Masaya Mannoh

Residual impurities in GaN/Al 2 O 3 grown by metalorganic vapor phase epitaxy

Residual impurities in GaN films on sapphire (A12O3) substrates grown by two-step metalorganic vapor phase epitaxy (MOVPE) have been investigated. We have mainly investigated the incorporation of carbon into the GaN films with GaN buffer layers on A12O3 during MOVPE growth, comparing trimethygallium (TMGa) and triethygallium (TEGa) as the typical gallium precursors. The films were characterized by secondary ion mass spectroscopy analysis, photolu-minescence, and Hall measurements. The carbon, hydrogen, and oxygen concentrations increase with decreasing growth temperature in using TMGa. Especially the carbon concentration increases with decreasing a V/III ratio, for both TMGa and TEGa. There is about two times more carbon in the GaN films grown using TEGa than those using TMGa. The carbon from TMGa mainly enhances the D-A pair emission (∼378 nm), which shows the carbon makes an acceptor level at nitrogen sites in GaN. On the other hand, the carbon from TEGa enhances a deep emission (∼550 nm), which shows the carbon makes not only an acceptor level but deep levels at interstitial sites in GaN. The carbon impurities originate from methyl radicals for TMGa, or ethyl radicals for TEGa. It is supposed that, in the case of TEGa, the carbon impurities are not always located at nitrogen sites, but are also located at interstitial sites because of the C-C bonding in ethyl radicals.

Composition dependence of photoluminescence of AlxGa1−xAs grown by molecular beam epitaxy

Low‐temperature (∼4 K) photoluminescence of lightly Si‐doped AlxGa1−xAs grown by molecular beam epitaxy has been studied. The defect‐related emissions, due to the defect exciton (d, X) and the defect complex (d), have been identified. The peak energies of these emissions, which are 1.505 eV (d, X) and 1.474 eV (d) for GaAs, have been determined as a function of the mole fraction x ( x<0.45). The energy difference between the donor–bound‐exciton (BE) peak and the defect‐exciton peak is almost constant (∼9 meV). In contrast, the energy difference of the BE peak and the defect‐complex peak increases from 40 meV at x=0 to about 70 meV at x∼0.4. In addition, the ionization energies of carbon and silicon acceptors in AlxGa1−xAs have been determined as a function of x and compared with theoretical calculations.

Analysis of GaInP/AlGaInP compressive strained multiple-quantum-well laser

We have analyzed GaInP/AlGaInP compressive strained MQW lasers, with theoretical calculation and experimental results. Our calculations of TE polarized gain, where the valence subband mixing and the heterobarrier leakage current are taken into account, are in good agreement with the experimental results. When a compressive strain of up to 0.5% is induced in the quantum wells, the density of states near the valence band edge is decreased, due to the reduction of heavy-hole and light-hole subband mixing. At the threshold condition, the compressive strain reduces not only the radiative recombination current, but also the hetero-barrier leakage current. Therefore, the threshold current is reduced, and its temperature dependence is found to be small, In the analysis, we also show that when larger compressive strain of more than 0.5% is induced in the 40-/spl Aring/-thick quantum wells, the threshold characteristics are degraded. >

High power and high‐temperature operation of GaInP/AlGaInP strained multiple quantum well lasers

High power and high‐temperature operation of transverse‐mode stabilized 690 nm AlGaInP strained multiple quantum well lasers is described. Three 0.5% biaxially compressive strained GaInP (8 nm) wells were employed in the active region. Low threshold current of 36 mA and very high output power of 60 mW at high temperature up to 100 °C were obtained with 700 μm long lasers, whose facets were coated with antireflection–reflection films. This is, to the best of our knowledge, the first report that an AlGaInP laser has reached a cw output power of 60 mW at a high temperature of 100 °C. Very low degradation rate at 50 °C with 35 mW output power was confirmed.

