AlN Surface Passivation of GaN-Based High Electron Mobility Transistors by Plasma-Enhanced Atomic Layer Deposition
© The Author(s). 2017
Received: 2 January 2017
Accepted: 12 April 2017
Published: 27 April 2017
We report a low current collapse GaN-based high electron mobility transistor (HEMT) with an excellent thermal stability at 150 °C. The AlN was grown by N2-based plasma enhanced atomic layer deposition (PEALD) and shown a refractive index of 1.94 at 633 nm of wavelength. Prior to deposit AlN on III-nitrides, the H2/NH3 plasma pre-treatment led to remove the native gallium oxide. The X-ray photoelectron spectroscopy (XPS) spectroscopy confirmed that the native oxide can be effectively decomposed by hydrogen plasma. Following the in situ ALD-AlN passivation, the surface traps can be eliminated and corresponding to a 22.1% of current collapse with quiescent drain bias (V DSQ) at 40 V. Furthermore, the high temperature measurement exhibited a shift-free threshold voltage (V th), corresponding to a 40.2% of current collapse at 150 °C. The thermal stable HEMT enabled a breakdown voltage (BV) to 687 V at high temperature, promising a good thermal reliability under high power operation.
Recent progress in high-power field-effect transistors (FET) was focused on GaN-based wide band-gap semiconductors. GaN-based high electron mobility transistors (HEMTs) have demonstrated a great potential due to their high breakdown electric field, low on-state resistance (R on), and high thermal stability [1, 2]. Therefore, GaN-based HEMTs provide significantly better performance compared with traditional Si-based power devices. However, GaN-based HEMTs meet the demand of reduction in dynamic on-resistance, which is so-called “current collapse” phenomenon during the high power switching. Current collapse phenomenon can be attributed to the high density of traps in GaN-based materials. The traps capture electrons and then act as a virtual gate on the surface, which deplete channel electrons and increase on-resistance simultaneously . Previous studies suggest kinds of dielectric layer can be effective passivation layers, such as plasma-enhanced chemical vapor deposition (PECVD) grown SiNx , atomic layer deposition (ALD) grown Al2O3 high-κ dielectric layer , and plasma-enhanced ALD (PEALD) grown AlN . The effective passivation of PEALD grown AlN with in situ low-damage plasma pre-treatment enables to remove the surface native oxide with minimum surface damage. The surface native oxide is strongly related to the surface defects, leading to the current collapse and unreliable device performance . Therefore, an optimum passivation layer with surface oxide removal process is a key technology to fabricate reliable GaN-based HEMTs. In this paper, we demonstrate a high reliable GaN-based HEMT regarding the PEALD-AlN passivation. Prior to AlN deposition, the hydrogen plasma carried out the surface native oxide removal process. The hydrogen plasma promised a low dynamic on-resistance, revealing a 22.1% of current collapse with quiescent drain bias (V DSQ) at 40 V. Moreover, the ALD-AlN with in situ plasma pre-treatment showed a 687 V of high breakdown voltage (BV) at 150 °C, promising a good thermal reliability under high power operation.
The epi-wafers were separated into pieces by 2 × 2 cm2. First, the BCl3/Cl2-based inductively coupled plasma etching was employed to define the mesa isolation. The specific contact resistance (ρ c) of 2.34 × 10−6 Ω-cm2 was obtained by Ti/Al/Ni/Au (25/125/40/150 nm) ohmic metal and thermal metallization by a rapid thermal annealing (RTA) at 850°C for 30 sec. Before passivation, ex situ surface cleaning by HCl:DI = 1:1 and BOE:DI = 1:100 was carried out the residual carbon and native oxide removal. Afterward, the surface was secondly treated by remote plasma by Picosun TM R200 PEALD. Next, the high-κ insulator was in situly deposited. After the gate insulator was deposited, the Ni/Au (50/200 nm) Schottky gate metal was evaporated. The HEMT device was realized with a 2-μm gate length (L G), 2-μm source-to-gate distance (L SG), 10-μm gate-to-drain distance (L GD), and 100-μm gate width (W G). Finally, a 150-nm-thick SiNx was deposited by plasma-enhanced chemical vapor deposition (PECVD) at 300 °C. The process flow is presented in Fig. 1b.
