P-cladding Layer Research Articles

Optimum Design of InGaN Blue Laser Diodes with Indium-Tin-Oxide and Dielectric Cladding Layers.

The efficiency of current GaN-based blue laser diodes (LDs) is limited by the high resistance of a thick p-AlGaN cladding layer. To reduce the operation voltage of InGaN blue LDs, we investigated optimum LD structures with an indium tin oxide (ITO) partial cladding layer using numerical simulations of LD device characteristics such as laser power, forward voltage, and wall-plug efficiency (WPE). The wall-plug efficiency of the optimized structure with the ITO layer was found to increase by more than 20% relative to the WPE of conventional LD structures. In the optimum design, the thickness of the p-AlGaN layer decreased from 700 to 150 nm, resulting in a significantly reduced operation voltage and, hence, increased WPE. In addition, we have proposed a new type of GaN-based blue LD structure with a dielectric partial cladding layer to further reduce the optical absorption of a lasing mode. The p-cladding layer of the proposed structure consisted of SiO2, ITO, and p-AlGaN layers. In the optimized structure, the total thickness of the ITO and p-AlGaN layers was less than 100 nm, leading to significantly improved slope efficiency and operation voltage. The WPE of the optimized structure was increased relatively by 25% compared to the WPE of conventional GaN-based LD structures with a p-AlGaN cladding layer. The investigated LD structures employing the ITO and SiO2 cladding layers are expected to significantly enhance the WPE of high-power GaN-based blue LDs.

Performance improvement of 1.3 μm InAlGaAs MQW modulators grown by MOVPE using C-doped InAl(Ga)As cladding layers

We propose a novel epitaxial layer structure to prevent Zn from diffusing into a multiple quantum well (MQW) layer of an electro-absorption modulator (EAM). Zn is practically employed as a p-type dopant in an InP material system, but there is a technical issue of unintentional Zn diffusion. In this work, we introduce a novel C-doped p-type layer between a Zn-doped p-cladding layer of InP and MQW to both reduce Zn diffusion and accurately define a p-i-n junction interface. The Zn concentrations in the MQW for each device are compared by measuring secondary ion mass spectrometry (SIMS). To clarify how the reduction of diffused Zn affects the EAM characteristics, we fabricate the EAMs with each epitaxial structure and compare their extinction ratio characteristics. The evaluation of extinction ratio per EAM capacitance successfully demonstrates that employing the C-doped layer effectively improves the EAM characteristics.

High-Power and High-Efficiency 221 nm AlGaN Far Ultraviolet Laser Diodes

The optical features of far ultraviolet laser diodes (UV LDs) with peak wavelength emission of 221 nm have been numerically analyzed. Global research teams are developing aluminum gallium nitride (AlGaN)-based farUV LDs on Sapphire and AlN substrates as an alternative to Mercury lamps for air-water purification, polymer curing, and bio-medical devices. In this study, the light output power, internal quantum efficiency, stimulated recombination rate curve, and optical gain curve of the compositionally graded p-cladding layer (p-CL) were studied and show significant improvements. Therefore, the optimized structure can reduce the overflow of electrons and increase the injection of holes. This approach proves to be an efficient method for enhancing farUV LDs’ overall performance when compared to the reference structure.

III-Arsenide separate confinement heterostructure infrared laser diodes with engineered the cladding layers

In this work, III-Arsenide laser diodes (LDs) show better optoelectronic properties when the aluminum (Al) concentration in the cladding layer is increased up to 95 % in a separate confinement heterostructure (SCH). These laser diodes show their peak gain the infrared lasing wavelength of ∼840 nm. Enhancing the Al in the n- and p-cladding layers, both separately and simultaneously in LDs, shows that the optoelectronic properties are reasonably improved. The output light power has been increased to 50 mW at a current density of 13 × 104A/cm2, with 95 % aluminum in both the n- and p-claddings. The modal field intensity has increased by 5 %, and the optical confinement factor (OCF) has improved from 0.0102 to 0.0105 in the proposed device. Consequently, the engineered cladding design, incorporating varied aluminum concentrations, is recognized as pivotal for optimizing the performance of arsenide laser diodes.

Impact of unintentionally formed compositionally graded layer on carrier injection efficiency in AlGaN-based deep-ultraviolet laser diodes

Increasing the injection efficiency, a critical factor constraining the reduction in threshold current in AlGaN-based deep-ultraviolet laser diodes, represents one of the paramount remaining technical challenges. In this study, the impact of compositionally graded layers that were unintentionally formed at the interface between the p-cladding and the core layer on carrier injection efficiency was analyzed. Experimental evaluations using laser diodes have shown that the elimination of an unintentionally formed layer increases the injection efficiency above the threshold current, from the conventional 3% to 13%. It has been postulated that the electron overflow toward the p-side exerts a substantial deleterious effect on the injection efficiency. An improvement in this aspect is achieved by increasing the electron-blocking capability due to the improved interface abruptness between the p-cladding layer and the core layer. The lasing threshold was strongly reduced, and characteristic temperature increased from 76 to 107 K for the improved devices.

