Gate reliability enhancement of p-GaN gate HEMTs with oxygen compensation technique

Improved p-GaN gate reliability is achieved through a simple oxygen compensation technique (OCT), which involves oxygen plasma treatment after gate opening and subsequential wet etching. The OCT compensates for the Mg acceptors near the p-GaN surface, leading to an extended depletion region under the same gate bias and thus reducing the electric field. Furthermore, the Schottky barrier height also increases by OCT. Consequently, suppressed gate leakage current and enlarged gate breakdown voltage are achieved. Notably, the maximum applicable gate bias also increases from 4 V to 8.1 V for a 10 year lifetime at a failure rate of 1%.


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][3] In particular, Schottky-type p-GaN gate HEMTs (SG-HEMTs) can effectively suppress gate leakage owing to the reversely biased gate metal/p-GaN Schottky junction at on-state. 4)][7] Under forward gate bias, electrons injected from the 2DEG channel are accelerated in the depleted p-GaN region and become the most energetic near the p-GaN surface, where the electric-field (E-field) peak appears.10] Based on the hot-electron-induced TDGB model, the gate reliability can be improved by: (i) enhancing the hot electrons bombardment robustness where the E-field peak appears, and (ii) reducing the energy of hot electrons by alleviating the Efield.For example, following method (i), GaON, 5) Ga 2 O 3 , 11) or Al x Ga 1−x N 12,13) has been introduced on the p-GaN surface as a reinforcement layer which is more immune to the hotelectron bombardment.Meanwhile, following method (ii), an i-GaN layer has been epitaxially grown on the p-GaN surface to reduce the E-field. 14)Notably, a n-GaN/p-GaN epitaxial gate stack has been proposed to simultaneously realize a high immunity to hot electrons and a low E-field. 15,16)However, the existing techniques require either new epitaxial layers on the p-GaN surface or high-temperature annealing processes.During the growth of new epitaxial layers on the p-GaN surface, the Mg memory effect is difficult to control. 17)eanwhile, the high-temperature annealing process adds additional thermal budget to the fabrication process.Therefore, a much simpler and lower-cost method would be preferred for enhancing gate reliability.
In this work, a simple, low-cost and effective oxygen compensation technique (OCT) is proposed to enhance the gate reliability of SG-HEMTs.In OCT, oxygen plasma is used to compensate the Mg acceptors near the p-GaN surface, thereby extending the depletion region and reducing the Efield.By employing the OCT, suppressed gate leakage current, enlarged gate breakdown voltage, and improved gate reliability are achieved, demonstrating the effectiveness of the OCT.
The p-GaN gate HEMT epitaxial layers are commercially available, which are grown by metalorganic CVD (MOCVD) on 6 inch Si (111) substrates, featuring a 15 nm Al 0.15 Ga 0.85 N barrier layer and a 100 nm p-GaN layer with a Mg-doping concentration of 4 × 10 19 cm −3 .
During the devices fabrication, the Schottky contact region between the gate metal and the p-GaN surface was treated with OCT, as illustrated in Fig. 1(a).The oxygen plasma treatment of OCT facilitates the penetration of oxygen (O) into the p-GaN surface.The oxygen in p-GaN tend to form O N donors or inert Mg Ga O N complexes, both of which have relatively lower formation energy among all possible configurations. 18,19)These configurations effectively compensate for activated Mg dopants in the plasma-treated region, 20,21) forming a weakly doped or i-GaN layer.The depletion region (W D ) extends wider under the same gate bias due to the compensated p-GaN region, and the E-field decreases with an approximate trapezoidal distribution shown in Fig. 1(b).In addition to the reduced E-field, the Schottky barrier height (j B ) also increases [Fig.1(b)], which will be discussed in the next section.It should be noted that in previous work, the oxygen plasma was also used to form a GaON-based surface reinforcement layer on the p-GaN gate, thereby enhancing the gate reliability. 5)However, the application of the oxygen plasma in this work differs from that in GaON formation.For GaON, plasma treatment aims to supply oxygen for the subsequential high-temperature chemical reactions and help overcome the chemical inertness of the GaN surface. 22)In contrast, in this work, the oxygen plasma is used to compensate the Mg acceptors in the p-GaN gate region.This compensation function of oxygen plasma has also been demonstrated in the work of using oxygen plasma to passivate p-GaN outside the gate region to achieve normally-off HEMTs. 21)In addition, oxygen plasma treatment was also used to treat the interface between the gate dielectric and GaN to reduce the impact of the interface state on device performance. 23,24)he device fabrication commenced with the removal of the p-GaN stack outside the gate region, followed by source/ drain ohmic contact formation, planar isolation, and passivation deposition using the same process parameters as that reported before. 25)After gate opening, the OCT was conducted.Finally, Ni/Au gate metal deposition and lift-off were performed, followed by contact pad formation and common post-gate-annealing at 350 °C in N 2 ambient to complete the device fabrication [Fig.1(a)].For comparison, conventional SG-HEMTs were fabricated using the same process flow but without conducting OCT.
The OCT process consists of two steps.The first step is the oxygen-plasma treatment which oxidizes the p-GaN surface and facilitates the penetration of oxygen.The second step is soaking the sample in dilute hydrochloric acid (25% vol) to fully remove the p-GaN layer that is heavily oxidized or damaged during the preceding plasma treatment.The total etched depth is around 2 nm, and clear atomic steps and smooth surface are well maintained on the p-GaN surface after conducting OCT, as shown in Figs.2(a) and 2(b).It is essential to emphasize that the OCT can be directly conducted after the gate opening, without the need for additional lithography to define the treatment region.Moreover, the OCT does not involve a long-term high-temperature annealing process.Thus, OCT is a simple and low-cost method.
Although both the OCT process and GaON formation involve oxygen plasma treatment, the formation of highquality crystalline GaON is less likely.The study on the mechanism of GaON formation reveals that high-temperature annealing after oxygen plasma treatment is a necessary process to manipulate the kinetic-thermodynamic reaction pathways for the formation of the GaON layer. 22)][28] However, there is no high-temperature annealing process in the OCT process of this work.As a result, the possibility of high-quality crystalline GaON formation is low.
To determine the effectiveness of OCT in facilitating the penetration of oxygen into the p-GaN layer, the oxygen distribution in the p-GaN gate treated with OCT was characterized using secondary-ion-mass-spectroscopy (SIMS) [Fig.2(c The gate capacitance-voltage (C G -V G ) tests were conducted to reveal the impact of OCT on the Schottky junction depletion region [Fig.2(d)].Under forward gate bias, the gate region can be modeled as two capacitors in series, one a reverse-biased metal/p-GaN Schottky junction capacitance (C Schottky ) and the other a voltage-independent AlGaN barrier capacitance (C AlGaN ). 29,30)The total capacitance C G is given by:

