Fabrication and characteristics of n- / p- type ZnO thin films and p-n homo-junctions using an inductively coupled RF co-sputtering method

Abstract

본 연구는 n- and p- type ZnO 박막의 제작과 특성, 또한 n- and p- type ZnO 박막을 이용하여 Light Emitting Diode (LED) 에 응용 가능한 p-n homo-junction 대한 실험적 연구를 수행한 것이다. 첫번째로 아무 것도 첨가되지 않은 순수한 n-type ZnO의 온도에 따른 hall concentration, 광학적/구조적 특성등과 같은 것을 조사하기 위하여 n-type ZnO 박막을 제작하였다. 실온을 포함한 여러온도에서 reactive co-sputtering method with an external RF source를 이용하여 제작한 n-type ZnO 박막은 hexagonal columnar structure을 가지고 있다. 보다 향상된 박막을 얻기 위하여 trichloroethylene (TCE), acetone, methanol과 deionized (DI) water에 기판을 넣어 열판에 순차적으로 가열한 다음 챔버에 넣기 전에 질소를 불어주었다. 모든 샘플들은 증착하기 전 기판에 흡착되어져 있는 산소를 제거하기 위하여 600 ℃에서 전처리를 해주었다. 챔버의 기본 베이스 압력은 5 × 10^(-7) torr이고, 증착압력은 5 × 10^(-3) torr이었다. 타겟에 가해지는 rf 파워를 변화시켜 가며 실험을 하였으며, 특히 600 ℃에서 증착된 n-type ZnO 박막은 PL 특성이 우수하게 나타내었다. α-step으로 측정된 샘플들의 평균적인 두께는 약 200 ~ 400 nm이다. 두번째로, 우수한 특성의 p-type ZnO 박막을 제작하기 위하여 p-type의 dopant로 질소를 선택하였는데, 일반적으로는 ZnO 박막에 질소를 주입하는 것이 매우 어려운 것으로 알려져 있었다. 이에 우리는 pre-activated nitrogen plasma를 이용하여 소위, nitrogen diffusion mechanism 방식을 이용하여 우수한 특성을 가지는 p-type ZnO 박막을 얻어낼 수 있었다. 보다 자세히 말하자면 일단 질소의 주입박막으로 WN박막을 형성하였으며, 이후 ZnO 박막 제작 후 온도를 순차적으로 올려 자연스렵게 질소가 ZnO 박막 내부로 들어갈 수 있게 하였다. 이렇게 질소가 doping된 p-type ZnO 박막은 상온에서 Hall concentration 값이 3.69 × 10^(18) cm^(-3)이고 mobility가 2.35 ㎠/Vs 그리고 비저항이 10 Ωcm 이였다. 질소를 dopant로 사용하여 또다른 p-type ZnO 박막을 제작하였다. 링 모양의 석영관에 plasma가 주입될 수 있는 구멍을 낸 후 외부 rf bias가 인가될 수 있게 기본 챔버를 수정하여 Al과 ZnO 타겟을 동시에 증착하는 방법인 소위, pre-activated nitrogen (N) plasma sources with an inductively coupled dual target co-sputtering system을 도입하였다. 이러한 증착방법은 self-bias 효과를 최대한 줄일 수 있으며, 두 개의 rf 파워를 사용하여 질소 atom의 밀도를 종전의 장비들 보다 증가시킬 수 있었다. 이렇게 AlN가 doping된 ZnO 박막은 온도에 따라 극명하게 두 가지로 나뉘어 졌는데 상온에서 증착된 ZnO:AlN 박막은 hole concentration이 4 × 10^(18) cm^(-3) 이고 mobility가 2.31 ㎠ v-1 s-1 인 n-type ZnO가 되었으며, 600 ℃에서 증착된 ZnO:AlN 박막은 hole concentration이 3 × 1018 cm^(-3) 이고 mobility가 154 ㎠ V^(-1) s^(-1) 인 우수한 전기적 특성을 가지는 p-type ZnO 박막을 얻어낼 수 있었다. 마지막으로 이러한 일련의 n- and p- type ZnO 박막을 이용하여 향후 LED에 응용할 수 있는 homojunction을 제작하였는데 Al이 doping된 n-type ZnO와 intrinsic ZnO, 그리고 AlN가 doping 된 p-type ZnO을 이용하여 turn on voltage가 약 3 V이고 breakdown voltage가 약 6.5 V인 homojunction을 제작하였다; This doctoral dissertation describes an experimental study on fabrication and characteristics of n- and p- type ZnO thin films, and p-n homo-junction using n- and p-type ZnO thin films of its application for the unique Light Emitting Diode (LED) devices. At first, we have fabricated undoped n-type ZnO thin films in order to investigate the properties of undoped n-type ZnO thin film such like hall concentration, optical properties, and structural properties as a function of growth temperature. The unique n-type ZnO thin films which had a hexagonal columnar structure were synthesized by using a reactive co-sputtering method integrated with an external RF source at various temperatures including room temperature (RT). In order to prepare the high quality of ZnO thin films, the substrates were prudently cleaned on a heating plate with a sequence of trichloroethylene (TCE), acetone, methanol and deionized (DI) water, and finally dried up by nitrogen blowing before being put into the co-sputtering chamber. Prior to starting the process of deposition of ZnO thin film, substrates were pre-heated at 600 ℃ in order to vaporize the oxygen on the substrate and the base pressure of the growth chamber was about 5 × 10^(-7) torr. The working pressure during the deposition process is 5 × 10^(-3) torr. The RF power for the target was fixed as 90 W, and the RF power for gas mixtures was varied from 60 W to 120 W with a step of 10 W. The latter RF power was induced to make the mixture gases be much more activated. And the temperature of substrate was also increased from room temperature to 600 ℃. Finally, we have synthesized the outstanding n-type ZnO thin films that had a hexagonal columnar morphology, improving the optical properties of ZnO thin films. Especially, an extreme improvement was observed in the PL properties of as-grown ZnO thin films at 600℃. The obtained results were characterized with field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) using a DMAX 2000 system with Cu Кα radiation (λ = 15.4 nm), X-ray photoemission spectroscopy (XPS), the photoluminescence property (PL; He-Cd laser 325 nm, as the excitation source), and 