Double Heterostructure Structure Research Articles

Electron mobility in asymmetric GaN/AlGaN quantum well transistor structure: effect of alloy disorder scattering

We analyze the electron mobility μ of GaN/AlGaN based quantum well (QW) transistor structure. We consider the potential profile V(z) by including the potential due topolarization (V p ) and Hartree potential (V H ) owing to surface electron density N s . The low temperature mobility is governed by the alloy disorder (ad-) and interface roughness (ir-) scatterings. As N s increases, μ increases. However, for larger N s (N s > 0.6 × 1013 cm−2), there is a deviation showing decreasing trend of μ. We show that the ad- scattering plays a vital role in governing μ. An increase in N s causes narrowing of the polarization induced channel potential through V H and hence facilitates the larger extension of the subband wave function into the surface barrier. Accordingly, the ad-scattering increases, thereby reducing μ. we show that with an increase in well width there is a substantial rise in μ in quantum well (QW) structures while almost no change in μ in double heterostructures (DH). Enhancement of height of the barriers leads to different results, i.e., for the back barrier, there is a reduction in μ in both QW and DH structures, while for the surface barrier, there is a rise in μ. The fascinating trends of our results of μ in different GaN/AlGaN structures elucidate the importance of ad-scattering on low temperature μ.

Optimization of the defects and the nonradiative lifetime of GaAs/AlGaAs double heterostructures

We used Raman scattering and time-resolved photoluminescence spectroscopy to investigate the molecular-beam-epitaxy (MBE) growth parameters that optimize the structural defects and therefore the internal radiative quantum efficiency of MBE-grown GaAs/AlGaAs double heterostructures (DH). The DH structures were grown at two different temperatures and three different As/Ga flux ratios to determine the conditions for an optimized structure with the longest nonradiative minority carrier lifetime. Raman scattering measurements show an improvement in the lattice disorder in the AlGaAs and GaAs layers as the As/Ga flux ratio is reduced from 40 to 15 and as the growth temperature is increased from 550 to 595 °C. The optimized structure is obtained with the As/Ga flux ratio equal to 15 and the substrate temperature 595 °C. This is consistent with the fact that the optimized structure has the longest minority carrier lifetime. Moreover, our Raman studies reveal that incorporation of a distributed Bragg reflector layer between the substrate and DH structures significantly reduces the defect density in the subsequent epitaxial layers.

Enhanced direct-gap light emission from Si-capped n+-Ge epitaxial layers on Si after post-growth rapid cyclic annealing: impact of non-radiative interface recombination toward Ge/Si double heterostructure lasers.

Enhanced direct-gap light emission is reported for Si-capped n+-Ge layers on Si after post-growth rapid cyclic annealing (RCA), and impact of non-radiative recombination (NRR) at the Ge/Si interface is discussed toward Ge/Si double heterostructure (DH) lasers. P-doped n+-Ge layer (1 × 1019 cm-3, 400 nm) is grown on Si by ultra-high vacuum chemical vapor deposition, followed by a growth of Si capping layer (5 nm) to form a Si/Ge/Si DH structure. Post-growth RCA to eliminate defects in Ge is performed in N2 at temperatures between 900°C and 780°C, where the annealing time is minimized to be 5 s in each RCA cycle to prevent an out-diffusion of P dopants from the Ge surface. Direct-gap photoluminescence (PL) intensity at 1.6 µm increases with the RCA cycles up to 40, although the threading dislocation density in Ge is not reduced after 3 cycles in the present condition. The PL enhancement is ascribed to the suppression of NRR at the Ge/Si interface, where an intermixed SiGe alloy is formed. For Ge/Si DH lasers, NRR at the Ge/Si interface is found to have a significant impact on the threshold current density Jth. In order to achieve Jth on the order of 1 kA/cm2, similar to III-V lasers, the interface recombination velocity S is required below 103 cm/s in spite of S as large as 105 cm/s at the ordinary defect-rich Ge/Si interface.