G Schottky AlGaN
The acceptor concentration (N A ) of p-GaN can be extracted from the 1/C Schottky 2 versus V Schottky plot, where V Schottky is the voltage drop of the Schottky junction.As shown in Fig. 2(e), the devices with and without OCT exhibit a similar N A in the uncompensated p-GaN region.However, a noticeable reduction in C G is observed for the p-GaN gates with OCT in Fig. 2(d).The reduced C G demonstrates that OCT effectively widens the depletion region of the Schottky junction, thereby mitigating the E-field.
Figure 3 plots the transfer, transconductance (G M ) and output characteristics of the devices with and without OCT.Both devices feature the same dimensions, including a gate metal contact length (L G ) of 2 μm, a p-GaN gate length (L p-GaN ) of 3.5 μm, a gate-drain distance (L GD ) of 15 μm, a gate-source distance (L GS ) of 3 μm, and a gate width (W G ) of 20 μm.The threshold voltage (V TH ) is slightly affected by the OCT treatment, which positively shifts by less than 0.15 V from the initial value of 1.72 V defined at 10 −3 mA mm −1 [Fig.3(a)].Since the OCT process is conducted only on the p-GaN surface, it specifically affects the Schottky junction without impacting the p-GaN/AlGaN/GaN (p-i-n) junction, ensuring that the device does not exhibit an additional negative threshold voltage shift. 31,32)Moreover, both the leakage current and the capacitance of the Schottky junction are reduced after the OCT process, meaning that under the same gate voltage, more voltage is dropped across the Schottky junction based on either capacitors or diodes model. 7,29)To ensure an enough voltage drop across the pi-n junction to turn on the 2DEG channel, a higher gate Forward gate leakage and breakdown characteristics were measured at various temperatures from 25 °C to 175 °C with a step of 50 °C [Fig.4(a)].With OCT, the forward gate breakdown voltage (V GBD ) is boosted from 10.4 V to 14.5 V at 25 °C.V GBD increases as temperature increases and reaches 14.8 V at 175 °C for the devices with OCT [Fig.4(a)].As shown in the inset of Fig. 4(a), thermal stability of V TH is well maintained after conducting OCT.Up to 175 °C, the V TH variation of devices with and without OCT is less than 0.06 V, indicating that the configurations of oxygen in p-GaN (i.e.O N or Mg Ga O N ) are quite stable owing to the large migration energy barrier. 18,19)o further clarify the difference between OCT and GaON technology, a comparison is made between the thickness of the compensated p-GaN and the Schottky barrier height in this study and those in Ref. 5, where high-quality crystalline   051002-3 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd GaON was formed to improve gate reliability.The gate leakage at high V GS is dominated by Fowler-Northeim tunneling due to the weak temperature dependence [Fig.4 (a)]. 33,34)From the C G -V G and the gate leakage characteristic [Figs.2(c) and 2(d)] and Fig. 4, we can simultaneously estimate the compensated p-GaN thickness (i.e.i-GaN) and Schottky barrier height (j b ) by using Eqs.( 1)-( 5):

