Multiquantum-well Laser Diodes Research Articles

Exciton recombination in lasing contributing an opposite abrupt change of the electrical behavior near threshold between GaN- and GaAs- multi-quantum-well laser diodes

The opposite sudden change of electrical characteristics between narrow and wide bang-gap multi-quantum-well (MQW) laser diodes (LDs) in the threshold region (which is defined as a current region between two kinks of IdV/dI–I curve) shows an interesting phenomenon that the slope changes of IdV/dI–I or Vj–I curve between two adjacent regions (‘below’ and ‘in’, or ‘in’ and ‘above’ threshold region) display an approximate e-exponential relationship with the wavelengths of LDs. After comparing the exciton binding energy in different MQW LDs, and analyzing the temperature dependence of Vj–I and IdV/dI–I of GaN MQW LDs, we suggested that the fraction of exciton recombination into lasing is a reason causing the relationship of sudden changes of the electrical characteristics with wavelengths of LDs.

Effect of QW thickness and numbers on performance characteristics of deep violet InGaN MQW lasers

The performance characteristics of deep violet indium gallium nitride (InGaN) multiquantum well (MQW) laser diodes (LDs) with an emission wavelength of around 390 nm have been investigated using the integrated system engineering technical computer aided design (ISE-TCAD) software. A comparative study on the effect of quantum well (QW) thickness and number on electrical and optical performance of deep violet In 0.082 Ga 0.918 N/GaN MQW LDs have been carried out. The simulation results showed that the highest slope efficiency and external differential quantum efficiency (DQE), as well as the lowest threshold current are obtained when the number of wells is two. The different QW thickness values of 2.2, 2.5, 2.8, 3 and 3.2 nm were compared and the best results were achieved for 2.5 nm QW thickness. The radiative recombination rate decreases with increasing QW thickness because of decreasing electron and hole carrier densities in wells. By increasing QW thickness, output power decreases and threshold current increases.

Optically pumped lasing properties of InGaN/GaN stripe multiquantum wells with ridge cavity structure on patterned (001) Si substrates

We demonstrated lasing action and investigated the optical properties of multiquantum-well (MQW) stripe crystals on patterned (001) Si substrates. Longitudinal and higher order transverse modes were observed from a ridge waveguide structure. These results strongly suggest the possibility of fabricating InGaN MQW laser diodes on (001) Si.

Improvement of the performance characteristics of deep violet InGaN multi-quantum-well laser diodes using step-graded electron blocking layers and a delta barrier

The performance characteristics of deep violet In0.082Ga0.918N/GaN multi-quantum-well (MQW) laser diodes (LDs) with step-graded electron blocking layers (EBLs), a new EBL structure proposed by Zhang et al. and Yang et al., were numerically investigated. The simulation results indicated that the output emission wavelength was blue-shifted because of the change in the built-in polarization and internal electric field, although the use of step-graded EBLs effectively improved LD performance. To prevent the polarization effects and consequently a blue shift in the output emission wavelength, a new MQW LD structure with step-graded EBLs and a delta barrier as a last barrier is proposed. Simulation results indicated that the proposed LD structure exhibits higher values of output power, slope efficiency, and differential quantum efficiency, as well as lower threshold current, compared with conventional LD structures. The results also showed that the use of the delta barrier reduced the changes in the built-in polarization and internal electric field, and negligible changes in the output emission wavelength were observed.

The effects of quantum wells number and the built-in polarization on the performance of quaternary AlInGaN UV laser diode

The performance of quaternary Al0.08In0.08Ga0.84N multi-quantum well (MQW) laser diodes (LDs) using the simulation program of Integrated System Engineering Technical Computer Aided design (ISE TCAD) was studied. The simulation results show that the low threshold current, high output power and slope efficiency can be obtained when the quantum wells number is 4. Although, the fourth quantum well which placed in the right side (n-side) of the active region has a negative value of optical gain this means that the optical gain does not occur in this quantum well of laser structure. However, high external differential quantum efficiency (DQE) inside the active region was also observed. Optical gain and intensity were increased when the numbers of quantum wells increase reached 4. The built-in electric field effect inside the quantum well leads to the reduction of the overlap integral between the electrons and holes by separating their wave function was included. As well as, Al0.25In0.08Ga0.67N electron blocking layer (EBL) employed to enhance the performance of Al0.08In0.08Ga0.84N MQW LDs by increasing the optical confinement factor (OCF) inside the quantum wells.

Tapping-mode tuning-fork near-field scanning optical microscopy of low power semiconductor lasers.

