Enhanced Carrier Transport Properties in GaN-Based Metal-Insulator-Semiconductor High Electron Mobility Transistor with SiN/Al₂O₃ Bi-Layer Passivation

The GaN-based MISHEMTs were fabricated on a sapphire substrate with the epitaxial layer stack of AlN nucleation (100 nm), GaN buffer (1800 nm), unintentionally doped GaN channel (100 nm), AlN interfacial (1 nm), Al_0.26Ga_0.74N barrier (21 nm) and GaN capping (2 nm) layers grown by metal-organic chemical vapor deposition (MOCVD). The device processing started from the deposition of Ti/Al/Ni/Au (30/100/30/100 nm) followed by rapid thermal annealing for 150 sec at 900°C to form ohmic contacts. Then, phosphorus implantation was conducted for the device isolation. Three different dielectric systems (sample A: SiN 20 nm, sample B: SiN 5 nm/Al₂O₃ 15 nm, sample C: SiN 5 nm/Al₂O₃ 35 nm) were prepared to investigate the mechanical stress effects. A SiN layer was deposited at 250°C using a plasma enhanced chemical vapor deposition (PECVD) system. The process gases for PECVD were SiH₄ and N₂. An Al₂O₃ layer was deposited by atomic layer deposition (ALD) using trimethylaluminum (TMA) as a metal precursor and O₂ plasma as an oxidant at 150°C. For samples A and B, the net physical thickness of 20 nm was kept the same for the dielectric layer. After opening source and drain contact pads with buffered oxide etch, Ni/Au (30/370 nm) were deposited for gate electrode. The gate length (L_G), width (W_G), and distance between source and gate (L_SG) of the fabricated device were 4.5, 100, and 1 μm, respectively whereas the distance between gate and drain (L_GD) was varied from 6.5 to 21.5 μm.

To investigate the mechanical stress applied to the GaN, UV-visible micro-Raman spectroscopy measurements were performed as shown in Fig. 1. The in-plane stress was calculated using the equation: Δω = K_Rσ, where K_R = 4.3 cm⁻¹ GPa⁻¹.^26,27 Before dielectric layer deposition, the E₂-high peak was obtained at 572.3 cm⁻¹. After dielectric layer deposition, the E₂-high peak was shifted according to the strength and type of the induced mechanical stress. With increasing SiN thickness, the compressive stress enhances. Therefore, the E₂-high peaks were moved to 573.2 cm⁻¹ and 574.2 cm⁻¹ for 5 nm thick SiN layer and sample A, respectively. By contrast, the deposition of Al₂O₃ layer resulted in tensile stress and compensated the compressive stress caused by the 5 nm thick SiN layer. Thus, the E₂-high peak was slightly recovered to 572.8 cm⁻¹ in sample B. In sample C, the tensile stress applied by the 35 nm-thick Al₂O₃ layer overcomes the compressive stress and consequently, provides tensile stress on the GaN channel, resulting in the shift of E₂-high peak to 572.1 cm⁻¹.

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The typical device properties of the GaN-based MISHEMTs were measured to compare the device performance for various passivation systems and corresponding mechanical stresses. Fig. 2a shows the transfer characteristics. The device threshold voltage depends on the thickness of the dielectric layer, reducing V_TH with increasing dielectric layer thickness.^28,29

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We note that the 2DEG concentration also influences the V_TH by reducing V_TH with increasing 2DEG concentration. However, in our case, the V_TH is mainly determined by the dielectric layer thickness. The reason is that in sample B with larger 2DEG concentration compared to that of sample A (discussed in Fig. 4), the V_TH is higher than that of sample A since the net electrical thickness of the dielectric layers is thinner because of higher dielectric constant of Al₂O₃ layer. The V_TH of sample C is −11.1 V as it has the thickest net electrical thickness due to the 35 nm thick Al₂O₃ layer. With reducing compressive stress, the transconductance maximum increases, as well as saturation current obtained at V_G = V_TH + 5 V. In accordance with the results, device with tensile stress (sample C) shows the best performance among the three samples.

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The on-resistance (R_on) was extracted from the output characteristics shown in Fig. 2b. The R_on is an important factor which depends on the fundamental carrier transport for a fixed device geometry.²⁵ R_on was extracted from the slope of the linear regime (V_D < 5 V) at V_G ≈ V_TH + 5 V. Compared to the device with compressive stress (sample A), the device with tensile stress exhibits lower R_on (29.7 and 22.4 Ω·mm for samples A and C, respectively.). At higher drain bias (V_D > 10 V), the drain current is lower in sample C than in sample B as the thermal emission capability is deteriorated with thicker dielectric layers,³⁰ reinforcing self-heating effects.