Demonstrating the electron blocking effect of AlGaN/GaN superlattice cladding layers in GaN-based laser diodes

Electron leakage currents seriously limit the power conversion efficiencies (PCEs) of gallium nitride (GaN)-based laser diodes (LDs). To minimize the leakage currents, electron blocking layers are generally applied in the p-type region. However, few works have discussed the electron blocking effect of a p-cladding layer, which is found to be critical in suppressing the leakage currents of an LD. In this work, we compare the blocking performance of uniform AlGaN p-cladding layers and AlGaN/GaN superlattice (SL) p-cladding layers with the same average Al component respectively. Both light-emitting diodes (LEDs) and LDs with the same epitaxy structures are characterized by light–current (L–I) and current–voltage (I–V) measurements. The latest analytical model of leakage currents is applied to fit the L–I curves of LEDs, where smaller leakage coefficients are observed in the SL structures compared with the uniform-layer structures. Eighty LDs with varying ridge widths are studied by comparing the threshold current densities, slope efficiencies, and PCEs. The SL-based p-cladding layer shows statistically significant advantages over a uniform AlGaN layer. The blocking effects of both scattering- and bound-state electrons in SLs are investigated theoretically. Repetitive reflection and thermal relaxation are responsible for the blocking effect of scattering-state electrons. Simulation results indicate that the tunneling effect of bound-state electrons through a miniband mechanism is insignificant at a large injection level due to a negative differential conductivity by the Esaki–Tsu effect. We demonstrate a better electron blocking performance of p-cladding layers based on SLs than uniform AlGaN layers in GaN-based LDs.

Graded Cladding Layers in Infrared III-Arsenide Separate Confinement Heterostructure Laser Diodes

Graded aluminum composition in AlGaAs n-cladding and p-cladding layers have been shown to exhibit improved optoelectronic performance in the single quantum well separate confinement heterostructure laser diode (SQW SCH LD) in the infrared region. The output power of the laser diode is increased up to 45 mW by using graded aluminum composition in n-/p- cladding layers. In addition, the lasing threshold current density is also lowered which results in a significant optical gain. The stimulated emission rate also improves due to improved electron-hole recombination and, thus, resulting in improved lasing.

Continuous wave room temperature operation of the 2 μ m GaSb-based photonic crystal surface emitting diode lasers

Continuous wave room temperature operation of 2 μm GaSb-based photonic-crystal surface-emitting diode lasers has been realized. The deep etched square mesa devices showed threshold current densities of 500 A/cm2 at 20 °C. The epi-side down mounted lasers generated above 10 mW of output power in the continuous wave regime and tens of milliwatts in pulses from the 200 × 200 μm2 aperture. The breakthrough in the device performance parameters was achieved thanks to a highly homogeneous air-pocket retaining epitaxial regrowth process optimized for a specifically designed antimonide diode laser heterostructure. The nanofabrication method utilizing low temperature atomic hydrogen surface cleaning yielded low disorder square lattice of droplet-shaped voids covered by uniform p-cladding layer. The laser emission spectrum as well as near/far field patterns demonstrated peculiar features presumably linked to deformation of the void shape during regrowth and formation of the array of filaments.

Improvement of the optoelectronic characteristics in deep-ultraviolet laser diodes with tapered p-cladding layer and triangular electron blocking layer

Improvement of the optoelectronic characteristics in deep-ultraviolet laser diodes with tapered p-cladding layer and triangular electron blocking layer

Effects of the Stepped-Doped Lower Waveguide and a Doped p-Cladding Layer on AlGaN-Based Deep-Ultraviolet Laser Diodes

Effects of the Stepped-Doped Lower Waveguide and a Doped p-Cladding Layer on AlGaN-Based Deep-Ultraviolet Laser Diodes

Reduced threshold current density of GaN-based green laser diode by applying polarization doping p-cladding layer

Absorption induced by activated magnesium (Mg) in a p-type layer contributes considerable optical internal loss in GaN-based laser diodes (LDs). An LD structure with a distributed polarization doping (DPD) p-cladding layer (CL) without intentional Mg doping was designed and fabricated. The influence of the anti-waveguide structure on optical confinement was studied by optical simulation. The threshold current density, slope efficiency of LDs with DPD p-CL, and Mg-doped CL, respectively, were compared. It was found that LDs with DPD p-CL showed lower threshold current density but reduced slope efficiency, which were caused by decreasing internal loss and hole injection, respectively.

