T. Suyama

Near‐ideal low threshold behavior in (111) oriented GaAs/AlGaAs quantum well lasers

Fundamental characteristics of (111) oriented GaAs/AlGaAs graded‐index separate‐confinement‐heterostructure single quantum well lasers have been compared with conventional (100) oriented lasers. In particular, the threshold current density Jth of (111) oriented lasers does not change with the well width Lz in the range of Lz=30–100 A, which corresponds to an ideal extreme. The lowest Jth of 145 A/cm2 together with a high characteristic temperature T0 of 186 K in the threshold‐temperature dependence has been achieved for an Lz of 40 A and a cavity length of 490 μm. The dependence of T0 on Lz showed that T0 is maximum at Lz∼60 A for both (111) and (100) oriented lasers.

Interface disorder in GaAs/AlGaAs quantum wells grown by molecular beam epitaxy at high substrate temperature

The interface disorder of quantum wells grown by molecular beam epitaxy at high substrate temperature is investigated by low‐temperature photoluminescence. The excitonic emission from a single quantum well is a single sharp peak, and the well width precisely determined from the emission peak energy does not equal to integral multiples of one‐monolayer width in almost all samples. These results indicate that the lateral size of growth islands with a one‐monolayer height is much smaller than the exciton diameter and a one‐monolayer interface acts as a layer with smaller AlAs mole fraction than the barrier layer.

Improvements in AlGaAs laser diodes grown by molecular beam epitaxy using a compositionally graded buffer layer

The effect of a compositionally graded buffer layer (CGBL) in AlGaAs laser diodes grown by molecular beam epitaxy is presented. The threshold current and the differential quantum efficiency are improved by a factor of ∼2. The lifetime is markedly improved by using a CGBL and it is comparable to that of lasers grown by liquid phase epitaxy.

Interface disorder in GaAs/AlGaAs quantum wells grown by molecular beam epitaxy on 0.5°‐misoriented (111)B substrates

The interface disorder in quantum wells (QW’s) grown by molecular beam epitaxy on 0.5°‐misoriented (111)B GaAs substrates was characterized by low‐temperature photoluminescence and by transmission electron microscopy. It was found that the abruptness of the heterointerface of (111)B QW’s is as sharp as that of (100) QW’s. The effect of growth interruption at a high substrate temperature of 720 °C on the interface disorder was also studied and was found to be detrimental.

Polarization-dependent gain-current relationship in (111)-oriented GaAs/AlGaAs quantum-well lasers

Detailed measurements of polarization‐dependent gain‐current characteristics have been carried out on (111)‐ and (100)‐oriented GaAs/AlGaAs single‐quantum‐well lasers by using Hakki and Paoli’s method. Polarization‐dependent gain‐current characteristics have been found to strongly depend on the well width and the quantization direction. The saturation of gain with the injection current and the width of the gain spectrum depends on the well width, and they are also affected by the transition between the higher‐order subbands. Experimental results of polarization‐dependent spontaneous emission are also presented. The simple selection rule and the effective‐mass theories do not quantitatively account for the experimental results.

Low current threshold AlGaAs visible laser diodes with an (AlGaAs)m(GaAs)n superlattice quantum well

Very short period (AlGaAs)m(GaAs)n superlattices (SL’s) have been used for single quantum wells (SQW’s) of visible laser diodes emitting in the wavelength region of 680–785 nm for the first time. The threshold current of graded‐index separate‐confinement‐heterostructure (GRIN SCH) lasers with SL SQW’s is lower than that of lasers with AlGaAs alloy SQW’s. The ridge‐waveguide structure GRIN SCH SL SQW laser emitting at 785 nm shows the low threshold current of 11 mA.

Effect of group V/III flux ratio on deep electron traps in AlxGa1−xAs (x=0.7) grown by molecular beam epitaxy

The effect of group V/III flux ratio γ on deep electron traps in AlxGa1−xAs (x=0.7) grown by molecular beam epitaxy at 720 °C has been studied by deep level transient spectroscopy. The photoluminescence characteristics of a GaAs single quantum well sandwiched by Al0.7Ga0.3As are determined by the electron traps denoted as E4–E6(E6’) in Al0.7Ga0.3As with the activation energies of 0.77 eV (E4), 0.72 eV (E5), 0.90 eV (E6), and 1.00 eV (E6’). The concentrations of these traps are minimized to the order of 1013 cm−3 at γ∼2 in spite of high Al content.

Energy‐band structure of (AlAs)(GaAs) superlattices

The energy‐band structure of (AlAs) (GaAs) short‐period superlattices (SLs) has been investigated by using photoluminescence and photoluminescence excitation spectroscopy. In the indirect transition region, both direct and indirect transitions are simultaneously observed and the band structure is elucidated. Indirect‐direct crossover occurs in the region of 15–18 A GaAs thickness for SLs with 10‐A AlAs thickness.

Low‐threshold room‐temperature cw operation of (AlGaAs)m(GaAs)n superlattice quantum well lasers emitting at ∼680 nm

Low cw threshold current of 45 mA and high differential quantum efficiency of 78% have been achieved at room temperature in an (Al0.6Ga0.4As)2(GaAs)2 superlattice quantum well laser emitting at 681 nm. The quantum well structure has been optimized to minimize the threshold current density at 680 nm. The new ridge‐waveguide structure with a current‐blocking supporting region is employed to reduce the thermal resistance.

Influences of thin active layer in (GaAl)As double-heterostructure lasers grown by liquid-phase epitaxy

Studies have been carried out on the influences of thinning the active layer in (GaAl)As double‐heterostructure (DH) lasers grown by liquid‐phase epitaxy (LPE). The measured peak gain varies sublinearly with current in the laser with a thin active layer of d∼0.05 μm while it varies linearly for d≲0.07 μm. This sublinear gain‐current relationship results from the rather wide graded‐band‐gap interface region formed by LPE and increases the lasing threshold. The photoluminescence (PL) peak of a DH sample with a thin active layer shifts to higher energies with the excitation due to the wide interface width, which agrees with the calculated results. High‐energy bumps observed in the PL spectra are more noticeable in the sample with the thinner active layer, which demonstrates that carriers recombine at the high‐energy portion of the interface region. The excitation dependence of PL intensity has shown that the nonradiative recombination rate is higher for the thinner active layer. Discussion is given concernin...