Interplay of sidewall damage and light extraction efficiency of micro-LEDs

Abstract

This Letter describes the impact of shape on micro light-emitting diodes (µLEDs), analyzing 400 µm2 area µLEDs with various mesa shapes (circular, square, and stripes). Appropriate external quantum efficiency (EQE) can yield internal quantum efficiency (IQE) which decreases with increasing peripheral length of the mesas. However, light extraction efficiency (ηe) increased with increasing mesa periphery. We introduce analysis of Jpeak (the current at peak EQE) since it is proportional to the non-radiative recombination. Etching the sidewalls using tetramethylammonium hydroxide (TMAH) increased the peak EQE and decreased the sidewall dependency of Jpeak. Quantitatively, the TMAH etching reduced non-radiative surface recombination by a factor of four. Hence, shrinking µLEDs needs an understanding of the relationship between non-radiative recombination and ηe, where analyzing Jpeak can offer new insights.

Impact of Sidewall Conditions on Internal Quantum Efficiency and Light Extraction Efficiency of Micro‐LEDs

“…Figure a shows the measured average EQEs of 16 points of 400 µm 2 ‐ and 100 µm 2 ‐µLEDs before and after TMAH etching as a function of current density. For the 400 µm 2 µLEDs, TMAH etching increases the peak EQE by ≈10% in the low current regime, similar to our previous report, [ 21 ] whereas for the 100 µm 2 µLEDs, TMAH etching slightly decreases EQE by ≈3%. EQE was reported to decrease with shrinking µLEDs size and attributed to increased Shockley–Read–Hall (SRH) non‐radiative recombination rate at the sidewall.…”

“…The longer lifetime demonstrates that the TMAH treatment reduces the non‐radiative recombination. Since the total lifetime at room temperature is mainly dominated by the non‐radiative lifetime [ 28–29 ] and the non‐radiative recombination rate is the inverse of the lifetime, one could try to quantify the sidewall recombination [ 15,21 ] : $$\[ \begin{array}{*{20}{c}}{A\; = {A_0}\; + \frac{{{A_{\rm{s}}}\lambda l}}{S}}\end{array} \]$$where A is the non‐radiative recombination rate, A 0 is the non‐radiative recombination rate in the QW, A s is the surface recombination rate, λ is the carrier diffusion length, l is the peripheral length of device, and S is the area of the device. A s depends on the sidewall surface and should be higher without TMAH treatment.…”

“…The sample fabrication processes are similar to those presented in the previous work. [ 21 ] To investigate the effect of ICP‐RIE on the surface states of the (10‐10) GaN surface, m‐plane GaN substrate (purchased from Mitsubishi chemical) was etched by ICP‐RIE for 5 min under conditions similar to those described above.…”

“…The longer lifetime demonstrates that the TMAH treatment reduces the non-radiative recombination. Since the total lifetime at room temperature is mainly dominated by the non-radiative lifetime [28][29] and the non-radiative recombination rate is the inverse of the lifetime, one could try to quantify the sidewall recombination [15,21] :…”

Investigations of Sidewall Passivation Using the Sol-Gel Method on the Optoelectronic Performance for Blue InGaN Micro-LEDs

“…For example, Wong et al used atomic-layer deposition (ALD) for sidewall passivation, after which devices’ EQE increased from 24% to 33% for 20 × 20 µm [ 18 ]. Wet chemical treatment such as potassium hydroxide (KOH) or TMAH solution, also used for reducing the sidewall damage induced from the inductively coupled plasma (ICP) etching, can achieve less than 10% improvement for EQE [ 19 , 20 ]. However, as the size decreased to less than 10 µm, EQE sharply dropped to less than 20% [ 21 ].…”