Influence of Surface Passivation on AlN Barrier Stress and Scattering Mechanism in Ultra-thin AlN/GaN Heterostructure Field-Effect Transistors
© The Author(s). 2016
Received: 27 May 2016
Accepted: 16 August 2016
Published: 23 August 2016
Ultra-thin AlN/GaN heterostructure field-effect transistors (HFETs) with, and without, SiN passivation were fabricated by the same growth and device processes. Based on the measured DC characteristics, including the capacitance-voltage (C-V) and output current-voltage (I-V) curves, the variation of electron mobility with gate bias was found to be quite different for devices with, and without, SiN passivation. Although the AlN barrier layer is ultra thin (c. 3 nm), it was proved that SiN passivation induces no additional tensile stress and has no significant influence on the piezoelectric polarization of the AlN layer using Hall and Raman measurements. The SiN passivation was found to affect the surface properties, thereby increasing the electron density of the two-dimensional electron gas (2DEG) under the access region. The higher electron density in the access region after SiN passivation enhanced the electrostatic screening for the non-uniform distributed polarization charges, meaning that the polarization Coulomb field scattering has a weaker effect on the electron drift mobility in AlN/GaN-based devices.
Attributed to the high critical field and electron velocity, nitride heterostructures have attracted great attention because of the excellent potential application in high-voltage and high-power operations at microwave/sub-microwave frequency [1–5]. Thanks to the large band-gap energy and conduction-band offset to GaN, AlN/GaN heterostructures with ultra-thin barrier layer (~3 nm) are expected to be important in three-dimensional device scaling in order to obtain high frequencies, enabling the realization of millimeter-wave and/or even sub-millimeter-wave power devices [6–8]. Using device-scaling technologies, the HRL Laboratory has reported a D-mode AlN/GaN heterostructure field-effect transistor (HFET) with an ultra-high f T exceeding 450 GHz and a f max close to 600 GHz, which are the best frequency characteristics yet found in GaN-based HFETs . Nowadays, the surface passivation usually uses an SiN dielectric grown by a plasma enhanced chemical vapor deposition (PECVD) system, which has been demonstrated to be an effective material when mitigating against current collapse in Al x Ga1-x N/GaN HFETs, to a certain extent regulating the two-dimensional electron gas (2DEG) density [10–19]. It has been proven that the increase of 2DEG density is not due to the induced stress in the barrier layer but the influence of SiN passivation on the surface properties of the AlGaN barrier layer: the potential barrier height of the AlGaN barrier layer is then changed after SiN passivation, resulting in a 2DEG density change [18–21]. However, in the AlN/GaN heterostructure, the AlN barrier layer is ultra thin (c. 3 nm) and has large piezoelectric polarization, so the crystal lattice of the AlN layer may be more sensitive to SiN passivation. Whether the SiN passivation induces additional stress in the ultra-thin AlN barrier layer remains unknown. Moreover, the polarization Coulomb field (PCF) scattering related to the non-uniform distribution of polarization charges in the barrier layer has been demonstrated to be an important mechanism in AlGaN/GaN HFETs [22–25]. The PCF scattering exerts a dominant influence on electron drift mobility in AlN/GaN HFETs due to the thin barrier layer [26, 27]. Whether the SiN passivation induces additional stress in the AlN barrier layer or just affects the surface properties of the AlGaN barrier layer, the increase in 2DEG after SiN passivation will affect the PCF scattering, which influences electron mobility in AlN/GaN HFETs. As a result, it was deemed worthwhile to investigate the influence of SiN passivation on the ultra-thin AlN barrier layer and the transport properties in AlN/GaN HFETs.
In this work, ultra-thin AlN/GaN HFETs with, and without, SiN passivation were fabricated with the same growth and device processes, respectively. Using the measured DC characteristics, including the capacitance-voltage (C-V) and output current-voltage (I-V) curves, it was found that the electron mobility varied with gate bias quite differently for devices with, and without, SiN passivation. Based on Raman and Hall measurements of the AlN/GaN heterostucture with different SiN thicknesses, SiN passivation was proved to exert no significant influence on the piezoelectric polarization of the AlN barrier layer, but to have affected the surface properties of the AlN/GaN heterostructure. As a result, the increase in electron density in the access region weakened the effect of PCF scattering in ultra-thin AlN/GaN HFETs after SiN passivation.
