Compact Multi-Junction VCSEL Stack for Lower Current Spreading
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Solution Overview
Problem
Existing VCSEL devices face limitations in output power due to lateral current spreading and modal instability, particularly when using multiple active regions, which also result in higher electrical resistance, self-heating, and reduced optical output power.
Innovation Solution
A compact multi-junction VCSEL structure is developed with active regions aligned at intensity peaks and tunnel junctions at troughs, reducing lateral current spread and optical loss, and incorporating a carrier confinement element like an oxide aperture layer or conductive disk to minimize current spreading.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If multiple active regions are stacked to increase output power, then optical output power is improved, but lateral current spreading increases causing modal instability
Solution Approach 1:
The device divides the active region into multiple discrete quantum well segments (first, second, and third quantum wells) separated by tunnel junctions. Each quantum well acts as an independent active region that can be selectively populated, allowing the device to achieve higher output power through multiple regions while maintaining modal stability by controlling current distribution across the segmented structure.
Solution Approach 2:
The patent applies different doping concentrations to different regions: highly doped tunnel junctions (1E19 to 1E21 atoms/cm³) for efficient carrier transport between quantum wells, and lightly doped or undoped quantum well regions for optimal optical gain. This local differentiation of material properties enables simultaneous achievement of high output power and modal stability.
2Power
If multiple active regions are stacked to increase output power, then optical output power is improved, but electrical resistance increases causing self-heating
Solution Approach 1:
The patent replaces conventional ohmic contacts with tunnel junctions that utilize quantum mechanical tunneling for carrier transport. The highly doped tunnel junctions provide low-resistance electrical pathways (electrical conductivity improvement) that reduce Joule heating, enabling the multi-region structure to achieve higher output power without excessive self-heating that would normally result from increased electrical resistance.
3Reliability
If oxide aperture layer is used to confine current, then lateral current spread is reduced, but fabrication complexity increases due to matching requirements
Solution Approach 1:
The patent extracts the current confinement function from the oxide aperture layer and implements it through the quantum well structure itself, which naturally confines carriers through its lower doping concentration and physical boundaries. This eliminates the need for precise oxide aperture fabrication and matching, reducing fabrication complexity while maintaining effective current confinement for modal stability.
Solution Approach 2:
The quantum well structure serves multiple functions simultaneously: it acts as the active region for optical gain, provides current confinement through its doping profile and physical structure, and enables wavelength selection through quantum confinement effects. This multi-functionality eliminates the need for separate oxide aperture layers, reducing fabrication complexity.
4Reliability
If oxide aperture layer is added to limit current spread, then current confinement is improved, but electrical resistance increases reducing efficiency
Solution Approach 1:
The patent replaces the oxide aperture layer (which increases electrical resistance) with highly doped tunnel junctions that provide both current confinement and low-resistance carrier transport. The tunnel junctions achieve current confinement through their doping profile and positioning between quantum wells, while their high doping concentration (1E19 to 1E21 atoms/cm³) ensures low electrical resistance, thereby maintaining high wall plug efficiency.
5Reliability
If active regions are spaced apart to reduce current spread, then modal stability is improved, but optical loss increases due to longer cavity length
Solution Approach 1:
The patent segments the active region into multiple closely-spaced quantum wells separated by thin tunnel junctions, rather than using widely-spaced active regions. This segmentation allows modal stability to be achieved through controlled current distribution across adjacent quantum wells, while the short distances between them minimize optical loss in the cavity.
Solution Approach 2:
The patent transitions from spatial separation of active regions in the vertical dimension to functional separation through doping profile differentiation in the horizontal dimension. The quantum wells are positioned close together vertically, but current confinement is achieved through lateral doping variations and tunnel junction positioning, allowing modal stability without increased cavity length and associated optical loss.
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 configuration enhances optical confinement, reduces threshold current, increases slope efficiency, and achieves higher optical output power while maintaining a compact physical size, thus overcoming the limitations of prior art VCSEL designs.
Implementation Method 1
These 'multiple active region' devices electrically couple one active region to another via a tunnel junction that is formed between adjacent active regions.
Implementation Method 2
A set of process steps is used to oxidize the majority of this layer, leaving a central portion in its original composition to form an 'aperture' for confining the beam emitted from the active region.
Implementation Method 3
At a sufficiently high bias current (referred to as the threshold current), the injected minority carriers form a population inversion in the quantum wells, producing gain.
Implementation Method 4
As light passes from a layer of one index of refraction to another, a portion of the light is reflected, creating a diffractive Bragg reflector (DBR) structure.
Data Source
AI summary
A multi-junction VCSEL is formed by as a compact structure that reduces lateral current spreading by reducing the spacing between adjacent active regions in the stack of such regions used to from the multi-junction device. At least two of the active regions within the stack are located adjacent peaks of the intensity profile of the VCSEL, with an intervening tunnel junction positioned at a trough between the two peaks. The alignment of the active regions with the peaks maximizes the generated optical power, while the alignment of the tunnel junction with the trough minimizes optical loss. The close spacing on adjacent peaks forms a compact structure (which may even include a cavity having a sub-λ optical length) that lessens the total path traveled by carriers and therefore reduces lateral current spread.