Light-emitting diodes with AlN polarization-induced buried tunnel junctions: A second look

Interband Zener tunneling of electrons has been recently used in III-nitride semiconductor based light emitters to efficiently inject holes into p-cladding layers. Zener tunneling probabilities can be significantly enhanced if crystal symmetry-induced internal polarization fields assist the dopant-induced built-in electric fields of tunnel junctions because of the large reduction of the tunneling distance. In a metal-polar buried tunnel junction geometry, such electric field alignment needs an AlN interlayer at the tunnel junction. Because AlN is a larger bandgap semiconductor than GaN, it is not clear a priori if the net tunneling probability is reduced or enhanced compared to a homojunction. By combining theoretical modeling with experimental blue light emitting diodes, we find that the large tunneling enhancement due to the polarization field and band realignment overcome the reduction in tunneling due to the larger bandgap of AlN. Compared to a homojunction tunnel-junction, the inclusion of AlN in the tunnel junction is found to lower the turn-on and operating voltages and increase the wall-plug efficiency. This proves that polarization-induced AlN tunnel junctions are superior to homojunctions at low injection currents, resulting in higher optical emission intensity and superior uniformity.

High-power hybrid GaN-based green laser diodes with ITO cladding layer

Green laser diodes (LDs) still perform worst among the visible and near-infrared spectrum range, which is called the “green gap.” Poor performance of green LDs is mainly related to the p-type AlGaN cladding layer, which on one hand imposes large thermal budget on InGaN quantum wells (QWs) during epitaxial growth, and on the other hand has poor electrical property especially when low growth temperature has to be used. We demonstrate in this work that a hybrid LD structure with an indium tin oxide (ITO) p-cladding layer can achieve threshold current density as low as 1.6 kA / cm 2 , which is only one third of that of the conventional LD structure. The improvement is attributed to two benefits that are enabled by the ITO cladding layer. One is the reduced thermal budget imposed on QWs by reducing p-AlGaN layer thickness, and the other is the increasing hole concentration since a low Al content p-AlGaN cladding layer can be used in hybrid LD structures. Moreover, the slope efficiency is increased by 25% and the operation voltage is reduced by 0.6 V for hybrid green LDs. As a result, a 400 mW high-power green LD has been obtained. These results indicate that a hybrid LD structure can pave the way toward high-performance green LDs.

Proposal of novel structure for wide wavelength tuning in distributed Bragg reflector laser diode with single grating mirror

We report a novel structure that is capable of wide wavelength tuning in the distributed Bragg reflector laser diode (DBR-LD) with a single grating mirror. This device's DBR section has two tuning elements, plasma, and heater tunings, which are implemented simultaneously on the top of a single waveguide by using an in-between dielectric layer. For the proposed structure, a three-dimensional thermal simulation was conducted. The results showed that the temperature profile within the waveguide is highly affected by the position of heater metal and thermal conductivity of the p-cladding layer. As a result, it is important to use a uniform temperature region in the DBR section for a wide tuning range and stable single-mode operation. For a 550-μm long DBR-LD with a 250-μm long DBR section, a tuning range of 26 nm (i.e., 7 nm for plasma tuning and 19 nm for heater tuning); an SMSR of more than 45 dB; and a peak power variation of less than ± 2.5 dB were obtained. From the comparisons of two DBR-LDs with only one tuning element, we confirmed that using the dielectric layer is a very effective way of achieving a wide tuning under the independent tuning operation.

Light output performance of red AlGaInP-based light emitting diodes with different chip geometries and structures.

We investigated the optical and electrical properties of red AlGaInP light-emitting diodes (LEDs) as functions of chip size, p-cladding layer thickness, and the number of multi-quantum wells (MQWs). External quantum efficiency (EQE) decreased with decreasing chip size. The ideality factor gradually increased from 1.47 to 1.95 as the chip size decreased from 350 μm to 15 μm. This indicates that the smaller LEDs experienced larger carrier loss due to Shockley-Read-Hall nonradiative recombination at sidewall defects. S parameter, defined as ∂lnL/∂lnI, increased with decreasing chip size. Simulations and experimental results showed that smaller LEDs with 5 pairs of MQWs had over 30% higher IQE at 5 A/cm2 than the LED with 20 pairs of MQWs. These results show that the optimization of the number of QWs is needed to obtain maximum EQE of micro-LEDs.

