Nanoscale imaging of reduced forward bias at V-defects in green-emitting nitride LEDs

Technical Summary: Nanoscale Imaging of Reduced Forward Bias at V-defects in Green-Emitting Nitride LEDs

Problem Statement
While III-nitride light-emitting diodes (LEDs) have achieved record wall-plug efficiencies (WPE) exceeding 90% in the blue spectrum, long-wavelength (green and yellow) devices suffer from a significant efficiency drop known as the "green gap." Recent breakthroughs have shown that incorporating high densities of V-defects (pyramidal structures emerging from threading dislocations) into green-emitting LEDs can restore WPE to record levels. This improvement is largely attributed to enhanced electrical efficiency (EE), defined as the ratio of photon energy to the input electrical energy ($h\nu/qV_F$). Theoretical models propose that this EE enhancement results from a reduction in the forward bias ($V_F$) required for charge injection. This reduction is hypothesized to occur because V-defects expose semi-polar $\{10\bar{1}1\}$ facets with thinned quantum wells and lower Indium content, thereby lowering internal potential barriers. However, prior to this work, there was no direct experimental evidence confirming this charge injection mechanism at the nanoscale, as existing techniques lacked the spatial resolution to isolate V-defect properties from the surrounding material.

Methodology
To address this gap, the authors utilized a Scanning Tunneling Luminescence Microscope (STLM) to perform local optoelectronic measurements on a commercial green-emitting InGaN/GaN multi-quantum well (MQW) heterostructure.

1. Sample Preparation: A 60 nm p-GaN capping layer was removed via atomic layer etching (ALE) to expose the V-defect rims while leaving the $\{10\bar{1}1\}$ facets covered by remnant p-GaN. This exposed the V-defect rims at the surface.
2. Local Injection: The STLM tip was used as a local hole injector (positive bias, $V_{TIP} > 0$) to create a local Schottky contact on the exposed heterostructure. The device was configured as a two-terminal system where the tip acted as the p-contact and the macroscopic n-type neutral zone served as the n-contact.
3. Measurement Protocol:
   • Current Mapping: The team mapped the injection current ($I_{TIP}$) and the differential current ($\Delta I = I_n - I_p$) to confirm that injected holes traverse the MQW heterostructure to the n-contact rather than leaking through the surface.
   • Local I-V Characterization: The tip was paused at predefined grid points to measure local current-voltage characteristics. The local forward bias ($V_F$) was defined as the tip voltage required to achieve a specific injection current (e.g., 10 $\mu$A).
   • Spectroscopy: Local electroluminescence (EL) spectra were collected at specific tip positions to analyze photon energy shifts.

Key Results

1. Reduced Forward Bias at V-Defect Rims: Statistical analysis of local $V_F$ maps revealed a systematic reduction in the forward bias required for current injection at V-defect rims compared to the flat (0001)-plane surfaces. The measured difference ($\Delta V_F$) was approximately -1 V, independent of the specific current set point chosen.
2. Current Path Confirmation: Differential current measurements confirmed that over 99.9% of the tip-injected current flows through the MQW heterostructure to the n-contact, validating that the observed electroluminescence originates from the active region.
3. Electroluminescence Blue Shifts: Local EL spectra collected at V-defect rims exhibited small but systematic blue shifts of approximately 10 meV relative to the (0001)-plane emission.
4. Correlation with Macroscopic Behavior: The observed 1 V local reduction in forward bias aligns closely with the macroscopic forward bias reduction observed in high-efficiency green LEDs containing V-defects.

Significance and Claims
The paper claims to provide the first direct experimental evidence supporting the charge injection hypothesis for V-defect-enhanced LED efficiency. The key conclusions are:

• Mechanism Verification: The ~1 V reduction in local forward bias at V-defect rims confirms that these defects act as preferential pathways for charge injection by lowering the internal potential barriers, thereby increasing electrical efficiency (EE).
• Transport Dynamics: The small magnitude of the observed blue shift (10 meV) indicates that the majority of holes injected at the V-defect rims are laterally transported to the (0001)-plane quantum wells before radiative recombination occurs. This supports models suggesting that recombination primarily happens in the standard quantum wells, but injection is facilitated by the V-defect facets.
• Methodological Advance: The study demonstrates the unique capability of STLM to simultaneously perform local electrical and optical spectroscopy at the nanoscale (<10 nm resolution), bridging the gap between macroscopic device performance and microscopic physical mechanisms.

The authors conclude that these findings unambiguously confirm that V-defects improve the wall-plug efficiency of long-wavelength nitride LEDs by reducing the forward bias necessary for charge injection, a mechanism driven by the specific geometry and composition of the V-defect facets.