Electro-Optic Modulator Using Accumulation Space Charge Regions
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Solution Overview
Problem
Electro-optic modulators using cadmium telluride or cadmium zinc telluride crystals face issues with space charge induced polarization effects due to deep level traps, leading to reduced modulation efficiency and optical leakage, especially at high photon fluxes, which are not effectively addressed by existing methods.
Innovation Solution
The use of high-quality cadmium zinc telluride single crystals with low deep level trap densities and metal electrodes with work functions compatible with the semiconductor's surface states to create accumulation space charge regions, allowing for efficient charge carrier transport and minimizing built-in potential opposing the applied electric field.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If metal electrodes are applied on semiconductor crystal surfaces, then electrical contact is established for electro-optic modulation, but space charge-induced polarization effects occur due to incompatible work functions, reducing modulation performance
Solution Approach 1:
The patent changes the work function parameter of the metal electrodes by selecting specific metals (aluminum, silver, gold) whose work functions are lower than the electron affinity of CdTe. This parameter change prevents the formation of depletion regions and space charge accumulation at the metal-semiconductor interface, thereby eliminating the harmful polarization effects while maintaining electrical contact for electro-optic modulation
Solution Approach 2:
The patent converts the potentially harmful space charge effects into a beneficial configuration by intentionally designing the metal-semiconductor contact to form accumulation regions instead of depletion regions. The accumulation of majority carriers at the interface creates a favorable electric field distribution that enhances charge carrier transport and eliminates the detrimental polarization effects that would otherwise occur
2Power
If deep level traps are present in the crystal bulk, then charge carrier generation occurs, but prolonged birefringence decay times result due to slow de-trapping from deep levels
Solution Approach 1:
The patent extracts or removes the harmful deep level traps from the crystal bulk through careful crystal growth techniques and material selection. By eliminating these deep traps, the patent prevents the formation of prolonged space charge fields that would extend birefringence decay times, while still maintaining the necessary charge carrier generation for electro-optic modulation through shallow level processes
Solution Approach 2:
The patent changes the energy level parameter of trap states in the semiconductor crystal by selecting materials and growth conditions that minimize deep level defects. This parameter change ensures that charge carrier de-trapping occurs rapidly rather than being prolonged by deep level occupancy, thereby reducing birefringence decay times to nanosecond scales
3Illumination intensity
If high optical flux is applied for modulation, then amplitude modulation efficiency increases, but space charge field counter balances the applied electric field, causing field collapse and loss of birefringence
Solution Approach 1:
The patent converts the harmful space charge field that would normally counter balance the applied electric field into a beneficial accumulation field. By designing the metal-semiconductor contacts to form accumulation regions, the photogenerated carriers are accumulated at the interfaces rather than creating opposing space charge fields in the bulk, allowing high optical flux to be applied without field collapse
Solution Approach 2:
The patent changes the charge distribution parameter at the metal-semiconductor interfaces by controlling the work function mismatch. This parameter change ensures that space charge regions are of the accumulation type rather than depletion type, fundamentally altering how the crystal responds to high optical flux and preventing the counter-balancing effect that leads to field collapse
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances electro-optic modulation by reducing optical leakage and achieving almost instantaneous extinction ratio changes, with a transition of at least 90% of the extinction coefficient difference within 1 nanosecond, thereby improving the overall performance of the modulator.
Implementation Method 1
Cadmium telluride (CdTe) single crystals and Cadmium Zinc Telluride (CZT) single crystals exhibit high electro-optic coefficients... In these crystals, the electro-optic effect is realized either by amplitude modulation or by frequency modulation
Implementation Method 2
Semiconductors or semi-insulators such as CdTe, CZT and Gallium Arsenide (GaAs) absorb photon energies in the near infrared (NIR) frequency ranges close to the band edge region, and generate photon-induced charge carriers in the crystal bulk
Data Source
AI summary
An electro-optic modulator includes a doped semiconductor crystal having a crystallographic surface having an amplitude modulation orientation, a first metal electrode located on a first surface of the doped semiconductor crystal, a second metal electrode located on a second surface of the doped semiconductor crystal, and accumulation space charge regions located within surface regions of the doped semiconductor crystal that are proximal to the first metal electrode and the second metal electrode and including excess charge carriers of a same type as majority charge carriers of the doped semiconductor crystal.