Demonstration of AlGaN/GaN MISHEMT on Si with Low-Temperature Epitaxy Grown AlN Dielectric Gate

Abstract

AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MISHEMT) with a low-temperature epitaxy (LTE)-grown single crystalline AlN gate dielectric were demonstrated for the first time and the post-gate annealing effects at 400 °C were studied. The as-deposited LTE-AlN MISHEMT showed a maximum drain current (I_Dmax) of 708 mA/mm at a gate bias of 4 V and a maximum extrinsic transconductance (g_mmax) of 129 mS/mm. The 400 °C annealed MISHEMT exhibited an increase of 15% in g_mmax, an order of magnitude reduction in reverse gate leakage and about a 3% suppression of drain current (I_D) collapse. The increase of g_mmax by post-gate annealing is consistent with the increase of 2DEG mobility. The suppression of I_D collapse and the reduction of gate leakage current is attributed to the reduction of interface state density (5.0 × 10¹¹ cm⁻²eV⁻¹) between the AlN/GaN interface after post-gate annealing at 400 °C. This study demonstrates that LTE grown AlN is a promising alternate material as gate dielectric for GaN-based MISHEMT application.

1. Introduction

AlGaN/GaN based high-electron-mobility transistors (HEMTs) have demonstrated excellent high-frequency and high-power performance owing to their excellent material properties, such as large breakdown field, wide band gap and high electron mobility [1,2,3,4]. However, two of the major limiting factors that conventional GaN HEMTs with Schottky metal gates suffer from are a high gate leakage current, and current collapse [5]. The high gate-leakage occurs due to the Schottky metal contact, while current collapse is caused by charge trapping at the surface states present on the AlGaN surface. To solve these issues, various materials such as Al₂O₃ [6,7], HfO₂ [8] or ZrO₂ [9] have been used as both a passivation layer and gate dielectrics_. Among non-oxide insulators, AlN is an attractive high-k dielectric material for III–N metal-insulator-semiconductor high-electron-mobility transistors (MISHEMTs) due to its high breakdown field and high dielectric constant [10,11]. In addition, AlN is of interest due to its high thermal conductivity (200 WK⁻¹m⁻¹), which makes it suitable for use as a passivation layer to suppress the self-heating [12]. AlN has also been reported to help reduce current collapse [13]. There are two main methods used for the deposition of AlN namely metal-organic chemical vapor deposition [14] (MOCVD) and plasma-enhanced atomic layer deposition [15,16] (PEALD). However, the growth temperature of MOCVD (>600 °C) is not desirable for the fabrication of AlGaN/GaN HEMTs. Furthermore, using a lower growth temperature also has the advantage of preventing tensile strain-induced cracking of AlN layer in AlN based MIS-HEMTs [17,18]. PEALD is a different approach to grow AlN films which can form a better interface with GaN at 350 °C but with a low deposition rate [19]. Recently, Dikme et al. [20] realized thick single crystalline AlN layers on Si and sapphire substrates at 200 °C using a novel technique called low-temperature epitaxy (LTE). In this technique, AlN is deposited as a combination of physical vapor deposition (PVD) and chemical vapor deposition (CVD). LTE also allows for thick (~1 µm) crystalline films to be grown at low-temperatures which is compatible with III-V device processing. We have recently reported the properties of interface states for AlGaN/GaN metal-insulator-semiconductor diodes (MIS-diodes) using the LTE grown AlN [21,22] So far, no reports have discussed the AlGaN/GaN MISHEMTs with LTE-AlN and its post-gate annealing effects. In this paper, we report AlGaN/GaN MISHEMTs on Si substrate with LTE grown AlN through DC, pulsed I-V and interface trap characterization and analysis.

2. Materials and Methods

The AlGaN/GaN HEMT structure on Si (111) substrate was grown by MOCVD. It consists of i-GaN (2 nm) cap layer, i-Al_0.27Ga_0.73N (18 nm) barrier layer, i-GaN (800 nm) buffer layer and transition layer (1400 nm). The resistivity of the Si substrate is >10,000 Ω.cm. Hall samples of (i) as-grown HEMT without LTE-AlN, (ii) HEMT with as-deposited ~8 nm LTE-AlN, (iii) HEMT with LTE-AlN annealed at 400 °C and (iv) HEMT with LTE-AlN annealed at 450 °C were prepared and their results at room temperature are summarized in

Table 1. 2DEGproperties of AlGaN/GaN with and without AlN and its post deposition annealing at 400 °C and 450 °C for 300 s in N₂.

