本論文主要的研究方向是：採用濺鍍(sputtering)系統，以直流磁控濺鍍法(DC magnetron sputtering deposition)沉積氮化鈦(TiN)薄膜。所使用的製程氣體為氬氣(Argon)，並以氮化鈦(TiN)作為濺鍍靶材(target)，進行濺鍍反應後，生成氮化鈦(TiN)薄膜。基材為切割後的藍寶石基板(Al2O3，sapphire)與 n 型氮化鎵(n-GaN)試片，氮化鈦(TiN)薄膜則在基材上生成。

利用快速升溫熱爐管系統(Rapid thermal anneal，RTA)來進行退火，在氮氣(Nitrogen)環境下經過不同溫度與持溫時間熱處理後，對其光電與結構特性作一系列之研究，並研究氮化鈦(TiN)薄膜在 n 型氮化鎵(n-GaN)上之歐姆接觸(Ohmic contact)的特性與蕭特基能障高度(Schottky barrier height，SBH)的變化。

由實驗結果得知，使用濺鍍功率 100W 所沉積厚度為 60nm 的氮化鈦(TiN)薄膜，經由快速升溫熱爐管系統(RTA)在氮氣環境下 900℃ 熱處理 60 秒後，氮化鈦(TiN)薄膜有較低的電阻率為 1.25E-2(ohm-cm)，其載子濃度提高為 6.93E21(1/cm3)，在 510nm 之綠光波段具有 70% 以上之穿透率，且沉積在藍寶石基板(sapphire)上之氮化鈦(TiN)薄膜其熱穩定性良好。

然後將氮化鈦(TiN)薄膜沉積在 n 型氮化鎵(n-GaN)試片上研究其歐姆接觸特性，雖然將氮化鈦(TiN)薄膜直接沉積在 n 型氮化鎵(n-GaN)試片上無法得到線性之歐姆接觸(Ohmic contact)特性，但是利用快速升溫熱爐管系統(RTA)來進行退火，可以有效降低氮化鈦(TiN)薄膜與 n 型氮化鎵(n-GaN)試片之間的蕭特基能障高度(SBH)，並成功改善其歐姆接觸(Ohmic contact)特性，然而經過快速升溫熱爐管系統(RTA)在氮氣環境下 800℃ 熱處理 90 秒後，氮化鈦(TiN)薄膜的蕭特基能障高度(SBH)從原本未退火前的 0.513eV 降低為 0.454eV 有顯著地改善，且經過快速升溫熱爐管系統(RTA)在氮氣環境下 900℃ 熱處理 60 秒後，其傳輸線模型(Transmission line model，TLM)之特徵接觸電阻值(ρc)降低為 1.44E-2(Ω－cm2)，有助於日後將氮化鈦(TiN)薄膜應用在相關之光電與半導體元件上。

關鍵字：直流磁控濺鍍；氮化鎵；氮化鈦；蕭特基能障；歐姆接觸

Characteristics of TiN thin film and theirs contact on n-type GaN

Chia-Shing Chen

Wei-Chih Lai

Department of Photonics, College of Science

SUMMARY

In this study, a sputtering system was adopted and the direct current magnetron sputtering method was employed to deposit TiN thin films. Argon gas and a TiN target were used to produce TiN thin films through a sputtering reaction. The substrates comprised trimmed sapphire (Al2O3) plates and n-type GaN (n-GaN) samples on which TiN thin films were grown. A tube furnace system was utilized to perform rapid thermal annealing (RTA) . According to heat treatments performed using various temperatures and temperature holding times in a nitrogen environment, a series of studies on the thin film optoelectronic and structural properties were executed. In addition, the variations of ohmic contact and the Schottky barrier of the TiN thin films grown on the n-GaN substrates were assessed.

The results indicated that 60-nm thick TiN thin films sputtered using 100 W for 60 seconds at 900 oC heat treatment in a nitrogen environment yielded the minor resistivity 1.25E-2 (ohm-cm) , an increased carrier concentration 6.93E21 (1/cm3) , a transmittance exceeding 70 % between the green wavelengths of 510 nm , and a satisfactory thermal stability on the sapphire substrate.

Key words: magnetron-sputtering ; GaN ; ohmic ; Schottky ; TiN

INTRODUCTION

Sputtering is mainly due to the sputtering phenomenon of discovery, sputtering phenomenon as early as a hundred years ago had been discovered and applied plating areas. So far, sputtering is mature and popular technology in the industry. Ways of generating plasma-based sputtering can be divided into: DC sputtering, RF sputtering, magnetron sputtering, unbalanced magnetron sputtering, enclosed unbalanced magnetron sputtering and etc. For different needs and properties of materials, a different sputtering process must be chose in order to meet the requirements.