An ultra-thin AlN/GaN heterostucture, from top to sapphire substrate, was formed with a 1-nm GaN cap layer, a 3-nm AlN barrier layer, a 2.5-μm S. I. GaN buffer layer, and a low-temperature AlN nucleation layer, which was grown by metal organic chemical vapor deposition (MOCVD). From room-temperature Hall measurements, the sheet carrier density and electron drift mobility were found to be around 8.92 × 1012 cm−2 and 1510 cm2 V.s−1, respectively. The device mesa was isolated by reactive ion etching with Cl2/BCl3 gas. Ohmic contacts with Si/Ti/Al/Ni/Au metal stacks were deposited by e-beam evaporation and lift-off and then annealed rapidly in a nitrogen atmosphere to form good Ohmic contacts. The specific resistivity was found to be 5.9 × 10−5 Ω cm2 by transmission line method (TLM). The rectangular Ohmic contacts were 50 μm long and 100 μm wide, with a source-to-drain distance of 100 μm. Through e-beam evaporation and lift-off technology, Schottky contacts with Ni/Au metal stacks were deposited in the center between the drain and source contacts, and the size of each Schottky contact was 20 μm long by 100 μm wide. Finally a 100-nm-thick SiN passivation layer was deposited by PECVD. To compare devices with, and without, SiN passivation, unpassivated AlN/GaN HFETs were also prepared with the same growth and device processes. Besides, classical van der Pauw Hall structures were fabricated on the same wafer during processing. Each pattern was a 500 μm × 500 μm square mesa.
Results and Discussion
In the SiN passivated AlN/GaN HFETs, the SiN passivation induced no additional tensile stress in the AlN layer and had no significant enhancing effect on the PCF elastic potential and scattering of the 2DEG electron mobility. However, the 2DEG density under the access region greatly increased after SiN passivation. Once the 2DEG density increased, the electrostatic shielding on the non-uniform region of the polarization charges among ρ Mat, ρ G , and ρ S/D was enhanced, which weakened the effect of PCF scattering on electron mobility under the gate area. Thus, as shown in Fig. 3, the deviation of electron mobility between the gate biases of −0.8 and 0 V became much smaller after SiN passivation. Moreover, the increased electron mobility, after SiN passivation, at the same gate bias was also due to the weaker PCF scattering effect.
Rectangular HFET devices with, and without, SiN passivation were fabricated on ultra-thin AlN/GaN heterostuctures. Based on the measured DC characteristics, the changes in 2DEG electron mobility underneath the Schottky contacts with applied gate voltage for the fabricated AlN/GaN HFET devices was obtained. Based on Hall measurements of AlN/GaN heterostuctures with different SiN thicknesses, the electron mobility, electron density, and sheet resistance were found to have remained quasi-constant with increasing SiN thickness, which demonstrated that the stress induced by the presence of the SiN film should not be an essential reason for the increased 2DEG density. No wavenumber shift in the GaN buffer further indicated that the SiN passivation exerted no significant influence on the piezoelectric polarization of the AlN barrier layer. Pulse output characteristics further demonstrated that the increased 2DEG density was mainly caused by the reduction of surface states after SiN passivation. The higher electron density in the access region after SiN passivation enhanced the electrostatic screening for the non-uniformly distributed polarization charges, meaning that PCF scattering had a weaker effect on the electron mobility in the AlN/GaN HFETs after SiN passivation.
This work was supported by the National Natural Science Foundation of China (Grant No. 61306113).
YL designed this study and drafted the manuscript. XB performed the sample preparation. YG carried out the experimental measurements. YL performed the material growth. ZH instructed this study and revised the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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