2DEG Parameters	AlGaN/GaN HEMT Structure
	Without LTE-AlN [22]	With LTE-AlN
		As-dep. [22]	Annealing Temperature °C
			400	450
Sheet Resistance (Ω/□)	591	523	520	512
Hall Mobility (cm2V−1s−1)	1440	1210	1330	1360
Sheet Carrier Concentration (×1012 cm−2)	7.35	9.89	9.02	8.76

. The MISHEMT fabrication process started with mesa isolation by reactive ion etching (RIE) using a Cl₂/BCl₃ mixture. The ohmic contacts consisting of Ti/Al/Ni/Au (20/120/40/50 nm) was deposited followed by rapid thermal annealing at 825 °C for 30 s in an N2 atmosphere. Transmission line measurements showed a contact resistance of 0.4 Ω mm. Next, the gate dielectric layer using single crystalline AlN with a thickness of ~8 nm was deposited at 200 °C by LTE. The thickness of the deposited LTE-AlN has previously been confirmed by TEM and is reported elsewhere [22]. The novel growth method combines physical vapor deposition (PVD) and chemical vapor deposition (CVD). The Al source is solid Al with a purity of 5N, while the N source is N₂ gas with purity of 5N8. The N was activated by a linear ion gun close to the sample surface and the Al was sputtered in a way that its beam overlaps with the ion gun beam. The substrate temperature was in the range of 200–225 °C and the deposition pressure was in the upper 10⁻³ mbar range with a total power density of around 5–7 W/cm². Before the deposition, the sample was cleaned with a weak Ar/H₂ plasma to remove the native oxide. More details of the growth conditions can be found in the paper by Dikme et al. [20,21].

The gate metal stack Ni/Au (50/200 nm) was subsequently formed on the LTE grown AlN by electron beam evaporation. Finally, metal thickening (Ti/Au 10/400 nm) was also performed after AlN etching by Cl2/BCl3/Ar (40/20/10 sccm) plasma. The inset of Figure 1a shows the cross-sectional schematic diagram of the fabricated MISHEMTs with LTE-grown AlN. For this study, we have used device dimensions of L_g/L_sg/L_gd/W_g = 2/2/2/(2 × 100) µm. To study the post-gate annealing effects a MISHEMT sample with ~8 nm of LTE-AlN were annealed at 400 °C in a N₂ atmosphere using rapid thermal annealing process. A post-gate annealing temperature of 400 °C was chosen as there was minimal changes to the Hall parameters after post deposition annealing at 450 °C (see Table 1). A lower temperature is also beneficial, as higher temperatures have previously been shown to cause degradation of Ni/Au gates [23,24].

3. Results and Discussion

Figure 1 shows (a) the capacitance-voltage (C-V) and (b) gate leakage current (I_gleak) characteristics of Schottky diode (MS-diode), LTE-AlN MIS-diode with and without post-gate annealing. At zero-bias, the capacitance density of 373 nF/cm² and 302 nF/cm² for 200 µm diameter conventional Schottky diode and MIS-diode were obtained, respectively. After annealing, there is no significant change in capacitance density at 0 V. With reference to Schottky diode, the LTE-AlN MIS-diode exhibited 2 orders of magnitude lower I_gleak at −20 V (Figure 1b). After post-gate annealing at 400 °C, MIS-diodes exhibited about an order of magnitude further reduction in I_gleak. The improvement in I_gleak is attributed to the improvement of interface properties of LTE-AlN on GaN/AlGaN after the 400 °C annealing.

Figure 2 shows (a) current–voltage (I_ds-V_ds) and (b) transfer characteristics of LTE-AlN/AlGaN/GaN MISHEMTs without and with post-gate annealing at 400 °C. The as-deposited AlN MISHEMT showed a maximum drain current (I_Dmax) of 708 mA/mm at a gate bias of 4 V and a maximum extrinsic transconductance (g_mmax) of 129 mS/mm. After annealing, MISHEMT exhibited I_Dmax of 684 mA/mm at a gate bias of 4 V and g_mmax of 148 mS/mm. The decrease in I_Dmax after annealing originates from a change in two-dimensional electron gas (2DEG) carrier concentration (n_s), as I_D ∝ n_s. As shown in Table 1, after annealing at 400 °C, n_s was found to decrease by 9% (from 9.89 × 10¹² cm⁻² to 9.02 × 10¹² cm⁻²) which results in the 9% reduction in I_Dmax. Similarly, the 15% improvement of g_mmax after post-gate annealing is attributed to an increase in electron mobility as well as a reduction of interface states [25]. This is attributed to a reduction in Coulomb scattering from the dielectric layer near the AlGaN/GaN interface [26]. The enhanced mobility was confirmed by Hall measurements, which shows an ~10% improvement (from 1210 cm²V⁻¹s⁻¹ to 1330 cm²V⁻¹s⁻¹) in 2DEG Hall mobility (µ_n), as seen in Table 1. From the Figure 2b, it is clear that AlN MISHEMT exhibited an order of magnitude improvement in the device I_ON/I_OFF ratio after the post-gate annealing at 400 °C, which is due to the reduction of drain current at OFF-state. This is possibly caused by a reduction on traps at the AlN/GaN interface reducing the available leakage current conduction paths. The threshold voltages (V_th) of the devices were measured at −3.95 V and −3.8 V for as-deposited MISHET and post-gate annealed MISHEMT at 400 °C, respectively. V_th can be expressed as

$${\mathrm{V}}_{\mathrm{th}}={\mathrm{V}}_{\mathrm{th}0}-\frac{{\mathrm{Q}}_{\mathrm{it}}}{{\mathrm{C}}_{\mathrm{AlN}}}$$