To figure out the detailed mechanism for the performance improvement in the device with tensile stress, which presents the best performance, we performed the C(V) measurments and extracted carrier concentrations for various passivation systems at 1 MHz. The capacitance depends on the net electrical thickness of the dielelctric layers; thicker dielectric layer shows lower capacitance. As shown in Fig. 3a, the hysteresis was negligible, reflecting good interface quality between AlGaN/GaN and AlGaN/GaN capping/dielectric layers of the samples.

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The 2DEG concentration (N_a) as a function of the corresponding depth (D) in MISHEMTs was extracted^8,31 from the C(V) measurement by following equations:

and

where, t_OX, t_CAP, and t_BAR are the thickness of dielectric, GaN capping, and AlGaN barrier, respectively. ɛ_GaN, ɛ_OX and A are dielectric constants of the GaN and dielectric layer, and area of the gated region, respectively.

ɛ_OX is calculated as^8,31

where, C_MISHEMT and C_HEMT are capacitances of MISHEMT and HEMT, respectively, read from the plateau regions of the C(V) curves. Calculated ɛ_OX values for the three different samples are provided in Table I. The obtained ɛ_OX values are comparable with reported values of SiN³² and Al₂O₃⁸ deposited by PECVD and ALD, respectively. As shown in Fig. 3b, the carrier concentration at hetero-interface enhances with diminishing compressive stress, and the highest carrier concentraion is observed in the device with tensile stress. Piezoelectric field enhances when the compressive stress reduces and tensile stress is applied, improving polarization induced charges,²⁵ which results in enlarged carrier concentraion with reducing compressive stress.

The carrier mobility was also extracted since the carrier mobility is a key parameter which reflects the device performance. For the achievement of the precise carrier mobility (μ) behavior under the gated region, the impact of parasitic resistances (i.e., access and contact resistance) were taken into account using the following equation^33,34

where, N_s is achieved by the integration of the capacitance. W_G and L_G are gate width and length of the HEMTs, respectively. V_{D_ADJ} is the voltage across the gated region calculated as^33,34

where, R_ACC and R_C are the resistance for access region defined as the distance between gate and source/drain and contact resistance, respectively obtained from Fig. 5.

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As shown in Fig. 4, the maximum carrier mobility increases when the compressive stress reduces, exhibiting the highest carrier mobility in the device with tensile stress (sample C). This phenomenon is different from the conventional N_s - μ tendency^35,36 that requires further detailed investigation. Possible scenarios of improvement of the carrier mobility with reducing compressive stress as well as increasing carrier concentration are as follows. First, the conduction-band-edge is significantly varied with reducing compressive stress, generating launcher effect²⁷ and increasing carrier mobility. In addition, when the compressive stress diminishes, the distance between the lattices becomes longer and the mean free path directly related to the scattering time is increased,²⁵ boosting carrier mobility.

To confirm the simultaneous improvement of the carrier concentration and mobility, we have extracted the sheet resistance R_SH shown in Fig. 5. In Fig. 5a, I_D(V_G) curves were measured for various L_GD and passivation systems. When the L_GD increases, the drain current decreases due to the enlarged access resistance. Therefore, the total resistance linearly increases with increasing L_GD as shown in Fig. 5b. The slope of each line yields the R_SH and the intercept point with Y-axis indicates the contact resistance.^33,34 Compared to the samples A and B, the sample C exhibits dramatic reduction in R_SH. This is a clear evidence for the simultaneous improvement of the carrier concentration and mobility in the device with tensile stress because if the carrier mobility is deteriorated with enhancing carrier concentration, the R_SH should not result in such a large reduction according to R_SH ∼ 1/(N_a · μ).³⁶ The device characteristics related to the carrier transfer and device performance are summarized in Table I.

In summary, we have investigated the mechanical stress applied on the GaN channel via the deposition of the SiN/Al₂O₃ bi-layer passivation and device performance corresponding to the induced mechanical stress. The PECVD-SiN and ALD-Al₂O₃ results in the compressive and tensile stresses, respectively that compensate each other. The 2DEG transfer characteristics such as the carrier concentration at AlGaN/GaN interface and carrier mobility were improved with decreasing compressive stress, showing the best properties in device with tensile stress. Moreover, when the tensile stress is introduced on GaN channel, the device presents the best performance in terms of the transconductance, saturation current, on-resistance, and sheet resistance. As a result, the mechanical stress engineering through the bi-layer passivation system is an efficient technique for the improvement of the carrier transfer properties and device performance in GaN-based HEMTs.

This work was supported by the program of Defense Acquisition Program Administration and Agency for Defense Development.

S.-J. Chang 0000-0002-6973-8199

H.-W. Jung 0000-0001-5770-6387