Investigation of Bulk Traps by Conductance Method in the Deep Depletion Region of the Al2O3/GaN MOS Device
© The Author(s). 2017
Received: 22 February 2017
Accepted: 26 April 2017
Published: 10 May 2017
Conductance method was employed to study the physics of traps (e.g., interface and bulk traps) in the Al2O3/GaN MOS devices. By featuring only one single peak in the parallel conductance (G p/ω) characteristics in the deep depletion region, one single-level bulk trap (E C-0.53 eV) uniformly distributed in GaN buffer was identified. While in the subthreshold region, the interface traps with continuous energy of E C-0.4~0.57 eV and density of 0.6~1.6 × 1012 cm−2 were extracted from the commonly observed multiple G p/ω peaks. This methodology can be used to investigate the traps in GaN buffer and facilitates making the distinction between bulk and interface traps.
Owing to the superior properties of high electron mobility, high breakdown voltage, high-power density, low on-resistance, and high temperature operation capability, GaN heterojunction field-effect transistors (HFETs) have been considered as a promising solution for next-generation energy-efficient power electronics and attracted tremendous attention in the last two decades . For power switching applications, the enhancement-mode (E-mode) transistors are highly preferred rather than the depletion-mode (D-mode) devices for the inherent fail-safe operation and simple gate driver circuitry. Despite the various technologies proposed to realize E-mode, GaN HFETs such as p-cap gate [2, 3], fluorine plasma ion implantation , and cascode technology , the MOSFET with partially or fully recessed gate is considered as a promising candidate because of its high-threshold voltage (V TH), large gate swing for improved fail-safe capability [6, 7], and low on-resistance . Moreover, the MOS-gate is compatible with the mainstream gate driver ICs. However, the traps (e.g., interface and bulk traps) tarnish the advantages of GaN HFETs due to the stability and reliability issues such as V TH instability , drain lag or gate lag , and power slump. Besides the surface/interface traps, the GaN power HFETs’ performance such as the breakdown voltage and dynamic on-resistance could be substantially affected by the bulk traps in GaN buffer layer in high-voltage-switching applications [11, 12] since the high electric-field is prone to trigger the buffer traps for dynamic charging/discharging. Hence, it is of great significance to characterize the buffer traps of GaN MOS devices.
Bulk traps in GaN MOS devices have been studied by deep-level transient spectroscopy (DLTS)  and pulse measurement . However, though the dynamic charge/discharge process of both bulk and interface trap-induced transient behavior may simultaneously appear in the same spectrum, extra effort is required to differentiate between the bulk and interface traps when using DLTS-like techniques and pulse measurement [15, 16]. On the other hand, the conductance method has been widely used to evaluate the interface traps in AlGaN/GaN MIS structures as well as GaN-based MIS structures [17–19]. Moreover, it is possible to discriminate the bulk and interface traps in the conductance method by studying its bias dependence because the conductance loss is sensitive to the traps within a few kT/q around the Fermi level. In this letter, the conventional conductance method normally used to characterize the interface traps is employed to study the bulk traps (BT) in GaN buffer for the first time. Two trap-dominated regions were found in the Al2O3/GaN MOS structure with full barrier recess. In the deep depletion region, only one single Gp/ω peak is captured at the measured bias voltage ranging from −1 to 0 V, revealing the bulk traps with a single level in GaN buffer. The energy of the bulk trap was determined to be E C-0.53 eV. While in the subthreshold region, the interface traps with continuous energy levels that result in multiple Gp/ω peaks were observed within the measured bias range of 1.6~2.6 V. The energy levels were extracted to be in the range of 0.4 to 0.57 eV below GaN conduction band (CB).
Results and Discussion
where the capture cross section of the trap σ n = 4 × 10−13 cm−2, the electron thermal velocity v th =2.6 × 107 cm/s, the density of states at GaN CB N C = 2.2 × 1018 cm−3, the Boltzmann constant k B = 1.38 × 10−23J/K, and temperature T = 300 K were used . The interface trap levels are in the range of 0.4 to 0.57 eV below GaN CB with D IT decreased from 1.6 × 1012 to 0.6 × 1012cm−2.
In conclusion, for the first time, the conventional conductance method was used to study the buffer traps in the Al2O3/GaN MOS device with full barrier removal. The bulk traps with a single energy level and uniformly distributed in GaN buffer that leads to a single Gp/ω peak were detected by f-dependent conductance measurements in the deep depletion region. On the other hand, the interface traps with wide energies were measured in the subthreshold region, which corresponds to the multiple Gp/ω peaks observed in the f-dependent conductance measurements. Due to the different f-dependent conductance response originating from the different energy and spatial distributions, the demonstrated approach is much easier to be used to investigate the physics of the bulk traps in GaN buffer.
This work was supported in part by the National Natural Science Foundation of China under Project Nos. 61234006 and 61674024, and in part by the National Science and Technology Major Project 02 under Project No. 2013ZX02308-005, in part by the Natural Science Foundation of Guangdong Province, China, under project Grant No. 2015A030311016, in part by the Fundamental Research Funds for the Central Universities under project ZYGX2016J211 and in part by the opening project of State Key Laboratory of Electronic Thin Films and Integrated Devices under Grant KFJJ201609.
YYS jointly conceived the study with ZJL. YYS, ABZ, LYZ, and YS performed the experiments. YYS and ZJL performed all the data analyses and wrote the original draft of the manuscript. QZ, WJC, and BZ reviewed and edited the manuscript. All authors reviewed the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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