The newly developed inverted tapping-mode tuning-fork near-field scanning optical microscopy (TMTF-NSOM) is used to study the local near-field optical properties of strained AlGaInP/Ga0.4In0.6P low power visible multiquantum-well laser diodes. In contrast to shear-force mode NSOM, TMTF-NSOM provides the function to acquire the evanescent wave intensity ratio /I(2omega)/ / /I(omega)/ image, from which the evanescent wave decay coefficient q can be evaluated for a known tapping amplitude. Moreover, we probe the near-field stimulated emission spectrum, which gives the free-space laser light wavelength lambda(o) and the index of refraction nr of the laser diode resonant cavity. Once q, lambda(o), and n(r) are all measured, we can determine the angle of incidence theta(o) of the dominant totally internally reflected waves incident on the front mirror facet of the resonator. Determination of such an angle is very important in modelling the stability of the laser diode resonator.

Performance and optical gain characteristic of InGaN MQW laser diodes

This paper discusses the performance of InGaN multi-quantum well (MQW) laser diodes and the effects of composition fluctuations on the optical gain characteristics. Pulsed threshold current densities as low as 4.8 kA/cm 2 have been observed for ridge-waveguide laser diodes allowing room-temperature continuous-wave (cw) operation with threshold currents of 125 mA and operating voltages of 6.5 V. The effects of composition fluctuations in the InGaN alloy were investigated by comparing theoretical and experimental gain spectra obtained from true spontaneous emission measurements, cavity length studies and cw laser emission spectra. We find that the distributed losses in our structures are of the order of 45–50 cm −1. Using a simple logarithmic gain–current relationship, we were able to describe the experimental results with a characteristic gain g 0=72 cm −1 and a transparency current density J trans=2.5 kA/cm 2. Optical gain calculations have been performed for InGaN quantum-well structures, based on band structures generated with an effective-mass Hamiltonian and taking pseudomorphic strain into account. We find that a very modest amount of composition fluctuation (standard deviation in the In content <0.008), which produces inhomogeneous broadening, is consistent with the experimental observations. This is consistent with TEM structural studies, which show that there is negligible phase separation in In x Ga 1− x N MQW with indium content smaller than x=0.1.

Effect of source and load resistance on the performance of bistable lasers

The effect of source and load resistances on the bistable behavior of multiquantum-well laser diodes is studied. Lower values of source resistance are found to affect the threshold current with a slight change in the hysterisis width. Also, the turn ON delay is found to decrease exponentially with the increase in source resistance. Lower values of load resistance leads to smaller hysterisis width, and for values less than 125 /spl Omega/, for the device under study with an input resistance (R/sub in/) of 1 k/spl Omega/, an astable behavior is observed when triggered by an electrical pulse in the gain section.

Optical characterization of visible multiquantum-well semiconductor lasers by collection/excitation modes of scanning near-field optical microscopy

Both collection and excitation modes of scanning near-field optical microscopy were used to study a strained AlGaInP/Ga0.4In0.6P low power visible multiquantum-well laser diode. Collection and excitation modes provide the near-field optical propagating intensity distribution and local photoconductivity information, respectively, at the facet of laser diode. Results show highly localized spatial correlation of the laser diode structure and its optical performance at the facet. Defect level in the energy range of 60–380 meV below the conduction band in the highly n-doped (Al0.7Ga0.3)0.5In0.5P cladding layer was found by the excitation mode using the wavelengths of 543.5 and 632.8 nm. Spatially resolved near-field optical spectra of both stimulated and spontaneous emissions were acquired as well. Longitudinal modes of the stimulated emission of laser diode were observed locally.

InGaN/GaN/AlGaN-based laser diodes grown on free-standing GaN substrates

The InGaN multi-quantum-well (MQW)-structure laser diode (LD) was grown on an epitaxially laterally overgrown GaN on sapphire. The lowest threshold current densities between 1.2 and 2.8 kA cm −2 were obtained when the number of InGaN well layer was 2. The LDs showed an output power as high as 420 mW per facet under room-temperature (RT) continuous-wave (CW) operation. The lifetime of the LDs at a constant output power of 30 mW was 250 h under CW operation at an ambient temperature of 50°C. Another InGaN MQW LD was grown on a free-standing GaN substrate that was obtained by removing the sapphire substrate. The LDs on free-standing GaN substrate with cleaved mirror facets showed an output power as high as 30 mW under RT-CW operation. The stable fundamental transverse mode was observed by reducing the ridge width to as small as 2 μm. The lifetime of the LDs on free-standing GaN substrate at a constant output power of 5 mW was about 160 h under CW operation at an ambient temperature of 50°C, due to a high threshold current density of 6 kA cm −2.