Characteristics of molecular‐beam epitaxially grown pair‐groove‐substrate GaAs/AlGaAs multiquantum‐well lasers

Growth and device characteristics of an index‐guided GaAs/AlGaAs multiquantum‐well (MQW) laser, called a pair‐groove‐substrate (PGS) MQW laser, are described in detail. The laser structure is fabricated by using single‐step molecular‐beam epitaxy on a (001) GaAs substrate with a pair of etched grooves along the 〈110〉 direction. A mesa between a pair of grooves, where the lasing action occurs, becomes narrow during growth, and the narrow mesa offers lateral waveguiding that stabilizes a fundamental transverse mode. The superior crystalline quality of the mesa top, which is examined by a microprobe photoluminescence technique, serves to lower the lasing threshold currents. The lasers with mesa widths below 2 μm show stable transverse mode operation with a low threshold current of 20 mA, as well as a high external differential quantum efficiency of 68%. The low threshold and high characteristic temperature accomplish a high‐temperature continuous‐wave operation at 153 °C for the lasers mounted on silicon he...

Dry-etched-cavity pair-groove-substrate GaAs/AlGaAs multiquantum well lasers

A fabrication technique based on reactive ion beam etching is presented for the formation of the facet mirrors on GaAs/AlGaAs lasers called pair‐groove‐substrate (PGS) multiquantum well lasers. Laser cavities with vertical and smooth walls are achieved with this etching technique by using a high‐temperature baked photoresist mask. PGS lasers with 200‐μm‐long etched cavities show a low pulsed threshold current of 29 mA and a high external differential quantum efficiency of 43%. The threshold current is comparable to those of cleaved lasers. Room‐temperature cw operation is easily realized in junction‐up mounting.

Extremely high quantum efficiency (86%) operation of AlGaInP visible laser diode with lateral leaky waveguide structure

AlGaInP visible laser diode with lateral leaky waveguide structure has been demonstrated for the first time. The laser has differential quantum efficiency as high as 43% from one facet without coating in addition to stable fundamental transverse‐mode oscillation. The high differential quantum efficiency is due to the propagation loss as low as 4.2 cm−1 of this laser, which is the lowest value for AlGaInP visible diode to the best of our knowledge. This laser can be successfully fabricated by selective growth of AlGaInP quarternary alloy.

Defect‐related emissions in photoluminescence spectra of AlxGa1−xAs grown by molecular beam epitaxy

We have studied defect‐related emissions in low‐temperature (∼4 K) photoluminescence of lightly Si‐doped AlxGa1−xAs (x 0.27. This suggests different origins for the (d,X) and (d) emissions. The (d,X) and (d) emissions are tentatively associated with oxygen and carbon, respectively.

Ionized Mg doping in molecular‐beam epitaxy of GaAs

Using ionized Mg beams accelerated to energies from 130 to 500 eV, Mg doping was studied in molecular‐beam epitaxy of GaAs. The incorporation coefficient of Mg increases by a factor of about 100 when compared with the use of neutral Mg beams. Hole concentrations as high as about 1×1019 cm−3 have been achieved. Photoluminescence measurements suggest that the damage due to Mg‐ion bombardment is negligible when the ion accelerating voltage (Va) ≲130 V. For higher Va , the damage can be removed by postgrowth annealing.

Pair-groove-substrate GaAs/AlGaAs multiquantum well lasers by molecular beam epitaxy

We investigated molecular beam epitaxial growth characteristics on (001) GaAs substrates with a pair of etched grooves along the 〈110〉 direction. It is found that, as the growth proceeds, the mesa width between the pair of grooves gradually decreases and that the grown mesa surface becomes slightly concave. These results offer great advantages for precisely defining the lateral width of index guided lasers during growth. A pair‐groove‐substrate GaAs/AlGaAs multiquantum well laser has been newly developed, which shows stable fundamental transverse mode oscillation and a high external differential quantum efficiency of 68% as well as a low threshold current of 23 mA.