InGaN Multi-quantum Well Research Articles

Blue-Laser Structures Grown on Bulk GaN Crystals

Room temperature (RT) pulsed operation of blue (402–427 nm) InGaN multi-quantum well (MQW) structure laser diodes (LDs) grown on bulk GaN crystals have been demonstrated. Substrate crystals were grown with the high nitrogen pressure and high temperature solution method. After substrate preparation the laser structures were grown by the low pressure MOVPE process. The LD structures are fully strained on the whole area of GaN crystals, without mismatch-related defects, being confirmed by X-ray diffraction, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Stripe geometry cavity has a contact area of 10 × 500 μm2. The mirror facets were cleaved and coated with reflection dielectric layers.

Vertical Cavity InGaN LEDs Grown by MOVPE

A RCLED structure, composed of InGaN multi-quantum wells inserted in a 3λ GaN based micro-cavity, was grown by metalorganic vapor phase epitaxy. The bottom mirror was a GaN/AIGaN distributed Bragg reflector (DBR). The growth process was followed by in-situ laser reflectometry, thus allowing an accurate control of the optical thickness (OT) of the various layers in the RCLED structure. We used the 543 nm line of a HeNe laser, and both the DBR and 3λ microcavity were designed at this wavelength. Taking into account the OT decrease after cooling (induced by the thermal lattice contraction and the index decrease), a 3λ cavity at 504 nm (green) was obtained. The emission of the InGaN QWs was found to be detuned with respect to the resonance wavelength. Nevertheless, we observed a resonance effect with an enhanced EL emission near the reflectance dip within the stop-band, the width of the EL peak being as low as 17 nm. The resonance effect was further confirmed by angle-resolved electroluminescence measurements. This work was developed within the framework of the European AGETHA project.

Spatial inhomogeneity investigation of QW emission in InGaN MQW LEDs

The luminescence properties of InGaN multiquantum well (MQW) light emitting diodes (LEDs) with various emission wavelengths were investigated using electroluminescence (EL) microscopy (ELM) and micro-EL (μ-EL) spectroscopy. Spatial inhomogeneity of the QW emission was observed as a function of the emission wavelength in spectrally resolved ELM images. The results show that the emission from diodes with shorter emission wavelengths exhibit less spatial inhomogeneity, while bright features were observed on the surface of samples with longer emission wavelengths. μ-EL spectra obtained from 5×5 μm 2 regions show an increase in the full width at half maximum (FWHM) of the spectra with increasing injection current and emission wavelength. With increasing emission wavelength, there is a shift in the main emission area from the p-contact area to the n-contact. The μ-EL spectra obtained from different bright spots in the ELM images demonstrate the variation of main emission peak position and FWHM of the EL spectra. The results suggest that the band-tailing effect due to small self-organised In-rich regions play a key role in the emission of the InGaN MQWs.

Room-temperature lasing of InGaN multiquantum-well hexagonal microfacet lasers by current injection

InGaN multiquantum-well (MQW) hexagonal microfacet (HMF) lasers have been fabricated by electron cyclotron resonance dry etching of as-grown planar InGaN MQW laser structures followed by metalorganic vapor phase epitaxy regrowth of p-GaN to form mirror facets of the lasers. The mirror facets are smooth and vertical {112̄0} facets of the regrown p-GaN. The HMF lasers lase at 401 nm under pulsed operation at room temperature. There is no significant difference in threshold current density for different cavity lengths of 480 and 770 μm, indicating that the mirror facets act as total reflectors.

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.

Selective area deposited blue GaN–InGaN multiple-quantum well light emitting diodes over silicon substrates

We report on fabrication and characterization of blue GaN–InGaN multi-quantum well (MQW) light-emitting diodes (LEDs) over (111) silicon substrates. Device epilayers were fabricated using unique combination of molecular beam epitaxy and low-pressure metalorganic chemical vapor deposition growth procedure in selective areas defined by openings in a SiO2 mask over the substrates. This selective area deposition procedure in principle can produce multicolor devices using a very simple fabrication procedure. The LEDs had a peak emission wavelength of 465 nm with a full width at half maximum of 40 nm. We also present the spectral emission data with the diodes operating up to 250 °C. The peak emission wavelengths are measured as a function of both dc and pulse bias current and plate temperature to estimate the thermal impedance.

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.

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.

Luminescence spectra from InGaN multiquantum wells heavily doped with Si

A systematic study has been carried out on emission spectra of heavily Si-doped InGaN multiquantum wells with different degree of potential fluctuation of InGaN in the lateral plane by the use of the various excitation sources. It is demonstrated that the quantum-confined Stark effect due to the piezoelectric field plays no serious role in optical spectra under appropriate doping conditions and, then, the degree of potential fluctuation of InGaN alloys is clearly reflected in spontaneous emission spectra.