Stimulated emission and optical gain in AlGaN heterostructures grown on bulk AlN substrates

Optical gain spectra for ∼250 nm stimulated emission were compared in three different AlGaN-based structures grown on single crystalline AlN substrates: a single AlGaN film, a double heterostructure (DH), and a Multiple Quantum Well (MQW) structure; respective threshold pumping power densities of 700, 250, and 150 kW/cm2 were observed. Above threshold, the emission was transverse-electric polarized and as narrow as 1.8 nm without a cavity. The DH and MQW structures showed gain values of 50–60 cm−1 when pumped at 1 MW/cm2. The results demonstrated the excellent optical quality of the AlGaN-based heterostructures grown on AlN substrates and their potential for realizing electrically pumped sub-280 nm laser diodes.

Impact of active layer design on InGaN radiative recombination coefficient and LED performance

The relative roles of radiative and nonradiative processes and the polarization field on the light emission from blue (∼425 nm) InGaN light emitting diodes (LEDs) have been studied. Single and multiple double heterostructure (DH) designs have been investigated with multiple DH structures showing improved efficiencies. Experimental results supported by numerical simulations of injection dependent electron and hole wavefunction overlap and the corresponding radiative recombination coefficients suggest that increasing the effective active region thickness by employing multiple InGaN DH structures separated by thin and low barriers is promising for LEDs with high efficiency retention at high injection. The use of thin and low barriers is crucial to enhance carrier transport across the active region. Although increasing the single DH thickness from 3 to 6 nm improves the peak external quantum efficiency (EQE) by nearly 3.6 times due to increased density of states and increased emitting volume, the internal quantum efficiency (IQE) suffers a loss of nearly 30%. A further increase in the DH thickness to 9 and 11 nm results in a significantly slower rate of increase of EQE with current injection and lower peak EQE values presumably due to degradation of the InGaN material quality and reduced electron-hole spatial overlap. Increasing the number of 3 nm DH active regions separated by thin (3 nm) In0.06Ga0.94N barriers improves EQE, while maintaining high IQE (above 95% at a carrier concentration of 1018 cm−3) and showing negligible EQE degradation up to 550 A/cm2 in 400 × 400 μm2 devices due to increased emitting volume and high radiative recombination coefficients and high IQE. Time-resolved photoluminescence measurements revealed higher radiative recombination rates with increasing excitation due to screening of the internal field and enhanced electron and hole overlap at higher injection levels. To shed light on the experimental observations, the effect of free-carrier screening on the polarization field at different injection levels and the resulting impact on the quantum efficiency were investigated by numerical simulations.

Photoconductivity decay in metamorphic InAsP∕InGaAs double heterostructures grown on InAsyP1−y compositionally step-graded buffers

Lattice-mismatched InAs0.32P0.68∕In0.68Ga0.32As∕InAs0.32P0.68 double heterostructures (DH) were grown on compositionally graded InAsyP1−y∕InP substrates by solid-source molecular-beam epitaxy (MBE) out to a misfit of ∼1%. The kinetics of carrier recombination were investigated in the nearly totally relaxed MBE-grown DH structures using photoconductivity decay (PCD) measurements. High minority carrier lifetimes of 4–5μs close to the radiation limit were measured, indicating the ability of MBE-grown InAsyP1−y buffers in achieving high-electronic-quality, low-band-gap mismatched InGaAs layers. Analysis suggests that very low interface recombination velocities are achieved. A photogenerated carrier diffusion model is presented to explain the initial nonlinear decays observed in PCD data for these heterostructures.

