Enhancement of n-type GaN (20–21) semipolar surface morphology in photo-electrochemical undercut etching

Abstract

An ice bath photo-electrochemical (PEC) undercut etching technique to separate devices from substrates is described. Smoothly etched Si-doped ( ) GaN is produced by etching a 40 nm relaxed sacrificial layer single quantum well. This has potential for improving the active region quality of semipolar green-emitter. Removal of unetched misfit dislocations revealed an RMS surface roughness decreasing from σrms = 5.136 to 0.25 nm. In view of the development of green-light emitters, the interplay between the effects of reactant diffusion-limited etch process and defect-selective etching is demonstrated by enhancing PEC etching performance toward a smooth n-type semipolar GaN surface.

Progress in GaN Single Crystals: HVPE Growth and Doping

“…Key words: gallium nitride; hydride vapor phase epitaxy; doping; crystal growth; review Ⅲ族氮化物作为第三代半导体材料的代表, 在 光电子和微电子等领域具有重大的应用前景, 与之 相关的材料生长和器件研制受到了广泛关注, 并取 得了长足进步。相比于第一代和第二代半导体材料, 以碳化硅(SiC)、氧化锌(ZnO)、氮化镓(GaN)和氮化 铝(AlN)为主的第三代半导体材料具有更高的击穿 电场、电子饱和速率、热导率以及更宽的带隙, 更 适用于研发制造高频、大功率、抗辐射、耐腐蚀的 电子器件、光电子器件和发光器件。 相比于间接带隙半导体 SiC 以及受限于 p 型掺 杂的 ZnO, GaN 作为第三代半导体材料的代表属于 直接带隙半导体, 具有带隙宽、击穿电压高、热导 率高、 介电常数小等特性 [1] , 已广泛应用于蓝绿激光 器、射频微波器件等光电子器件和电力电子器件中, 在激光显示、5G 通信、相控阵雷达以及智能电网等 领域具有重要应用前景 [2] , 并逐渐成为第三代半导 体产业的核心支撑材料。 根据衬底材料的不同, [4] Fig. 1 Schematic diagram of GaN [4] (a) Hexagonal unit call (left) and the bond structure of GaN (right), with green balls indicating Ga atoms and blue balls indicating N atoms; (b) Polar face (left), non-polar face (middle) and one kind of semi-polar faces (right) of GaN crystal 原子, 潜在的半极性面有: {1010}, {1012}, {1013}, {1014}, {1015}, {1122}, {1124}, {2021}, {2023}, {3032}, {3130}, {2132}和 {2133}; 目前以{1010}, {1013}, {1014}, {1122}, {2021}以及{3130}为代表 的半极性面已被发现并研究 [4][5][6][7] 。 完美晶体中的原子严格按周期性规则排列, 但 生长过程中产生的缺陷不仅破坏了晶体结构的完整 性, 还会影响晶体的性质。因此研究晶体中缺陷的 产生、 相互作用以及对性能的影响对于提升 GaN 晶 体的光电性能, 提升 GaN 基器件的效率和稳定性具 有重要的意义。GaN 晶体生长过程中缺陷的产生是 不可避免的, 根据尺度和形貌的不同, 缺陷可分为 四种 [4] 。 (1)零维缺陷, 即点缺陷, 与单个原子的位置 有关, 如空位(V Ga 、V N )、间隙原子(N i 、Ga i 、间隙 杂质原子)、替代原子(N Ga 、Ga N 、替代杂质原子), 掺 杂就是通过晶体中杂质原子形成点缺陷进而影响晶 体的光电性质。不同的点缺陷作为施主、受主或等 电子杂质发挥作用, GaN 中常见的施主有 Ga 格点位 置上的 Si、Ge 以及 N 格点位置的 O、S、Se 等; GaN 中常见的受主有 Ga 格点位置上的 Mg、Ca、Zn 以 及 N 格点位置的 Fe、 C、 Si、Ge 等。(2)一维缺陷, 也 称线缺陷, 与某一个方向有关, 如位错。(3)二维缺 陷, 也称面缺陷, 与某个晶面有关, 如晶界、晶面、 堆垛层错。(4)三维缺陷, 也称体缺陷, 与体积相关, 如空洞、裂纹、凹坑 [4] 。…”

Study of surface roughness of lifted-off epitaxial lateral overgrown GaN layers for the n-DBR mirror of a III-nitride vertical-cavity surface emitting laser

“…25) With the exception of PEC etching, all previous substrate removal or thinning techniques have required a polishing step to smooth the surface morphology to sub-nanometer level, so as to prevent optical scattering loss at the surface, and in the dielectric mirror fabricated above the surface, as well as to fine tune the cavity thickness. 5,12,[26][27][28][29] Achieving precise control of cavity thickness through polishing, with good uniformity across the whole wafer, can be a challenging task. The use of PEC etching has the advantage of allowing precise control of the cavity thickness, while maintaining smooth surface morphology; 12,[26][27][28][29] however, PEC etchants such as KOH may still impose a limit on the crystal orientation of growth substrates.…”

“…5,12,[26][27][28][29] Achieving precise control of cavity thickness through polishing, with good uniformity across the whole wafer, can be a challenging task. The use of PEC etching has the advantage of allowing precise control of the cavity thickness, while maintaining smooth surface morphology; 12,[26][27][28][29] however, PEC etchants such as KOH may still impose a limit on the crystal orientation of growth substrates. In particular, with polar c-plane (0001) substrates, the flip-chip side N-polar surface morphology could degenerate under PEC treatment.…”

Damage-free substrate removal technique: wet undercut etching of semipolar 202¯1 laser structures by incorporation of un/relaxed sacrificial layer single quantum well

“…During low-temperature wet etching, type I induces the transition between the carrier-diffusion-limited and reactantdiffusion-limited etch processes within the diffusion-limited etch regime to remove unetched MDs. 17) Type II unrelaxed SL offers undercut wet etching at RT PECE and is preferable, for instance, in the short-wavelength range light emitters with reduced need to tailor AR toward longer emissions. However, the FC-LDs, with the selected Au-In solid-liquid interdiffusion (SLID) bonding technology, are used to expand the large-scale production 18) prior to the removal of the semipolar substrate to access the n-or p-GaN side.…”

“…5(c) after bonding and ice bath wet etching; the surface was characterized by atomic force microscopy to exhibit reduced roughness, which approaches the standard epitaxial roughness. 17) The SEM image shows the EELDs after Au-In SLID bonding system using a Finetech Fineplacer Lambda flip-chip bonder for the significant enhancement of yield related to mechanical stability without cracking as in Fig. 5(d).…”