Y. Kan

Extremely wide modulation bandwidth in a low threshold current strained quantum well laser

Lasing characteristics of strained quantum well (QW) structures such as InGaAs/AlGaAs on GaAs and InGaAs/InAlAs on InP were analyzed by taking into account the band mixing effect in the valence band. A relaxation oscillation frequency fr, which gives a measure of the upper modulation frequency limit, was found increased three times in a 50 A In0.9Ga0.1As/In0.52Al0.48As QW structure compared with that in a 50 A GaAs/Al0.4Ga0.6As QW structure for the undoped case. One of the main factors for this improved frequency bandwidth is attributed to the reduced subband nonparabolicity as well as the reduced valence‐band density of state in the strained QW structure. The corresponding lasing threshold current is one order of magnitude smaller than that of the GaAs/AlGaAs QW structure. With a p doping in the QW the fr value increases, and the 3 dB cutoff frequency of about 90 GHz will be expected with an acceptor concentration of 5×1018 cm−3 in the In0.9Ga0.1As/In0.52Al0.48As QW.

Field effects on the refractive index and absorption coefficient in AlGaAs quantum well structures and their feasibility for electrooptic device applications

Electroreflectance and electroabsorption measurements have been carried out to clarify field effects on the refractive index and absorption coefficient in AlGaAs quantum well structures. The observed electroreflectance spectra show very clear exciton-induced features at room temperature. A maximum variation of the refractive index in each quantum well at a photon energy near the lowest excitonic transition gap is obtained to be 4 percent induced by a 105V/cm field modulation. Electroreflectance and electroabsorption spectra are shown to demonstrate a relation between dispersion curves of the field-induced variations in the refractive index and absorption coefficient in the quantum well structure. Based on the obtained results, operation of an electroabsorption modulator with the capability of small frequency chirping and an efficient optical switch are discussed from the practical point of view.

Incidence angle effect of a hydrogen plasma beam for the cleaning of semiconductor surfaces

Incidence angle effect of a hydrogen plasma beam with an ion energy of about 20 eV was observed in a cleaning process for GaAs and Si surfaces for the first time. An atomically flat (001) GaAs substrate surface which was observed by clear Laue spots was prepared with a glancing angle of incidence. Similar improvement of smoothness was observed with the glancing angle of incidence on a Si surface when it was compared with perpendicular incidence. The mechanism is discussed considering momentum transfer parallel to the surface in the collision process and the resultant migration enhancement on the surface.

Lasing in a ZnS0.12Se0.88/ZnSe multilayer structure with photopumping

Photopumped lasing in a ZnS0.12Se0.88/ZnSe multilayer structure up to 180 K is reported for the first time. The films were grown by metalorganic vapor phase epitaxy on (001) GaAs. The purpose of using the multilayer structure is to prevent the diffusion of the photoexcited carriers to have population inversion sufficient for lasing. The possibility of lasing at the higher temperature is briefly discussed.

New low-temperature process for growth of GaAs on Si with metalorganic molecular beam epitaxy assisted by a hydrogen plasma

A new low‐temperature cleaning as well as a growth method using a hydrogen (H) plasma was successfully applied to the growth of GaAs on Si. The H plasma is expected to play several roles, such as cleaning the Si surface just prior to growth, decomposition of the metalorganic sources introduced without thermal cracking for the GaAs growth, supplying H radicals to terminate the organic‐alkyl radicals, and enhancement of the mobilities for the surface migration. The almost atomically flat Si clean surface was prepared at a temperature as low as 300 °C using the H plasma. A single‐domain GaAs epilayer was successfully realized at 400 °C after growth of 100 nm on a two‐domain (001) Si surface using triethylgallium and triethylarsenic introduced without thermal cracking. The films were grown just after surface cleaning with the H plasma in the same chamber. Therefore, the reduction of the temperature for the whole growth process is possible with this new method.

Electroreflectance Spectra and Field-Induced Variation in Refractive Index of a GaAs/AlAs Quantum Well Structure at Room Temperature

Observed electroreflectance spectra for a GaAs/AlAs quantum well structure show very clear exciton-induced features at room temperature. A maximum rate for the field-induced modulation of refractive index, Δn/n/E in each quantum well is deduced to be 4%/105 V/cm at a photon energy close to the excitonic gap. Such a large variation in the refractiveindex is attributed to excitonic transitions.

Low-temperature GaAs epitaxial growth using electron-cyclotron resonance/metalorganic-molecular-beam epitaxy

A low‐temperature GaAs epitaxial growth method called electron‐cyclotron resonance molecular‐beam epitaxy was newly developed. Triethylgallium (TEGa) and triethylarsine (TEAs) were used as source gases and were introduced without thermal decomposition. The method has the advantage of cleaning the GaAs substrate at the growth temperature just prior to growth as well as to decompose metalorganics with the hydrogen plasma activated by the cyclotron resonance. The epitaxial GaAs film was successfully grown at a temperature as low as 300 °C. All samples grown at 400 °C exhibited p‐type conductivity for (arsine/gallium) ratios between 4 and 13. The p‐type carrier concentration was strongly dependent on the (arsine/gallium) ratio and was in the range of 1016 –1019 cm−3 .

Observation of optical bistability by charge‐induced self‐feedback in biased AlGaAs multiple quantum well structures

A new type of optical bistable device is demonstrated. We report the first observation of optical bistable switching caused by charge‐induced self‐feedback due to field screening in GaAs/AlGaAs multiple quantum well structures, both at room temperature and at low temperature (110 K). A bias voltage supply is directly connected to the p‐i‐n diode with the multiple quantum well structure to attain optical bistability. Since no external series resistance is required, the device is attractive from the standpoint of a practical application. The mechanism for the self‐feedback underlying the operation of the device is elaborated. The theoretical consideration about the resolvable spot size on the device operation is shown.

Control of ZnSe Film Stoichiometry at ZnSe/GaAs Interface Grown by MOCVD

ZnSe film stoichiometry at a ZnSe/GaAs interface was found to be critically dependent on the surface conditions of the GaAs substrate. It is shown for the first time that the main factor governing the interface stoichiometry is the competition between the Zn–As bondings and the Ga–Se bondings at the interface.

Switching of photoluminescence by pulsed electric field in GaAs/Al 0.7 Ga 0.3 As single quantum well structure

Transient photoluminescence measurements for pulsed electric fields at liquid nitrogen temperature on a GaAs/AlGaAs single quantum well structure have been carried out to clarify the field-dependence of the recombination lifetime of carriers and to demonstrate a fast switching of the luminescence intensities. The lifetime increases with the increasing field in a marked contrast to the previously reported results. The transient characteristic for a short pulsed voltage is observed to be free from lifetime limitation. The results are semiquantitatively interpreted in terms of the field-induced reduction in the overlap between electron and hole wave functions inside the GaAs well, combined with biomolecular recombination model.