modulation doped p-AlGaN/GaN superlattices (SLs) were investigated. Hole sheet charge density and mobility were studied as a function of the concentration of the p-type dopant Mg in the SL and the number of SL periods. Room temperature Hall measurements were carried out to determine the hole mobility and the sheet charge density. While the hole density increased with increasing number of SL periods, the hole mobility was largely unaffected. Hole mobilities as high as 18 cm 2 /Vs at a simultaneous high hole density of 6.5 x 10 13 cm -2 were observed for N-polar SLs with a Mg modulation doping of 7.5 x 10 18 cm -3 . For comparable uniformly doped Ga-polar SL samples, a mobility of 11 cm 2 /Vs was measured. These results confirm the presence of abrupt Mg doping profiles in N-polar p-type GaN/AlxGa(1-x)N SL allowing the demonstration of SLs with properties comparable to those of state-of-the-art Ga-polar modulation doped AlGaN/GaN SLs grown using MBE. Lowest sheet resistance in the GaN/AlGaN materials system of 5kΩ/□ is also reported. Test-structure transistors were also fabricated to investigate the applicability of these SL structures, with planar device resulting in a current of 5mA/mm, and a FinFET structure resulting in a current of over 100mA/mm. Wide-Bandgap (Al,Ga)N electronic devices are at the forefront of realizing the needs of next generation communication systems, power conversion and energy conservation, thus enabling compact and affordable electronic systems 1-6 . The main reasons for this include: (a) Wide-bandgap of GaN (~3.4eV) and III-nitride alloys, enabling higher breakdown voltages 7 and (b) the existence of built-in polarization fields leading to very high mobility and charge in two dimensional-electron gas (2DEG) structures 8 . But, to tap into the full potential of this materials system, the fabrication of p-type GaN based devices is desired 9 . Some of the challenges associated with p-type GaN include: -(a) the low mobility of holes in GaN, with a maximum observed value of 40 cm 2 /Vs at a hole concentration of 2×10 12 cm −2 in a two dimensional-hole gas (2DHG), and 20 cm 2 /Vs in bulk p-GaN (p= 1×10 17 cm −3 ) 10,11 ; (b) the deep nature of the common acceptor dopant in GaN (160 -220 meV) 12,13 ; (c) the passivation of MOCVD grown p-GaN material, like the one in this report, by hydrogen in the as-grown state, and the need for annealing at a high temperature (here, 800 0 C) for activation 14 ; (d) the challenges involved with making ohmic contacts to p-type GaN because of the high p-GaN work function at typical doping levels 15 . Due to the above listed challenges, in the past p-channel GaN devices have received significantly less attention compared to GaN-based n-channel field effect transistors (FETs) 10,16-24 .
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