Room-temperature continuous-wave operation of GaInN multiquantum well laser diodes with low indium content

Based on the indium content dependence of optical gain in a GaInN multiquantum well (MQW) structure, room-temperature continuous-wave operation of GaInN MQW laser diodes is demonstrated. Amplified spontaneous emission spectra for Ga/sub 1-x/In/sub y/N (x=0.07 0.11) MQWs suggest that lower indium content leads to smaller bandgap inhomogeneity and an improvement in the laser performance. Ga/sub 0.93/In/sub 0.07/N MQW laser diodes were operated with a threshold current density of 11 kAcm/sup 2/.

A DIL-type miniature optical transceiver module using an InGaAsP half-transmittance photodiode for TCM optical access networks

A compact DIL-type miniature optical transceiver module has been developed without using a conventional 3 dB optical coupler. This transceiver is composed of a newly designed InGaAsP half-transmittance photodiode (HT-PD) and a 1.3-/spl mu/m multiquantum-well laser diode (MQW-LD). An HT-PD is used both as a photodetector and as a window for laser diode (LD) light. Fiber output power of 0 dBm at the drive current of 33.8 mA and responsivity of 0.58 A/W at the reverse-voltage of 2 V were obtained. The fundamental characteristics of the HT-PD are studied and discussed.

InGaN/GaN/AlGaN-Based Leds and Laser Diodes

InGaN quantum-well-structure blue LEDs were grown on epitaxially laterally overgrown GaN (ELOG) and sapphire substrates. The output power of both LEDs was as high as 6 mW at a current of 20 mA. The LED on sapphire had a considerable amount of leakage current in comparison with that on ELOG. These results indicate that In composition fluctuation is not caused by threading dislocations (TDs), free carriers are captured by radiative recombination centers before they are captured by nonradiative recombination centers in InGaN, and that the dislocations form the leakage current pathway in InGaN. Red LED with an emission peak wavelength of 650 nm was fabricated by increasing the In composition and thickness of InGaN well layer. When the laser diodes (LD) was formed on the GaN layer above the SiO2 mask region, the threshold current density was as low as 3 kAcm−2. When the LD was formed on the window region, the threshold current density was as high as 6 to 9 kAcm−2. There is a possibility that a leakage current due to a large number of TDs caused the high threshold current density on the window region. InGaN multi-quantum-well (MQW) structure LDs grown on the ELOG substrate showed an output power as high as 420 mW under RT-CW operation. The longest lifetime of 9,800 hours at a constant output power of 2 mW was achieved. The InGaN MQW LDs were fabricated on a GaN substrate. The fundamental transverse mode was observed up to an output power of 80 mW.

InGaN-BASED LASER DIODES

▪ Abstract Continuous-wave operation of InGaN multi-quantum-well (MQW) structure laser diodes (LDs) has been demonstrated at room temperature with output power up to 50 mW, operating temperature up to 100°C, emission wavelength of 400–420 nm, and a lifetime up to 300 h. InGaN MQW LDs with a lifetime of more than 1000 h are expected soon. Commercialization will begin in 1998 if research on the bluish-purple InGaN-based laser diodes continues to progress. The stimulated emission of the InGaN-based LDs originates from localized energy states of 100–250 meV depth, which are equivalent to quantum dot energy states, probably arising from from InGaN composition fluctuation in the InGaN well layers.

Lasing mechanism of InGaN-GaN-AlGaN MQW laser diode grown on SiC by low-pressure metal-organic vapor phase epitaxy

We studied the lasing mechanism of an InGaN-GaN-AlGaN multiquantum-well (MQW) laser diode by making various optical characterizations on the diode. Excitation power dependence of photoluminescence (PL) intensity was obtained to investigate the carrier recombination process of the laser. Surface emission and edge emission were compared by optical pumping to clarify where the lasing lines were located in relation to the absorption continuum. From the results, we demonstrate that lasing phenomena in our laser are dominated by free carriers. PL mapping was also taken on the same laser chip to examine the in-cavity bandgap inhomogeneity. We found a very large bandgap scattering of 100 meV. We also found that the wavelength distribution has a periodic modulation. We clarified that the various stimulated emission lines observed in our lasers are caused by the in-cavity spatial bandgap inhomogeneity of the InGaN MQW.

Cleaved and etched facet nitride laser diodes

Room-temperature (RT) pulsed operation of blue (420 nm) nitride-based multiquantum-well laser diodes grown on a-plane and c-plane sapphire substrates has been demonstrated. Structures investigated include etched and cleaved facets as well as doped and undoped quantum wells. A combination of atmospheric and low-pressure metal organic chemical vapor deposition using a modified two-flow horizontal reactor was employed. Threshold current densities as low as 12.6 kA/cm/sup 2/ were observed for 10/spl times/1200 /spl mu/m lasers with uncoated reactive ion etched facets on c-plane sapphire. Cleaved facet lasers were also demonstrated with similar performance on a-plane sapphire. Laser diodes tested under pulsed conditions operated up to 6 h at RT. Lasing was achieved up to 95/spl deg/C and up to a 150-ns pulselength (RT). Threshold current increased with temperature with a characteristic temperature T/sub 0/ of 114 K.