Surface Pre-Treatment and AlN Passivation
Surface treatment and high-κ dielectric growth condition by PEALD
(Ar: 160 sccm)
(Ar: 110 sccm)
(Ar: 30 sccm)
(Ar: 30 sccm)
Pre-treatment (300 °C)
N2-based AlN (350oC)
NH3-based AlN (300 °C)
The X-ray photoelectron spectroscopy (XPS) using a PHI QuanteraII system was employed to determine the stoichiometry of the dielectric films. The narrow scan mode with Ar sputtering investigated the depth profile of the elemental composition during the interface. The minimum spot size is 7.5-um-diameter. The J.A. Woollam M-2000 spectroscopic ellipsometer was used to determine the refractive index (n) spectrum of AlN. The deuterium light source with the minimum beam size is 0.3 mm.
Results and Discussion
The values of I DS,max for the HEMT with plasma pre-treatment were relatively high regarding the mobility enhancement due to surface passivation eliminated carrier scattering [13, 14]. The better performance implies that the H2/NH3 plasma-pretreatment leads to a high quality interface between ALD-AlN and III-nitrides. To further investigate the gate control characteristics for both devices, the subthreshold swing (SS) is a parameter which clearly indicates the interface quality. The SS is defined to be the inverse slope of the log (I DS) versus V GS characteristic in the subthreshold region. The values of SS are 80 and 125 mV/dec for the HEMT with and without plasma pre-treatment, respectively. The lower SS value confirms a lower interfacial state density after plasma pre-treatment. The interfacial states can be attributed to the native Ga-O bonds, which result in a high density of surface traps. Hence, it is essential to remove the native oxide on HEMTs surface. The hydrogen plasma enables to remove native oxide on the GaN surface, following the ammonia plasma will passivate the surface by plasma nitridation simultaneously. In addition, surface nitridation also isolates the Ga and O atoms, preventing the formation of Ga-O bonds.
The current collapse was 22.1% for the HEMT with plasma pre-treatment but the absent of pre-treatment HEMT shows 44.9% at V DSQ = 40 V. The current collapse phenomenon can be effectively reduced by plasma pre-treatment for the GaN-based HEMTs.
Figure 5b shows the temperature dependent BV and gate leakage (I GS) for plasma pre-treated HEMT. The soft BV was defined as off-state I DS at 1 μA. The soft BV was reduced from 660 to 153 V as we increased the temperature from RT to 150 °C. The device BV can be reached to 858 V at RT but decreased to 687 V at 150 °C. We should notice that the gate leakages suggest the BVs were limited by impact ionization, as shown in the figure with void lines. Despite the soft BV was reduced to 153 V, a thermal stable HEMT enabled a BV > 600 V at high temperature, promising a good thermal reliability under high power operation. The ALD approaches improve device performance and provide an effective and easy methods for preventing undesirable phenomena, producing reliable devices for high power applications.
In summary, we fabricated a low current collapse GaN-based HEMT with an excellent thermal stability. The ALD-AlN shows a refractive index of 1.94 at 633 nm of wavelength. Prior to deposit AlN on III-nitrides, the H2/NH3 plasma pre-treatment resulted in a low trap density of the surface. The hydrogen plasma enables to effectively decompose native gallium oxide. The XPS spectroscopy reveals that the native oxide was removed. Following the in situ ALD-AlN leading to a well-passivated surface results in a low current collapse of 22.1% with quiescent drain bias (V DSQ) at 40 V. Furthermore, the high temperature measurement exhibited a shift-free of V th, corresponding to a 40.2% of current collapse at 150 °C. The thermal stable HEMT enabled a BV > 600 V at high temperature, promising a good thermal reliability under high-power operation.
This study was funded by the Ministry of Science and Technology of Republic of China under grant number MOST 105-2622-E-009 -023 -CC2. The authors would like to acknowledge the supporting by EPISTAR Corporation for Industry-university Collaboration project.
AJT, BWW, CHS, JMS, WKY, CYC, and HCK discussed the topic. AJT and KHC fabricated the different devices and performed the measurements. EØ, IFL, XPW, and YSF did the ALD process by NCTU-Picosun Joint Lab. All authors discussed the data analysis and interpretation and contributed equally to the writing of the manuscript. All authors approved the final version of the manuscript.
The authors declare that they have no competing interests.
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