Study of AlGaInP multiquantum-well/double heterostructure light-emitting diodes with In-added GaP window layer regrown by antimony-based liquid phase epitaxy

The characteristics of surface-emitting AlGaInP multiquantum-well (MQW) and double heterostructure (DH) light-emitting diodes (LEDs) with a GaP window layer regrown by a GaP source melt with the addition of indium in the liquid phase epitaxy (LPE) system are studied. Using antimony in the source melt instead of gallium eliminates the phenomenon of melt back at the top layer of the LED during the LPE regrowth process. Meanwhile, adding In and Zn to the source melt significantly increases the GaP growth rate and effectively reduces residual melt during the regrowth process. Thus, the AlGaInP LED with a Zn-doped GaP window layer regrown by In being added, Sb-based LPE exhibits a lower turn-on voltage and higher light output power than that without the regrown GaP window layer. However, the emission peak wavelength of the MQW LED shifts a little after the LPE regrowth process. The DH LED does not shift. Thus, the DH structure is more suitable than the MQW structure for the LPE regrowth process of GaP.

Optically pumped lead–chalcogenide midinfrared lasers on Si substrates

Double heterostructure (DH) and quantum well (QW) EuSe/PbSe/Pb1−xEuxSe edge-emitting laser structures on Si substrates are grown by molecular-beam epitaxy. They operate up to 250 K when pumped with 870 nm low-cost laser diodes with peak powers of ∼7 W, and emit up to 200 mW peak output power at ∼5 μm wavelength. Differential quantum efficiencies are up to 20%. The threshold powers are limited by Shockley–Read recombination due to the high dislocation densities (108 cm−2) in the active layers. Nearly similar maximum operation temperatures were observed when employing (111)—instead of (100)—oriented layers, as well when using QW rather than DH structures. Reduction of dislocation densities to 107 cm−2 is feasible and will lead to nearly an order of magnitude lower threshold powers.

Gas source MBE growth of TlInGaAs/InP DH structures for the application to WDM optical fiber communication systems

TlInGaAs/InP double heterostructures (DHs) were grown on (1 0 0) InP substrates by gas source MBE. The photoluminescence (PL) peak energy and its variation with temperature decreased with increasing Tl composition. For the DH with a Tl composition of 13%, the PL peak energy varied only slightly with temperature (−0.03 meV/K). This value corresponds to a wavelength variation of 0.04 nm/K and is much smaller than that of the lasing wavelength of InGaAsP/InP distributed feedback laser diodes (0.1 nm/K). The TlInGaAs/InP light emitting diodes were also fabricated and the similar small temperature variation for the electroluminescence peak energy was observed. The results are a great first step to realize the laser diodes with temperature insensitive lasing wavelength characteristics, which are important in the wavelength division multiplexing (WDM) optical fiber communication systems.

Recombination lifetimes in undoped, low-band gap InAsyP1−y/InxGa1−xAs double heterostructures grown on InP substrates

High-quality, thin-film, lattice-matched (LM) InAsyP1−y/InxGa1−xAs double heterostructures (DHs) have been grown lattice mismatched on InP substrates using atmospheric-pressure metalorganic vapor-phase epitaxy. The low-band gap InxGa1−xAs layers in the DHs have room-temperature band gaps that range from 0.47 to 0.6 eV. Both the optical and electronic properties of these films have been extensively measured. The band-to-band photoluminescence is quite strong and comparable to that found for LM InP/In0.53Ga0.47As DHs grown on InP. Recombination lifetime measurements of undoped DH structures show minority-carrier lifetimes in excess of 1 μs in most cases. The earlier properties make the band gap-flexible InAsyP1−y/InxGa1−xAs DH system attractive for applications in high-performance, infrared-sensitive devices.

MBE growth of AlGaAs/GaAs double- heterostructure light emitting diodes on GaAs(111)A and (211)A substrates using all-silicon doping

Al 0.3Ga 0.7As/GaAs double-heterostructure (DH) light emitting diodes have been grown on GaAs (111)A and (211)A substrates by molecular beam epitaxy using only Si dopant. Scanning electron microscope observations and current-voltage characteristics for these samples show that the DH structures have been successfully formed. The peak wavelength of the emission spectra for both the (111)A and (211)A samples is 875 nm at 300 K, and this peak is attributed to the emission in the i-GaAs layer. The peak intensity of the (211)A sample is stronger than that of the (111)A sample owing to an improved AlGaAs/GaAs interface structure.