Titanium nitride (TiN) plating is used extensively in electronics, aviation, consumer products and precision machinery industries. The ceramic plating has been developed for decades, so the titanium nitride (TiN) plating the theoretical basis and practical applications gradually perfected. Titanium nitride (TiN) thin film technique is used due to its characteristic of adhesion, anti-oxidation, anti-corrosion, thermal stability and easy deposition etch. Therefore it is very suitable in the semiconductor device fabrication application.

Increasingly, furnace anneals are being supplanted by rapid thermal anneal system (RTA) in today semiconductor manufacturing process. The main reason is the RTA process will make good ohmic contact of the bonding junction in both metal and semiconductor.

MATERIALS AND METHODS

Titanium nitride (TiN) is part of the crystal structure of face-centered cubic (Face-Centered Cubic Crystal Structure) sodium chloride (NaCl) structure. Wherein the titanium atoms are arranged one side center structure, and nitrogen atoms in the lattice are interspersed gap position (interstitial sites) on. Generally speaking face-centered cubic lattice structure coordination unit cell is divided into two categories: one tetrahedral coordination unit cell, and the other is a unit cell ligand octahedron. In this titanium nitride for the nitrogen atoms in tetrahedral unit cell space is too small, it is the nitrogen atom of titanium nitride, titanium atom is octahedral coordination unit cell arranged in the way. Titanium nitride (TiN) is a non-stoichiometric substance, wherein the nitrogen (N) in the range of atomic percentage change of 33 % ~ 55 % , the titanium nitride (TiN) due to the lattice constant of the coefficient deviation integer while lowering phenomenon.

The test pieces were prepared in this experiment: mainly by sputtering system, a DC magnetron sputtering deposition of titanium nitride (TiN) thin film. After the process gas used is argon gas, and titanium nitride (TiN) as a sputtering target, for sputtering the reaction of titanium nitride (TiN) thin film. After cutting the substrate is a sapphire substrate (Al2O3) n-type gallium nitride (n-GaN) test piece, a titanium nitride (TiN) thin film is generated and the substrate. Then using rapid thermal anneal system (RTA) to be annealed in a nitrogen environment through different heating treatment temperature and holding time.

RESULTS AND DISCUSSION

Experiments, the sputtering power was set 100 W , 150 W , sputtered film thickness is fixed at 60-nm , the test piece 100W-60nm , 150W-60nm , using rapid thermal anneal system (RTA) is annealed, under nitrogen environment, fixed nitrogen flow rate is 3 slpm , temperature rising rate of 10 oC per second, and change to a different heating treatment temperatures were 90 seconds at 700 oC , 90 seconds at 800 oC , 60 seconds at 900 oC , and waits for the chamber to cool to room temperature before removing the test piece, using the transmission line model (TLM) structure, measuring the specific contact resistance (ρc) and the observed current value is changed in voltage relationship.

Then observe the heat treatment conditions for 90 seconds at 700 oC , 90 seconds at 800 oC , 60 seconds at 900 oC on the n-type gallium nitride (n-GaN) , on the relationship between the current and voltage whether the ohmic contact characteristics, the measured results were shown in figure 1 and figure 2 , it is seen from the figure a titanium nitride (TiN) sputtering conditions regardless 100W-60nm or 150W-60nm , after 60 seconds at 900 oC heat treatment, on the relationship between current and voltage for resistivity contact characteristics, and characteristics of the specific contact resistance values (ρc) were 1.44E-2 (Ω-cm2) , 1.58E-2 (Ω-cm2) , when the heat treatment temperature was raised from room temperature to 60 seconds at 900 oC , and wherein the specific contact resistance values (ρc) were reduced.

Corollary possible reasons are : after the heat treatment condition for 60 seconds at 900 oC , which greatly enhancing carrier concentration, as shown in table 1 and table 2 . According to the literature : the distance between the lower edge of the conduction band and Fermi level can be the carrier concentration of the logarithmic function, when the carrier concentration increases, the Fermi level moves closer to the conduction band, thus making the Schottky barrier height (SBH) decreased, thus causing the reverse saturation current (Is) is increased for enhancing ohmic contact characteristics of great help.

CONCLUSION

In this study, titanium nitride (TiN) films were deposited onto n-type gallium nitride (n-GaN) test pieces to investigate the ohmic contact characteristics. Although the linear ohmic contact characteristics of the TiN films were unobtainable by direct deposition onto the n-GaN test piece, the Schottky barrier height (SBH) between the TiN films and n-GaN test pieces was effectively reduced through rapid thermal annealing (RTA) , improving the TiN film ohmic contact characteristics successfully.

After RTA at 800 °C for 90 seconds in a nitrogen-based environment, the SBH of the TiN films was reduced from 0.513 eV to 0.454 eV exhibiting substantial improvement. In addition, after RTA at 900 °C for 60 seconds in a nitrogen-based environment, the specific contact resistance (ρc) of the transmission line model decreased to 1.44E-2 (Ω-cm2) ; thus, the TiN films are a feasible option for use in optoelectronics and semiconductor components in the future.

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