Atomic layer etching process for GaN-based light emitting diodes

다국어 초록 (Multilingual Abstract)

In this study, the effect of an optimized atomic layer etching (ALE) process for removing surface damage induced by reactive ion etching (RIE) in the multiple quantum well (MQW) structure of GaN-based LED devices was investigated and compared with KOH-based wet etching. For GaN and InGaN, which are components of the MQW layer etched by a conventional RIE process, the optimized ALE process (etching per cycle (EPC) of ~2.4 Å/cycle) was applied. As a result, RIE induced etch residues were effectively removed, and the surface atomic composition of both GaN and InGaN was fully recovered similar to the reference samples. In contrast, KOH-based wet etching did not completely remove the etch residues, nor fully restore the surface atomic composition.
In particular, the optimized ALE process nearly completely eliminated the stress and defects present on the sidewalls of the MQW layer in patterned GaN structures. Moreover, even for the smallest device pattern size which exhibited the lowest photoluminescence (PL) intensity after RIE, the PL intensity became the highest after the ALE process. This indicates that ALE is especially effective for damage recovery in small sized devices with a high surface-to-volume ratio.
Therefore, in next generation GaN-based device fabrication where device size continues to scale down, ALE is expected to play a crucical role as an essential process for surface damage removal and device performance improvement, becoming more important than conventional wet etching.

국문 초록 (Abstract)

본 연구에서는 GaN 기반 LED 소자에 존재하는 MQW(Multiple Quantum Well) 구조에서의 RIE(Reactive Ion Etching) 공정에 의한 표면 손상을 제거하기 위한 최적화된 원자층 식각 (ALE, Atomic Layer Etching) 공정 효과를 KOH 용액 기반의 습식 식각과 비교하여 연구하였다. 기존 RIE 공정으로 식각된 MQW 층의 구성물질인 GaN 및 InGaN에 대해, 사이클당 약 2.4Å의 EPC (Etching Per Cycle)를 나타내는 최적화된 ALE 공정을 적용한 결과, RIE로 인해 발생된 식각 잔여물이 효과적으로 제거됨과 동시에 GaN 및 InGaN의 reference 시료와 유사한 수준으로 표면 원자 조성이 완전히 회복되는 것을 확인하였다. 반면, KOH 기반 습식 식각 결과 표면 원자 조성 및 식각 잔여물이 완전히 제거되지는 못하였다.
특히, 최적화된 ALE 공정은 패터닝된 GaN 구조의 MQW 층에서 측벽에 존재하는 응력과 결함을 거의 완전히 제거하였으며, RIE 이후 가장 낮은 PL 발광 강도를 보였던 가장 작은 패턴 크기의 소자에서도 ALE 적용 후 가장 높은 PL 강도를 나타냈다. 이는 표면 대 부피비 (surface to volume ratio)가 높은 소형 소자에서 ALE 공정이 손상 회복에 특히 효과적임을 의미한다.
따라서, 소자 크기가 지속적으로 감소하는 차세대 GaN 기반 소자 공정에서, ALE는 표면손상을 제거하고 소자 성능을 향상시키기 위한 필수 공정으로서, 기존 습식 식각보다 더 중요한 역할을 하게 될 것이다.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1 Purpose of Research 1
• Chapter 2. Research Background 4
• 2.1 Plasma etching process 4
• 2.1.1 Reactive Ion Etching (RIE) 4
• 2.1.2 Atomic Layer Etching (ALE) 6
• 2.2 Analytic tools 9
• 2.2.1 Scanning Electron Microscopy (SEM) 9
• 2.2.2 Surface Profilometer 11
• 2.2.3 X-ray Photoelectron Spectroscopy (XPS) 13
• 2.2.4 Raman Spectroscopy 15
• 2.2.5 Photoluminescene Spectroscopy (PL) 16
• 2.2.6 Transmission Electron Microscopy (TEM) 17
• Chapter 3. Experimental 18
• 3.1 ICP etch system and GaN sample 18
• 3.2 Experimental Conditions 21
• Chapter 4. Results and Discussion 25
• 4.1 ALE Characteristics of GaN and InGaN 25
• 4.1.1 ALE Characteristics depending on Adsorption and Desorption 25
• 4.1.2 ALE window and ALE synergy depnding on Bias voltages 29
• 4.2 Comparison of Etching Methods on blank GaN and InGaN 31
• 4.2.1 XPS Analysis on blank GaN and InGaN Surfaces 31
• 4.3 Comparison of Etch Damage Removal in Patterned GaN Structures 33
• 4.3.1 XPS Analysis on Patterned GaN Structures 33
• 4.3.2 Raman Analysis on Patterned GaN Structures 35
• 4.3.3 TEM Analysis on Patterned GaN Structures 37
• 4.3.4 PL Analysis on Patterned GaN Stuructures 39
• Chapter 5. Conclusions 41
• References 43
• 논문요약 46