Composition-Graded Nitride Semiconductor Layer for Green Laser Emission
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Solution Overview
Problem
Nitride semiconductor laser elements face challenges in emitting light at longer wavelengths due to increased optical confinement and threshold current, leading to a shift in emission wavelength to shorter wavelengths, which affects their application range and efficiency.
Innovation Solution
A semiconductor laser element with a composition-graded n-side semiconductor layer and an n-side barrier layer, where the composition-graded layer has decreasing band-gap energies and a specific dopant concentration, is used to enhance optical confinement and reduce threshold current, allowing for longer wavelength emission while maintaining low operating voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the laser emission wavelength is increased from blue to green range, then the application range is expanded, but the optical confinement to active layer decreases due to wavelength dispersion of refractive index
Solution Approach 1:
The patent applies local quality by creating a composition-graded layer with spatially varying composition (In content increasing from bottom to top) and varying dopant concentration (Si content decreasing from bottom to top). This gradient structure provides locally optimized properties: the higher In content regions provide better optical confinement for green wavelength, while the dopant gradient controls carrier distribution to maintain low threshold current. Each layer position has tailored properties suited to its functional requirement.
Solution Approach 2:
The patent employs parameter changes by systematically varying the composition parameters (In content) and dopant concentration (Si content) through the layer thickness. The In content increases from 0% at the bottom to 6% at the top, while Si concentration decreases from 2×10^18/cm³ to 5×10^17/cm³. These continuous parameter changes enable optimization of both optical confinement and electrical properties for green wavelength emission.
2Adaptability or versatility
If the laser emission wavelength is increased to green range, then the emission wavelength is extended, but the threshold current increases due to decreased optical confinement
Solution Approach 1:
The patent uses parameter changes by implementing a dopant concentration gradient where Si content decreases from 2×10^18/cm³ at the bottom to 5×10^17/cm³ at the top of the composition-graded layer. This gradient optimizes carrier distribution: higher dopant concentration at the bottom ensures good electrical contact and low series resistance, while lower concentration at the top reduces free carrier absorption and screening effects, thereby reducing threshold current for green wavelength emission.
Solution Approach 2:
The patent employs composite material structure by combining multiple layers with different compositions and dopant concentrations into a single n-side semiconductor layer. The composition-graded layer integrates varying In content (0-6%) and varying Si concentration, creating a composite structure that simultaneously addresses optical confinement requirements and electrical transport requirements, resulting in reduced threshold current for green emission.
3Reliability
If the current density is increased to compensate for decreased optical confinement, then the lasing can be maintained, but the emission wavelength shifts to shorter wavelength due to screening of localized state and band filling
Solution Approach 1:
The patent applies local quality through the dopant concentration gradient, where lower Si concentration (5×10^17/cm³ to 2×10^18/cm³) is positioned at the top region near the active layer to minimize screening effects and band filling, while higher concentration is at the bottom for electrical transport. This spatial differentiation allows maintaining lasing at green wavelength without significant wavelength shift.
Solution Approach 2:
The patent employs parameter changes by creating a gradient in Si dopant concentration from 2×10^18/cm³ at the bottom to 5×10^17/cm³ at the top. This parameter variation optimizes the balance between electrical conductivity (requiring high dopant) and reduced screening/band filling effects (requiring low dopant), thereby stabilizing the emission wavelength in the green range.
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
The solution effectively reduces the threshold current and voltage, enabling lasing at longer wavelengths with improved optical confinement and reduced screening of localized states, enhancing the semiconductor laser element's performance and application range.
Implementation Method 1
The composition-graded layer has band-gap energies decreasing upward, with the band-gap energy of upper side being smaller than the band-gap energy of the n-side barrier layer
Implementation Method 2
The composition-graded layer has an n-type dopant concentration in a range between greater than 5×10^17/cm³ and not greater than 2×10^18/cm³
Implementation Method 3
optical confinement to an active layer generally decreases with an increase in laser emission wavelength from blue wavelength range to green wavelength range, due to wavelength dispersion of refractive index
Data Source
AI summary
A semiconductor laser element includes an n-side semiconductor layer, an active layer, and a p-side semiconductor layer, layered upward in this order, each being made of a nitride semiconductor. The active layer includes one or more well layers, and an n-side barrier layer located lower than the one or more well layers. The n-side semiconductor layer includes a composition-graded layer located in contact with the n-side barrier layer. The composition-graded layer has a band-gap energy that decreases toward an upper side of the composition-graded layer, with a band-gap energy of the upper side being smaller than a band-gap energy of the n-side barrier layer. The composition-graded layer has an n-type dopant concentration greater than 5×1017/cm3 and less than or equal to 2×1018/cm3. The n-side barrier layer has an n-type dopant concentration greater than that of the composition-graded layer and a thickness smaller than that of the composition graded layer.