Blue-Green Stimulated Emission in Lattice-Matched ZnHgSSe/ZnSSe Double Heterostructures by Optical Pumping

The fabrication and characterization of ZnHgSSe/ZnSSe double heterostructure (DH) laser structures, lattice-matched on GaAs (100) substrates, are described. The structures were grown by molecular beam epitaxy with a ZnHgSSe active layer which includes a small amount of Hg (∼1.2%). The occurrence of stimulated emission was observed at 77 K in the blue-green wavelength region by optically pumping the DH structures.

Minority carrier lifetimes in molecular beam epitaxy grown AlxGa1−xAs/GaAs double heterostructures doped with aluminum

Both Al-treated pBN crucibles and Al-doped Ga melts have been used to reduce Ga-cell related oval defect densities in molecular beam epitaxy (MBE) grown epilayers. This practice, although effective in reducing the oval defect density, results in small amounts of Al contamination in the grown films. In this work, we have grown diagnostic Al0.3Ga0.7As/GaAs double heterostructure (DH) devices doped with Al in order to determine the effect of Al contamination on the minority-carrier lifetime. The active region of the DH structures was doped with Al at 5×1018 atoms/cm3 to emulate the levels observed when Al-treated pBN crucibles are used. Al impurity levels and distribution profiles were characterized by secondary ion mass spectrometry, and minority-carrier lifetimes of DH devices were characterized using time-resolved photoluminescence (PL) measurements. Device structures grown with various GaAs active layer thicknesses were used to calculate the interface recombination velocity (S) and the bulk lifetime (τB). We found that Al doping of the GaAs active layer adversely affects the measured PL decay time. GaAs active layers of DH devices doped with Al exhibited poor bulk lifetimes (τB≤120 ns) limited by Shockley–Read–Hall recombination. In contrast, DH devices grown with Al-free active layers resulted in significantly improved PL decay times with bulk lifetimes as high as τB=700 ns.

Strong signal analysis of optical nonlinearities in single-quantum-well and double-heterostructure lasers

Based on the numerical strong signal solution of the density-matrix formalism, the nonlinearities of the gain and the refractive index are discussed for both bulk double-heterostructure (DH) and single-quantum-well (SQW) InGaAs lasers. The results show that for both structures the nonlinear gain is well approximated by the recently proposed analytical expression and the modified two-level system approximation up to a range of photon density discussed herein, and the refractive index could either increase or decrease with the photon density depending on the wavelength detuning from the gain peak. The results of a new analytical expression for nonlinear refractive index is in qualitative agreement with the numerical ones; however, significant quantitative difference occurs between these two model results for high photon density values. Due to the more important nonlinear gain in SQW structures, the linewidth enhancement factor increases more rapidly with increasing photon density in these structures than in DH structures. >

Embossed grating lead chalcogenide distributed-feedback lasers

The successful preparation of double-heterostructure (DH) distributed-feedback (DFB) PbEuSe lasers using an embossing technique for the DFB sub-micrometer grating is reported here for the first time. The submicrometer grating was performed in (111) oriented silicon wafers by holographic exposure and argon ion milling. Using a hydrostatic press this grating was embossed into a lead chalcogenide wafer, covered by a DH structure grown by molecular beam epitaxy. Lasers made from wafers with grating periodicities of 0.678 and 0.755 μm show DFB characteristics at the corresponding emission wavenumbers around 1420 and 1590 cm−1.

Vertical cavity top-surface emitting lasers with thin Ag mirrors and hybrid reflectors

Three types of vertical cavity surface emitting lasers based on GaAs/AlGaAs have been grown by molecular beam epitaxy (MBE). The laser structures with top emission have been evolved from a simple double heterostructure (DH) with n-type distributed Bragg reflectors (DBR) as the bottom mirror and a semitransparent Ag as the top mirror, to a DH structure with a hybrid reflector consisting of a thin metal and a few pairs of p-DBR as the top mirror, and to a 3-QW structure again with a hybrid reflector as the top mirror. MBE growth of these three device structures is discussed, particularly on the in-situ growth of metals on GaAs. Devices have been fabricated and measured. A theoretical model has been used to gain an understanding of the parameters in each design. The MBE growth techniques have been improved to modify the heterostructural interface in the DBR to reduce the series resistance. The laser performance has been improved from a high threshold current in the first structure to a much lower threshold current in the second structure and finally to a cw room temperature operation with low threshold currents in the third structure.