Characteristics of indium-gallium-nitride multiple-quantum-well blue laser diodes grown by MOCVD

Room temperature (RT) pulsed operation of blue (420nm) nitride-based multi-quantum well (MQW) laser diodes grown on c-plane sapphire substrates has been demonstrated. Atmospheric pressure MOCVD was used to grow the active region of the device which consisted of a 10 pair In0.21Ga0.79N (2.5nm)/In0.07Ga0.93N (5nm) InGaN MQW. Threshold current densities as low as 12.6kA/cm2 were observed for 10×1200μm lasers with uncoated reactive ion etched (RIE) facets. The emission is strongly TE polarized and has a sharp transition in the far-field pattern above threshold. Laser diodes were tested under pulsed conditions lasted up to 6h at room temperature.

Lasing mechanism of InGaN/GaN/AlGaN multiquantum well laser diode

We investigated the lasing mechanism of an InGaN/GaN/AlGaN multiquantum well (MQW) laser diode on SiC grown by low-pressure metalorganic vapor phase epitaxy. Surface emission and edge emission are compared by optical pumping on the laser chip for which electrical pumping was made. Excitation power dependence of photoluminescence (PL) and PL mapping are also taken on the same chip. From the results, we demonstrate that lasing phenomena in our laser are dominated by free carriers and that the observed various stimulated emission lines are caused by the inhomogeneity of the InGaN MQW.

Gain spectroscopy of continuous wave InGaN multi-quantum well laser diodes

A systematic study of the optical gain of continuous wave InGaN multiple quantum well laser diode wafers has been made using the variable excitation-stripe length method. Experimental evidence is given of stimulated emission enhanced by resonance between degenerate states and non-degenerate ones, which co-exist in quantum wells having spatial potential undulation due to considerable fluctuation of the InGaN composition.

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.

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.

RT-CW Operation of InGaN multi-quantum-well structure laser diodes

The continuous-wave operation of InGaN multi-quantum-well (MQW) structure laser diodes (LDs) was demonstrated at room temperature with a threshold current of 25 mA, a threshold voltage of 5.8 V, an output power of 30 mW and a high operating temperature of 100°C. The energy differences between the absorption and the emission energy of the InGaN MQW structure LDs were as large as 220 meV at RT. A deep localized state (the localization energy is >100 meV) was formed in the InGaN well layer due to the InGaN phase separation during the growth. Both the spontaneous emission and the stimulated emission of the LDs originated from these deep localized energy states. The far field pattern showed a higher order transverse mode of the entire 5-μm-thick epitaxial layer stack, with air and sapphire as the upper and lower cladding layers, respectively.

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.

GaN-based blue/green semiconductor laser

High-power InGaN single-quantum-well (SQW) structure blue/green light-emitting diodes (LEDs) with an output power of 3-5 mW were fabricated. The continuous-wave operation of bluish-purple InGaN multiquantum-well (MQW)-structure laser diodes (LDs) was achieved at room temperature with a lifetime of 35 h. The threshold current and the voltage of the LD were 80 mA and 5.5 V, respectively. Photocurrent spectra of the InGaN SQW LEDs and MQW LDs were measured at room temperature. The Stokes shifts of the energy difference between the absorption and the emission energy of the blue/green InGaN SQW LED's and MQW LDs were 290, 570, and 190 meV. Both spontaneous and stimulated emission of the LDs originated from this deep localized energy state which is equivalent to a quantum dot-like state. When the temperature or the operating current of the LDs was varied, large mode hopping of the emission wavelength was observed. The carrier lifetime and the threshold carrier density were estimated to be 4.7 ns and 1/spl times/10/sup 20//cm/sup 3/, respectively. The optical confinement factor (/spl Gamma/) of the InGaN MQW LDs was estimated to be 0.025 from the measurement of the near-field radiation patterns.

First laser diodes fabricated from III–V nitride based materials

InGaN multi-quantum well (MQW) structure laser diodes (LDs) were fabricated on A-face sapphire substrates. The stimulated emission of the LDs was not observed up to the threshold current of 160 mA, which corresponded to a threshold current densityof 5 kA cm −2. The operating voltage at the threshold current was around 20 V. Polarization measurements showed that the transverse electric-polarized light output intensity increased to a much larger value than the transverse magnetic-polarized light, above the threshold current. At injection currents below the threshold, spontaneous emission, which had a full width at half-maximum of 20 nm and a peak wavelength of 406.8 run, appeared. Above the threshold current, strong stimulated emissions were observed at a wavelength of 406.3 nm.