Influence of the thickness change of the wave-guide layers on the threshold current of GaAs-based laser diode

The paper mainly deals with theoretical investigations of the effect of the thickness change of the waveguide layers on the threshold current. It is analyzed according to the result of a numerical simulation that asks how does the shift of the active region position affect the threshold current for a single quantum well (SQW) and double quantum well (DQW) laser diode (LD) with a relatively narrow waveguide. It is found that the variation trend of threshold current and optimum position of QW are different in SQW and DQW LD with 0.2 μm-thick waveguide, which may be due to the higher variation rate of optical loss in DQW LD with the shift of the active region. It is also found that in terms of either SQW or DQW LD, the variation tendency of the threshold current with a different loss coefficient of the p-cladding layer makes little difference for the relatively narrow waveguide LD. Moreover, the variation trend of the threshold current and the optimum position of QW is almost the same in SQW and DQW LD with 0.8 μm-thick waveguide, because the optical loss is small enough and the threshold current is dominated by the optical confinement factor (OCF) in QW.

Influence of barrier height and p-cladding layer on electroluminescent performance of AlGaN deep ultraviolet light-emitting diodes

The electroluminescent (EL) properties of AlGaN deep ultraviolet light-emitting diodes (DUV LEDs) are improved by increasing the Al composition of the barriers in the active region and decreasing the thickness of the p-cladding layer, respectively. Under 1.5 A cm−2 direct current (DC) injection, the intensity of the parasitic peak around 320 nm is 43 times more than that of the band-to-band peak at 265 nm in the LED with a lower Al-composition (55%) barrier, while the parasitic peak is suppressed efficiently in the LED with a higher Al-composition (70%) barrier. By increasing the Al composition from 55% to 70% and decreasing the thickness of the p-cladding layer from 25 to 10 nm, the light output power of the DUV LEDs improved 305 and 61 times at DC injection of 1.5 and 15 A cm−2, respectively. The band structure and the vertical conductivity were calculated. It was found that the increase of Al composition not only enhanced the barrier height for quantum wells but also decreased the barrier for hole injection. On the other hand, the thickness decrease of p-type cladding layers to some extent resulted in the improvement of both hole concentration and vertical tunneling conductivity. All of these contributed to the improvement of the EL properties of the LEDs.

Low-threshold-current yellow BeZnCdSe quantum-well ridge-waveguide laser diodes under continuous-wave room-temperature operation

Low-threshold-current yellow BeZnCdSe single-quantum-well (SQW) laser diodes (LDs) have been developed by using a ridge-waveguide structure. The top p-cladding layer was etched to suppress the leakage current that flowed laterally outside of the electrode. Ridge waveguides were covered with a SiO2 layer and planarized by chemical–mechanical polishing and reactive ion etching. Room-temperature lasing under continuous-wave condition was achieved with the laser cavity formed by cleaved waveguide facets coated with high-reflectivity dielectric films. Two types of LDs with different SQW thicknesses and Cd contents were developed and compared at various waveguide widths and lengths. Yellow LDs with sub-10 mA threshold current were obtained.

Leakage Current Analysis of GaN-Based Light-Emitting Diodes Using a Parasitic Diode Model

The leakage current of GaN-based light-emitting diodes (LEDs) was analyzed with a parasitic diode model. Arrhenius plots of forward leakage currents revealed that the threshold voltages of the main diode and the parasitic diode were 2.64 and 0.94 V, respectively. The parasitic diode, however, led to an insignificant change in the reverse leakage mechanism, e.g., a predominant reverse leakage mechanism was Poole-Frenkel conduction in the high-temperature range (>300 K) and the variable range hopping in the low-temperature range (<;300 K), as reported previously. The origin of the parasitic diode could be due to the incorporated hydrogen (H) atoms into the top LEDs layers, presumably enhancing the defect-assisted tunneling through the generated H-related deep-level states, particularly generated in the p-cladding or p-AlGaN electron blocking layer.

Design of Bottom-Emitting Metallic Nanolasers Integrated With Silicon-On-Insulator Waveguides

We propose and analyze a bottom-emitting metallic nanolaser with a cavity size smaller than one wavelength in three dimensions. We optimize the nanolaser performance by adjusting the thickness of SiN x insulator layer, pillar radius, p-cladding, and n-cladding layers. With a proper design, we can achieve the TE01 mode lasing with an extremely small effective mode volume of 0.135 (λ0/n)3 and an ultra low threshold gain of 247 cm−1. The nanolaser is integrated with a silicon-on-insulator waveguide through a bottom-connected grating coupler. By properly modifying the grating coupler period, fill factor, shift position, and grating height, a high coupling efficiency of 88.5% can be acquired.