Electron Holographic Study of Semiconductor Light‐Emitting Diodes

Abstract

Semiconductor light-emitting diodes (LEDs), especially GaN-based heterostructures, are widely used in light illumination. The lack of inversion symmetry of wurtzite crystal structures and the lattice mismatch at heterointerfaces cause large polarization fields with contributions from both spontaneous polarization and piezoelectric polarization, which in turn results in obvious quantum confined stark effect. It is possible to alleviate this effect if the local electrostatic fields and band alignment induced charge redistribution can be quantitatively determined across the heterostructures. In this Concept, the applications of electron holography to investigate semiconductor LEDs are summarized. Following the off-axis electron holography scheme, the GaN-based LED heterostructures including InGaN/GaN-based quantum wells, other GaN-based quantum wells, and other forms of GaN-based LED materials are discussed, focusing on the local potential drops, polarization fields, and charge distributions. Moreover, GaAs-based LED heterostructures are briefly discussed. The in-line electron holography scheme emphasizes the capability of large area strain mapping across LED heterostructures with high spatial resolution and accuracy, which is combined with quantitative electrostatic measurements and other advanced transmission electron microscopy characterizations to provide an overall nanometer scale perspective of LED devices for further improvement in their electric and optical properties.

Study of the Polarization Effect in InAs Quantum Dots/GaAs Nanowires

“…Figure a shows the formation of NWs and QDs following the VLS and S–K growth modes, respectively. The QD/NW heterostructure would lead to a polarization effect, which can be quantitatively characterized by off-axis electron holography, a powerful electron-interference technique for quantitative characterization of phase shifts due to electrostatic or magnetic fields, which has been successfully applied to characterization of a wide range of semiconductor heterostructures. − As shown in the setup image of off-axis electron holography (Figure b), the object wave passing through the sample contains both amplitude and phase information. By analyzing the phase information, quantification of electrostatic properties, including electrostatic potentials, charge distributions, polarization fields from NWs to QDs (Figure c), and band structures (Figure d), can be realized.…”

Study of Charge Distributions and Electrical Properties in GaAs/AlGaAs Single Quantum Well/Nanowire Heterostructures

“…Recently, electron holography as a unique phase-obtaining technique has been successfully applied to the characterization of the electrostatic potentials, − electric fields, and charge distributions − across semiconductor heterostructures, owing to its high spatial resolution and sensitivity to variations in phase shifts . In the current case, through electron holographic characterization of the GaAs/AlGaAs SQW/NW heterostructure which forms a type-I band alignment (Figure c), the detailed charge distributions across the hetero-interfaces and the nanometer scale mechanism for the observed novel optoelectronic properties are clearly revealed.…”

“…While the conventional TEM can only provide objectmodulated intensity of the electron wave, both amplitude and phase information can be acquired by electron holography. 32 Because electrostatic or magnetic fields can bring extra phase shifts to the electron wave passing through the sample, obtaining the variation of phase would be critical to studying its inner electrical or magnetic information. Figure 1b shows the setup of off-axis electron holography, with the introduction of a positively charged biprism, the objective wave including both amplitude and phase information of the sample interferes with the reference wave which transmitting through the vacuum region, and their interference pattern is recorded by a charge-coupled device (CCD) camera.…”

“…While the conventional TEM can only provide object-modulated intensity of the electron wave, both amplitude and phase information can be acquired by electron holography . Because electrostatic or magnetic fields can bring extra phase shifts to the electron wave passing through the sample, obtaining the variation of phase would be critical to studying its inner electrical or magnetic information.…”

Study of structure-property relationship of semiconductor nanomaterials by off-axis electron holography

“…Electron holography is an advanced electron-interference technique offering both amplitude and phase information of the exit electron wave. Although more than 20 approaches are possible for setting up electron holography [7] , off-axis electron holography in TEM mode is widely used for easy setup and accessible image construction processes (In the following, "off-axis electron holography" is simplified as "electron holography"). This approach requires highly coherent electron beam, biased metal coated quartz fiber situated at one of the selected area electron diffraction (SAED) aperture, and charge coupled device (CCD) for digital recording.…”