= +
e e e e + C 1 2 where W is the thickness of i-GaN, E is the value of peak Efield, I FN is the F-N tunneling current [Fig.4(a)], 33) j b is the Schottky barrier height, V bi is the build-in potential of Schottky junction, V Schottky is the voltage drop on the Schottky junction, V FV is the potential difference between the Fermi level of the undepleted p-GaN region and the p-GaN valence band, q is the elementary charge,  is the reduced Planck constant, ε 0 is the absolute dielectric constant, ε s is the relative dielectric constant of p-GaN and m * is the effective mass of holes in p-GaN.The calculations of E and C Schottky are based on trapezoidal electric field distribution. 35,36)he thickness of i-GaN is estimated to be 12.5 nm, which is significantly higher than the reported thickness of a highquality crystalline GaON layer (W = 4.3 nm). 5)The estimated 051002-4 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd j b is 1.02 eV, which is lower than the reported j b formed between the high-quality crystalline GaON layer and the gate metal (j b = 1.1 eV). 5)The thicker i-GaN and lower value of j b indicates that the proposed OCT is a technique that is different from GaON.In addition, from Eq. ( 3), the peak Efield at the p-GaN/metal junction is greatly reduced by the OCT (e.g.reduced for 26% at V GS = 10 V compared to that without OCT).The significant reduction on the E-field suggests that the primary reason for improving gate reliability should be the compensation of Mg dopants in the top portion of the p-GaN layer by O plasma.In addition to the reduced Efield, the presence of oxygen at the metal/p-GaN interface typically results in a higher j B . 37,38)After conducting OCT, the j B increases by 0.27 eV [Fig.4(b)].The increase in j B , combined with the reduced E-field, effectively suppresses the hole injection to the reverse-biased Schottky junction, resulting in a reduction of around three orders of magnitude in gate leakage at V GS of 6 V. Time-dependent gate breakdown (TDGB) tests were performed to evaluate the gate reliability, with a constant gate voltage applied and the source and drain grounded.During the tests, gate leakage is monitored, and time-tobreakdown (t BD ) is defined as when the gate leakage current suddenly increases and leads to a hard breakdown.The t BD of devices without and with OCT both follows the Weibull distribution as shown in Figs.5(a) and 5(b).The most conservative exponential law is used to predict the lifetime [Fig.5(c)].For a 10 year lifetime at a failure rate of 1%, devices with OCT have a much higher maximum applicable gate bias (V GMAX ) of 8.1 V compared to the devices without OCT, of which the V GMAX is 4 V.
[10]14,39,40) The devices with OCT deliver the highest V GMAX at a failure rate of 1%, demonstrating that OCT is an effective way to enlarge the safe gate-bias operation window.
In this work, p-GaN gate HEMTs with enhanced gate reliability is realized by employing a simple OCT.Compared to conventional Schottky-type p-GaN gate devices, the devices with OCT exhibit a larger forward gate breakdown voltage of 14.5 V and a higher maximum applicable gate bias of 8.1 V for a 10 year lifetime at 1% failure level.The proposed technique provides an attractive method to realize highly reliable p-GaN gate HEMTs for high-efficiency power conversion systems.
)].A sample without OCT was also measured to obtain the reference background signal of oxygen.With OCT, the obvious increase of oxygen concentration (N O ) near p-GaN surface is observed in Fig. 2(c), confirming that oxygen penetrates to the top portion of the p-GaN layer even after the removal of the heavily oxidized layer.The net N O introduced by OCT is estimated by subtracting the background signal from that measured on the sample with OCT.

Fig. 2 .
Fig. 2. AFM image of p-GaN surface (a) with and (b) without OCT.(c) SIMS depth profile of oxygen concentration (N O ) from p-GaN surface.Inset: SIMS depth profile in log scale.(d) C G -V G characteristics of p-GaN gates with and without OCT.(e) The extraction of N A from the 1/C Schottky 2 versus V Schottky plot, where C Schottky and V Schottky are the capacitance and voltage drop of the Schottky junction, respectively.

Fig. 4 .Fig. 5 .
Fig. 4. (a) Forward gate leakage and breakdown characteristics of SG-HEMTs with and without OCT at different temperatures.Inset: Dependence of V GBD and V TH shift on temperature.(b) The j B estimation from FN plots of log (I G /E 2 ) versus 1/E in both HEMTs.