SPICE simulation for analysis and design of fast 1.55 μm MQW laser diodes

A rate equation model for static and dynamic behavior of 1.55 /spl mu/m InGaAsP multiquantum-well (MQW) semiconductor lasers has been developed. A three level scheme for the rate equations has been chosen in order to model carrier transport effects. The introduction of quasi-two dimensional (quasi-2-D) gateway states between unbound and confined states has been used to calculate, for each well independently, carrier density and gain, allowing to take nonuniform injection into account. Starting from the formal identity between a rate equation and a Kirchoff current balance equation at a capacitor node, the model has been implemented on a SPICE circuit emulator, SPICE has granted an easy handling of parasitics and opens the possibility of integration with electrical components. The model's parameters have been directly derived from a complete set of measurements on real devices. Thanks to this characterization and the model accuracy, we have obtained good agreement between simulations and experimental data. The model was finally used to improve both static and dynamic properties of MQW devices. Based on this optimization, compressive strained InGaAsP-InP MQW Fabry-Perot lasers were realized, achieving low threshold current, high efficiency, and more than 10 GHz of direct modulation bandwidth.

InGaN/GaN/AlGaN-Based Leds and Laser Diodes

AbstractInGaN quantum-well-structure blue LEDs were grown on epitaxially laterally overgrown GaN (ELOG) and sapphire substrates. The output power of both LEDs was as high as 6 mW at a current of 20 mA. The LED on sapphire had a considerable amount of leakage current in comparison with that on ELOG. These results indicate that In composition fluctuation is not caused by threading dislocations (TDs), free carriers are captured by radiative recombination centers before they are captured by nonradiative recombination centers in InGaN, and that the dislocations form the leakage current pathway in InGaN. Red LED with an emission peak wavelength of 650 nm was fabricated by increasing the In composition and thickness of InGaN well layer. When the laser diodes (LD) was formed on the GaN layer above the SiO2 mask region, the threshold current density was as low as 3 kAcm-2. When the LD was formed on the window region, the threshold current density was as high as 6 to 9 kAcm-2. There is a possibility that a leakage current due to a large number of TDs caused the high threshold current density on the window region. InGaN multi-quantum-well (MQW) structure LDs grown on the ELOG substrate showed an output power as high as 420 mW under RT-CW operation. The longest lifetime of 9,800 hours at a constant output power of 2 mW was achieved. The InGaN MQW LDs were fabricated on a GaN substrate. The fundamental transverse mode was observed up to an output power of 80 mW.

InGaN-based blue laser diodes

The continuous-wave (CW) operation of InGaN multiquantum-well (MQW) structure laser diodes (LDs) was demonstrated at room temperature (RT) with a lifetime of 100 h. The threshold current and the voltage of the LDs were 50 mA and 5 V, respectively. The threshold current density was 8.8 kA/cm/sup 2/. The carrier lifetime and the threshold carrier density were estimated to be 3.5 ns and 1.8/spl times/10/sup 20//cm/sup 3/, respectively. The Stokes shift of the energy difference between the absorption and the emission energy of the InGaN MQW LD's were 140 meV. Both spontaneous and stimulated emission of the LD's originated from this deep localized energy state which is equivalent to a quantum dot-like state. From the measurements of gain spectra and an external differential quantum efficiency dependence on the cavity length, the differential gain coefficient, the transparent carrier density, threshold gain and internal loss were estimated to be 5.8/spl times/10/sup -17/ cm/sup 2/, 9.3/spl times/10/sup 19/ cm/sup -3/, 5200 cm/sup -1/, and 43 cm/sup -1/ respectively.

Subband emissions of InGaN multi-quantum-well laser diodes under room-temperature continuous wave operation

The emission spectra of InGaN multi-quantum-well (MQW) laser diodes (LDs) under continuous-wave (CW) operation at room temperature (RT) showed periodic subband emissions with an energy separation of 2 meV. The peak wavelength of the laser emission was measured as a function of the operating current. The peak wavelength showed mode hopping toward higher energy with increasing operating current. Each single-mode laser emission was located at a peak of each periodic subband emission. These periodic subband emissions probably result from the transitions between the subband energy levels of the InGaN quantum dots formed from In-rich regions in the InGaN well layers. High-power InGaN MQW LDs with an output power of 40 mW were performed under RT CW operation.