(1)

where V_th0 is the threshold voltages without any interface states, Q_it is the interface-trapped charge density and C_AlN is the capacitance of the AlN layer. After annealing at 400 °C there is a minimal positive shift in threshold voltage which could be caused by a slight reduction of interface traps at LTE-AlN/GaN interface. This is verified by the frequency-dependent conductance measurements discussed in the later section. A similar occurrence was also reported after post-gate annealing by Zhou et al. for Al₂O₃ and Shih et al. for HfO₂ [8,25]. In these cases, it was postulated that the positive V_th shift was caused by a reduction in positively charged traps and interface traps or positive fixed/mobile charges, and was confirmed by a reduction in calculated interface states after annealing. A benchmarking table between these devices and those published elsewhere can be seen in

Table 2. A benchmarking table for MISHEMT on AlGaN/GaN using AlN as a gate dielectric layer.

Reference	Thickness (nm)	Substrate	Deposition Method	Device Dimensions Lg/Wg (µm)	Idmax (mA/mm)	gmmax (mS/mm)	On/Off Ratio (Orders of Magnitude)
[15]	10.6	Si	ALD	2.5/60	563 @ 5V	87	~5
[27]	10	Sapphire	ALD	1/200	600 @ 4V	127	~9
[28]	20	Sapphire	PEALD	0.5/50	~1050 @ 2 V	289	~3
[29]	8	SiC	Reactive Sputtering	0.4/200	~1250 @ 2 V	260	~3
This work	8	Si	LTE	2/(2 × 100)	684 @ 4V	148	~8

[15,27,28,29].

Figure 3 shows pulsed I_D-V_D characteristics of (a) as-deposited MISHEMTs with LTE-AlN and (b) MISHEMTs with post-gate annealing at 400 °C. The devices were subjected to the pulse width/period of 100 µs/10ms and quiescent biases of (V_gs0, V_ds0) = (0, 0) and (−6, 20) V was used for the pulsed I-V measurements. The as-deposited LTE-AlN MISHEMT exhibited a I_D/I_Dmax ratio of 0.91 for both quiescent biases (V_gs0, V_ds0) = (0, 0) and (−6, 20). This indicates that the devices exhibited around 9% drain current (I_D) collapse. After annealing at 400 °C, the I_D/I_Dmax ratio of MISHEMT increases to 0.94. Therefore, about 3% I_D collapse was suppressed after post-gate annealing at 400 °C. The improvement in current collapse in 400 °C annealed MISHEMT is attributed to the reduction of interface states at the AlN/GaN interface.

In order to quantify the amount of interface states at the LTE-AlN/GaN interfaces, frequency-dependent conductance measurements were performed at selected biases to estimate the density of interface states (D_it) and trap time constant (τ_it). The frequency was varied from 1 kHz to 5 MHz over a wide range of gate voltages (V_g). Figure 4a shows the typical G_p/ω versus ω graph of LTE-AlN MISHEMT with post-gate annealed at 400 °C measured at different V_g values between −4.1 V to −3.5 V. The D_it calculations were performed using the conductance-frequency method, which is widely used for interface calculations [21,30,31]. The two peak regions in the G_p/ω plots correspondingly indicate the presence of both low frequency (slow traps) and high frequency (fast traps). The exhibited fast traps are associated interface traps of the AlGaN/GaN hetero-interface [15,32], while the observed slow traps are associated with the AlN/GaN interface. The estimated D_it is shown in Figure 4b for the as-deposited LTE-AlN MIS-diode, as well as the MIS-diodes with post-gate annealing at 400 °C. The minimum D_it were estimated as 7.6 × 10¹¹ cm⁻²eV⁻¹ and 5.0 × 10¹¹ cm⁻²eV⁻¹ for the as-deposited MIS-diode and MIS-diode with post-gate annealing at 400 °C, respectively. With reference to as-deposited MIS-diode, a reduction of D_it (24%) has been observed in the MIS-diode after post-gate annealing at 400 °C. This reduction of interface traps can be associated with the suppression of I_D collapse. Annealing at 400 °C helps to reduce the slow deep level traps thus reducing the remote Coulomb scattering from the AlN layer and improving the mobility.

4. Conclusions

In summary, AlGaN/GaN MISHEMTs on Si has been demonstrated for the first time with LTE grown AlN as a dielectric layer. The influence of post-gate annealing at 400 °C was also studied using DC, pulsed I-V and interface state characteristics. After the LTE-AlN deposition the LTE-AlN MISHEMT showed a maximum drain current (I_Dmax) of 708 mA/mm at a gate bias of 4 V and a maximum extrinsic transconductance (g_mmax) of 129 mS/mm. By employing a post-gate annealing scheme at 400 °C in an N₂ atmosphere there was about a 15% of increase in g_mmax and an order of magnitude reduction of gate leakage. About a 3% improvement of I_D collapse suppression was also observed after post-gate annealing at 400 °C. The reduction of I_gleak and I_D collapse could be due to the reduction of interface state density of about 25%. This study indicates that the optimized post-gate annealing, which in our case is at 400 °C, is a viable way to have improved device characteristics in AlGaN/GaN MISHEMTs with LTE grown AlN.