由於氮化鎵半導體具有優選c-自發極化特性(由試片表面指向基板)，在氮化鋁鎵/氮化鎵異質結構界面會吸引電子形成二維電子氣(2DEG)通道，此異質結構常用於空乏型高速電子遷移率電晶體之製作。本論文中，我們欲進行增強型高速電子遷移率電晶體之製作，先利用光電化學(photoelectrochemical，PEC) 濕式蝕刻法對氮化鋁鎵進行閘極掘入，提升閘極控制能力，同時降低掘入區域氮化鋁鎵的極化效應，使通道載子濃度下降，進而提升臨限電壓(threshold voltage，Vth)。為避免氮化鋁鎵過蝕而破壞到通道層，透過脈衝雷射沉積法(Pulsed laser deposition technique)製備高介電鈮酸鋰(Lithium Niobate，LiNbO3)薄膜作為閘極氧化層，對薄膜進行熱處理使產生具有優選c+自發極化特性(由基板指向試片表面)的鈮酸鋰(006)結晶相，利用其自發極化抵銷氮化鋁鎵/氮化鎵異質結構之殘餘極化效應以空乏2DEG通道，最後完成增強型高速電子遷移率電晶體之製作。
首先，以150 nm鈮酸鋰 (LiNbO3)薄膜作為閘極氧化層，在薄膜未經過熱處理直到提升退火溫度及時間至600°C 30分鐘時，Vth從初始的-2.6 V提升到最大值約-1.4 V，；對應之gmMAX為68.9 mS/mm；無閘極偏壓且VDS = 3 V時對應之IDS為9 mA ，得知鈮酸鋰薄膜在熱處理600°C 30分鐘後，達到最大極化強度，使通道具有最佳之空乏效果。
接著以40 nm二氧化矽(SiO2)薄膜作為閘極氧化層，探討氮化鋁鎵(初始厚度35 nm)在未進行閘極掘入以及掘入後厚度剩下約25、15及5 nm之元件電性比較，Vth分別為-3.2 V、-2 V、-1.2 V及0 V；gmMAX則分別為115.3 mS/mm、90.4 mS/mm、115.5 mS/mm及7.41 mS/mm；無閘極偏壓且VDS = 3 V時，IDS分別為15.9 mA、11.3 mA、6.7 mA及1.1mA，顯示提高氮化鋁鎵閘極掘入深度可降低通道載子濃度，進而提高Vth值，然而當氮化鋁鎵經閘極掘入使厚度降低至約5 nm時，因為通道層受到破壞的關係，使元件電流及轉導值嚴重衰退，所以氮化鋁鎵閘極掘入後的厚度控制在約15 nm較為適當。最後將上述對通道有最佳空乏效果之鈮酸鋰薄膜作為閘極氧化層，結合氮化鋁鎵閘極掘入之最適當條件進行元件製作，Vth及gmMAX分別可達到+0.4 V及55.2 mS/mm，達到增強型高速電子遷移率電晶體製作之目的。

Investigation of gate recessing E-Mode MOSHEMTs by PEC method combining LiNbO3 film

Chang-Lin Yang* and Ching-Ting Lee**
Institude of Microelectronics, Department of Electrical Engineering, National Cheng Kung University

SUMMARY:
In this study, LiNbO3 (LNO) films as gate oxide were incorporated with gate-recessing process for fabricating AlGaN/GaN E-HEMTs. The LNO films were deposited by pulsed laser deposition (PLD) system, and the gate-recessing process was carried out by photoelectrochemical (PEC) wet etching method. The LNO films on AlGaN layer were annealed to deplete the 2DEG channel by preferable direction of polarization. On the other hand, the gate-recessing process was utilized to enhance the depletion extent in 2DEG channel layer, so that the AlGaN/GaN E-HEMTs could be fabricated. The crystallinity, surface morphology, and domain structure of the LNO films were obtained by X-ray diffraction, scanning electron microscopy (SEM), and piezoelectric force microscopy (PFM), respectively. The AlGaN/GaN E-HEMTs with an etched 15-nm-thick AlGaN layer incorporating with LNO film annealed at 600 °C for 30 min was successfully fabricated without channel destruction. The threshold voltage (Vth) and the transconductance (gm(max)) of AlGaN/GaN E-HEMTs were about +0.4 V and 55.2 mS/mm, respectively.

Key words: LiNbO3；gate-recessing；AlGaN/GaN；E-HEMTs；photoelectrochemical wet etching

INTRODUCTION:
Due to inherent 2DEG layer, AlGaN/GaN system has often been used to fabricate depletion-mode high electron mobility transistors (D-HEMTs). However, from the practical applications in integrated circuits, enhancement-mode transistors (E-HEMTs) can offer capabilities of simplifying circuit design, promoting safety of operation and reducing unnecessary power consumption. Therefore, several approaches have been published to fabricate E-HEMTs, such as fluorine plasma treatment, gate recess and ferroelectric gate oxide, etc. In this study, we utilized LiNbO3 (LNO) ferroelectric films as gate oxide combining gate-recessing process to fabricate AlGaN/GaN E-HEMTs. LNO has the largest spontaneous polarization in all ferroelectric materials which could cause the maximum modulation to 2DEG theoretically. Through gate-recessing process, 2DEG could be partially decreased. Furthermore, with preferable polarizations (C+ domain structure) in LNO film, 2DEG was fully depleted, and AlGaN/GaN E-HEMTs could be successfully fabricated.