4-probe system for I-V measurement. Hall-effect measurements were carried out using the van der Pauw configuration (ACCENT HL 5500 Hall System) at RT. Depth profiles of the films were investigated by Auger spectroscopy. The PL properties of the films were measured using an ACCENT RPM2000 spectrophotometer with a 266-nm laser. The thickness of ZnO film that had been characterized by α-step was about 200 ~ 400 nm. Secondly, in order to make high quality p-type ZnO films, we have used the nitrogen (N) gas as a role of majority dopant. However, it is well known that it is quite difficult to make nitrogen doped p-type ZnO films, since the nitrogen-doped ZnO films are naturally unstable. Therefore, we have developed two kinds of growth technique; one is the nitrogen diffusion mechanism process and the other is the pre-activated nitrogen plasma method. For giving a full detail of the first process, called ‘the nitrogen diffusion mechanism’, the nitrogen atoms from a pre-grown tungsten-nitride (WN) was activated and diffused into a pure ZnO film during an in situ post-thermal annealing process. This process made high quality nitrogen doped p-type ZnO films on Si substrates be realized. The nitrogen atoms of WN film were easily diffused into the undoped ZnO films by partially substituting for oxygen sites of the pure ZnO film. The N doped ZnO film exhibited reproducible electrical properties including a Hall concentration of 3.69 × 10^(18) cm^(-3), a mobility of 1.35 ㎠/Vs, and a resistivity of 10 Ωcm at room temperature, along with corresponding structural results. The second process, pre-activated nitrogen (N) plasma sources produced with an inductively coupled dual target co-sputtering system led to efficient synthesis of wide band gap aluminum-nitride co-doped p-type ZnO:AlN. The AlN:ZnO films were prepared on sapphire and silicon (Si) substrates by means of separate ZnO (99.999 % purity) and Al (99.999 % purity) targets using a dual target sputtering system at RT and 600 oC. N plasma was generated by means of additional RF power applied using a ring-shaped quartz tube located inside the chamber. Additional external RF bias was applied during the sputtering process through the ring-shaped quartz tube in order to concentrate and supply a high density of N plasma around the sample for efficient growth of the AlN:ZnO films. This growth technique can also reduce the self-bias effect on the sample and increase the atomic N concentration by using two RF power sources, compared to a conventional RF reactive sputtering method using a single RF source. The base pressure in the deposition chamber was 5 × 10^(-7) torr, and the working pressure was 5 × 10^(-3) torr with Ar mixed with N and O₂ gas at various flow rates. All films were deposited for 30 mins at a sputtering power of 90 W, 90 W, and 80 W for ZnO, Al, and the external bias, respectively. Using an external power of 80 W, conversion of the conduction behavior of as-grown films was easily controlled by adjusting the growth temperature, without the need for a post-annealing process. While the as-grown films exhibited n-type conduction with a hole concentration of 4 × 1018 cm^(-3) and a mobility of 2.31 ㎠ v-1 s-1 at RT, the films revealed acceptable p-type conduction with a hole concentration of about 3 × 10^(18) cm^(-3) and a mobility of 154 ㎠ V-1 s-1 at 600 oC. The AlN co-doped ZnO films demonstrated p-and n-type conductivity with good electrical and optical properties simply by adjusting the growth temperature of the fabrication process. Thirdly, we investigated the properties of two kinds of p-n homojunctions formed by N-doped p-type ZnO and AlN doped p-type ZnO based on Al-doped n-type ZnO films. The former diode displayed good I-V characteristics with a turn-on voltage of about 3 V, demonstrated that our p-type ZnO growth process can simply form p-type ZnO:AlN. The latter one also showed gorgeous I-Vcharacteristics with a turn-on voltage of about 3.5 V. In conclusion, as-grown AlN:ZnO p-n homojunctions, which are produced without high temperature annealing, are belonging significant candidates for the applications in various devices. Our unique ZnO thin films with prominently structural and optical properties were easily synthesized by using a double RF co-sputtering method that would realize to fabricate the p-n homo junction by in-situ method with N2, Al-N2, P and etc, further.