Room-temperature operation of an InGaAsP double-heterostructure laser emitting at 1.55 μm on a Si substrate

The room-temperature operations of an InGaAsP double-heterostructure (DH) laser emitting at 1.55 μm on a Si substrate is reported. A pulsed threshold current as low as 46 mA has been measured for a ridge waveguide laser with a 4 μm strip width and a 200 μm cavity length. This successful laser operation is due to the high crystalline quality of the DH structure with full width at half maximum of x-ray rocking curves as low as 110 arcsec grown on a Si substrate by the organometallic vapor phase epitaxy/vapor mixing epitaxy hybrid method. A correlation between the optical property of an InGaAsP DH and its crystalline quality is also discussed.

Contribution of the band-filling effect to the effective refractive-index change in double-heterostructure GaAs/AlGaAs phase modulators

In this work we have theoretically estimated the effective refractive-index variation Δneff due to the removal of carriers in the depletion region of a double-heterostructure (DH) n-AlGaAs/n-GaAs/p-AlGaAs phase modulator when a reverse bias is applied to this structure. Δneff is calculated by a Kramers–Kronig analysis of the absorption spectrum, which changes around the absorption edge because of the band-filling effect associated with the presence of carriers. We also calculated the phase shift Δφ due to Δneff, and we showed that this contribution due to the band-filling effect can account for up to 28% of the total phase shift for a specific DH structure, showing in this way that the band-filling effect can no longer be ignored in the analysis of phase modulators.

Thermal Degradation of InP/InGaAsP/InP DH Structure

LPE-grown undoped InP/InGaAsP/InP samples were annealed under a phosphorus pressure produced by either an InP cover piece, phosphine or red phosphorus. The change in photoluminescence (PL) intensity of the quaternary layer before and after annealing was investigated as a function of the excitation power density. Even when annealed under a phosphorus pressure, a condition under which surface degradation is never observed, the PL intensity decreases. The rate of decrease of the PL intensity depends on the excitation power density. Because the rate is independent of the quaternary layer thickness, the decrease in the PL intensity is ascribed to a decrease in the internal quantum efficiency of the quaternary layer rather than to an increase in the interface recombination velocity. Furthermore, the dependence of the decrease in PL intensity on the excitation power was analyzed theoretically and the non-radiative carrier lifetime estimated. The thermal degradation of double heterostructure (DH) samples can be evaluated even when the surface remains unchanged.

Performance and reliability of high radiance InGaAsP/InP DH LED́s operating in the 1.15-1.5 µm wavelength region

The performance and reliability of InGaAsP/InP double heterostructure (DH) light emitting diodes (LED's) have been investigated over a wide range of emission wavelengths 1.15-1.5 \mu m. High radiance performance has been achieved by growing optimized DH structures and adopting a method of coupling a monolithic-lensed LED to a spherical-ended fiber. Interface instability previously encountered in the liquid phase epitaxy (LPE) to grow a DH for 1.5 μm wavelength has been eliminated by a new growth method using a reduced temperature. LED's exhibiting typical output powers of 50, 44, and 21 μW at 100 mA, measured at the end of a 50 μm core 0.20 NA graded-index fiber with a length of 1 m, have been realized, respectively, at wavelengths of 1.15, 1.27, and 1.5 μm. The spectral width, coupled power and its temperature dependence, and the cutoff frequency have been analyzed in terms of the emission wavelength. Operating lives of LED's at three different wavelengths have been estimated from the result of accelerated aging carried out at the ambient temperature of 200°C and at a constant current of 100 mA. It has been found that the degradation rate exhibits no dependence on the wavelength confirming the same value of half-lives in excess of 109h for the 60°C operation.