EXPERIMENT:
The 150-nm-thick LNO films were firstly deposited on AlGaN/GaN/sapphire substrates by using pulsed laser deposition technique (PLD) with KrF excimer laser ( λ = 248 nm, τ = 25 ns), deposition parameters were shown in Table 1. Before the films deposition, the substrates were dipped into HCl solution (~ 3%) for removing native oxide on the surface of AlGaN layer. The LNO films were annealed separately at 500 oC, 600 oC and 700 oC for 30 sec, 30 min and 1 h. The crystallinity, surface morphology, and domain structure of the LNO films were obtained by X-ray diffraction, scanning electron microscopy (SEM) and piezoelectric force microscopy (PFM), respectively. The LNO Films under the optimal annealing condition with the preferable direction of polarization and the better surface morphology were utilized as gate oxide on AlGaN/GaN E-HEMTs. Gate-recessed structure was performed on the AlGaN layer of AlGaN/GaN E-HEMTs by bias-assisted photoelectrochemical (PEC) wet etching method with the He–Cd laser (wavelength = 325 nm, power density = 10.0 mW/cm2) and H3PO4 solution (pH value = 0.7). Finally, the optimum annealed LNO film and gate-recess structure were combined, the transfer characteristics of AlGaN/GaN E-HEMTs were measured.

Table 1. Deposition parameters of LiNbO3 films by pulsed laser deposition.

RESULTS AND DISCUSSION:
Figure 1 shows the θ-2θ XRD pattern of the LNO films deposited on AlGaN/GaN/sapphire substrates annealed at different conditions. From the figure, (006) diffraction peak of LNO located at 2θ = 38.9◦ could be obtained after being annealed. The (006) crystalline is the ferroelectric phase of LNO which can deplete 2DEG channel by preferable spontaneous polarization. However, the Li2O was easily evaporated from LNO films during high temperature, therefore Li-deficient phase LiNb3O8 with no ferroelectric characteristics might be obtained, which would degrade the polarization effects of LNO film. The corresponding ( 02) diffraction peak intensity of LiNb3O8 located at 2θ = 38.1◦ increased compared with (006) LNO by raising annealing temperature and time.

Figure 1. θ-2θ XRD pattern of LNO films annealed separately at 500 oC, 600 oC and 700 oC for (a) 30 sec, (b) 30 min and (c) 1 h in oxygen ambient, respectively.

Figure 2 shows the surface morphology of LNO films deposited on AlGaN/GaN/sapphire substrates annealed at different conditions. The grains of LNO crystal were shown to aggregate while raising the annealing temperature and time. The grains distributed uniformly when annealed at 700 °C for 30 min. However, from Figure 2(c), LNO films started peeling (denoted by white arrows) when increased annealing time to 1 hour. This phenomenon was attributed to the lattice mismatch between LiNbO3 and GaN. Therefore, the LNO films with better surface morphology shown in Figure 2(a) and (b) were utilized as gate oxide on transistor.

Figure 2. Surface morphology of LNO films annealed separately at 500 oC, 600 oC and 700 oC for (a) 30 sec, (b) 30 min and (c) 1 h in oxygen ambient, respectively.

Figure 3 shows the vertical PFM images of the LNO film deposited on AlGaN surface. Out-of-plane domains in the LNO film shown in Figure 3(a) were almost C+ orientation. LNO film had random domain patterns ascribed to stochastic distribution of dipoles above frozen layer as shown in Figure 3(b).

Figure 3. The vertical PFM (a) phase image and (b) amplitude image of LNO film deposited on AlGaN surface. The bright and dark areas indicate the sections of C- domain and C+ domain, respectively.

Figure 4 shows the IDS-VGS characteristics of LNO/AlGaN/GaN HEMTs with the LNO films annealed at different conditions. The Vth of transistor was promoted to -1.4 V after annealed at 600 °C for 30 min and declined to -1.6 V after annealed at 700 °C for 30 min due to drastic Li2O evaporation.

Figure 4. IDS-VGS characteristics of LNO/AlGaN/GaN HEMTs with LNO film annealed separately at 500 oC, 600 oC and 700 oC for 30 sec and 30 min.

Figure 5. shows the IDS-VGS characteristics of SiO2/AlGaN/GaN HEMTs with different thickness of AlGaN layer under gate area. From the figure, 15 nm was optimum thickness of AlGaN layer without channel destruction.

Figure 5. IDS-VGS characteristics of SiO2/AlGaN/GaN HEMTs with thickness of AlGaN layer at 35, 25, 15 and 5 nm under gate area caused by PEC wet etching method.

Figure 6 shows the transfer characteristics of LNO/AlGaN/GaN HEMTs with the optimum LNO film and gate-recessing thickness of AlGaN derived from the above results. Finally, the AlGaN/GaN E-HEMTs was successfully fabricated, and the Vth and gm(max) of transistor were respectively about +0.4 V and 55.2 mS/mm.

Figure 6. The transfer characteristics of LNO/AlGaN/GaN HEMTs with LNO film annealed at 600 °C for 30 min and 15 nm AlGaN layer under gate area.

CONCLUSION:
In this study, to fabricate the high performance AlGaN/GaN E-HEMTs, the LNO films as gate oxide incorporating with gate-recessing process were utilized. The AlGaN/GaN E-HEMTs with an etched 15-nm-thick AlGaN layer incorporating with LNO film annealed at 600 °C for 30 min was fabricated without channel destruction. The Vth and the gm(max) of AlGaN/GaN E-HEMTs were about +0.4 V and 55.2 mS/mm, respectively.

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