Shigeyasu Tanaka

Defect structure in selective area growth GaN pyramid on (111)Si substrate

A GaN pyramid grown selectively on a (111)Si substrate with a patterned dot structure of a SiO2 mask, by metalorganic vapor phase epitaxy using AlGaN as an intermediate layer, was characterized by transmission electron microscopy. The dot pattern has an array of 5.0-μm-diameter window openings with a 10 μm period. The density of threading dislocations observed in the window region decreased gradually with increasing distance from the interface. This was mainly due to the dislocation reaction and bending of threading dislocations for the first 2 μm region from the interface and for the upper region, respectively. Dominantly observed defects in the lateral-growth part were dislocations parallel to the interface. An amorphous layer was formed at the interface in the window region. Nitride particles were observed at the interface in the mask region.

Structural characterization of GaN laterally overgrown on a (111)Si substrate

Using transmission electron microscopy, we have characterized defect structures in laterally overgrown GaN crystals, grown directly on SiO2 stripe-patterned (111)Si substrates by metalorganic vapor phase epitaxy using AlGaN as an intermediate layer. The width and the period of the stripe windows were nominally 1 and 2 μm, respectively. The average threading dislocation density for a completely coalesced 2-μm-thick GaN crystal obtained on the [112]-oriented stripe-patterned substrate was ∼2×109u2002cm−2. The reduction in threading dislocation density is a consequence of the lateral growth and dislocation reactions at the coalesced front of the mask. On the other hand, valleys and pits tend to remain on the mask during the growth on the [110]-oriented stripe-patterned substrate. Cracks were present in both crystals.

The initial stage of LPE growth of InGaAsP on GaAs in the region of immiscibility

Abstract The LPE growth process during the initial growth stage in the immiscible region of In x Ga 1− x As y P 1− y layers lattice-matched to GaAs was studied for several solid compositions as a function of growth time t g . It was found that the LPE process in the region of immiscibility is classified into three stages as follows: (1) an LPE layer with rather good crystal quality grows because of stabilization by the mismatch strain energy, (2) the crystal quality of the layer becomes gradually worse with increasing t g , and the interface spinodal decomposition is accelerated because the crystal defects weaken the substrate strain stabilizing effect, and then (3) it is impossible to grow the layer. The interfacial spinodal decomposition is more rapid at the solid composition near the center of the immiscible region, and it is slower on a (111)B GaAs than on a (100) GaAs substrate.

Raman scattering of InGaAsP lattice-matched to GaAs in the region of immiscibility

We have studied the Raman spectra of the InxGa1-xAsyP1-y quaternary alloys lattice-matched to GaAs in the immiscible region. The spectra exhibit four modes of behavior due to four binary compositions: GaP-, InP-, GaAs-, and InAs-like phonon modes. It was found that the phonon spectra in the immiscible region are well explained by the isodisplacement model of Inoshita [J. Appl. Phys. 56 (1984) 2056].

Growth of InGaP epitaxial layers by liquid phase electro-epitaxy

Abstract Liquid phase electro-epitaxy (LPEE) dominated mainly by electromigrative effects was employed to grow In x Ga 1t- x P( x ≁0.49) ternary alloys on (111)A GaAs substrates. At a growth temperature of 785°C, the expected linear dependence of the layer thickness on growth time was not observed. This is believed to be due to liquid compositional changes caused by a loss of P from the solution during growth. At a lower growth temperature of 690°C, the layer thickness increased approximately linearly with growth time. From the slope, the growth rate at a current density of 12 A/cm 2 was estimated to be about 4 μm/h. Good quality LPEE thin-layers were obtained at 690°C.

LPE growth of InGaP/InGaAsP multiple thin layers on (111)A GaAs substrates

Abstract LPE growth of InGaP/InGaAsP multiple thin layers on (111)A GaAs substrates was investigated by means of a conventional horizontal sliding boat technique. The growth rate, which also depends on the solid composition, was found to decrease with decreasing cooling rate. Using a low cooling rate of less than 0.1°C/min, we obtained multiple layers with a period shorter than 850 A, the best result being a period of about 500 A. The individual layer thickness were in the range of 250 to 600 A. The results suggest that melt-back and melt carry-over do not play an important role in the multiple layer growth.

Effect of lattice mismatch between epitaxial layer and substrate on immiscibility of InGaAsP/GaAs LPE layers

Abstract The liquid phase epitaxy (LPE) growth of InGaAsP on GaAs in the immiscible region has been studied in terms of the substrate induced stabilization. The results show that layers grown on (100) GaAs exhibit extraordinary behavior in the PL spectra as well as the surface morphology, while those on (111)A and (111)B GaAs do not. This suggests that the stabilization due to the substrate for the (111) orientation is stronger than that for (100), which is consistent with the theoretical prediction that the effect of strain energy induced by the substrate is greater for the growth on (111) than (100) oriented substrates. It is suggested, by examining the compositional variation at the onset of the LPE growth, that the effect which weakens the substrate induced stabilization for the (100) orientation is the presence of defects due to the lattice mismatch between the substrate and the transient layer, that grows within the initial short time of about 50 ms.

Behaviour of anion vacancy in InxGa1-xAsyP1-y grown on (111)A and (100) GaAs

Abstract The electron trapping levels in In x Ga 1- x As y P 1- y (0 ≦ y ≦ 0.58) grown on (111)A GaAs substrate were investigated and compared with those in (100) GaAs-based alloy layers. The activation energies of trapping levels (0.42–0.44 eV) agree well with the theoretical results (0.47–0.53 eV). A thermodynamical analysis further compared the arsenic fraction-dependent concentration N T . Taking into account the immiscible effect of the alloy atoms and the substrate-induced stabilization, the measured N T s for both (111)A- and (100)-based samples agree with the calculated one. These results lead to the conclusion that these levels originate from a phosphorous vacancy. In addition, the result of an aging test on the InGaP light emitting diode suggests that device degradation is caused by the generated defects containing the phosphorus vacancy.

Observation of the potential distribution in GaN-based devices by a scanning electron microscope

Mapping of the potential distribution using a scanning electron microscope (SEM) has been reported in recent years [1,2] for semiconductors such as Si, GaAs and InP. But, there are no such studies on GaN-based devices, to our knowledge. In this study, we observed two types of GaN-based devices by SEM to see if there is a condition that the contrast matches the potential distribution of the devices. The first device we studied was GaN p-n junction (p, n ∼5 × 10(17) cm(-3)). The device was cut, and polished from the cross-section to a flat surface. The cross-section was observed by SEM. Fig.xa01(a) shows an SEM image taken at 3 kV. The p-region appears bright and the n-region appears dark. The image intensity changes at the position of p-n junction, for which we used electron beam induced current (EBIC) technique to determine the p-n junction position. Fig.xa01(b) is a line profile across the p-n junction (broken line) of the SEM image together with a calculated potential distribution (solid line) using p and n concentrations. It can be seen that the contrast profile matches the potential distribution very well. The SEM observations were carried out for several accelerating voltages. But, best result was obtained at 3 kV. For lower accelerating voltages, the image seemed to reflect the surface potential. On the other hand, higher accelerating voltages resulted in blurred images. The second sample was a light emitting diode structure based on AlN where a multiple quantum well (MQW) structure was sandwiched by p- and n-AlGaN materials. In this case, the sample was obliquely polished from the surface (∼10°) to improve the lateral resolution. The SEM image could reveal the structure of MQW.jmicro;63/suppl_1/i22/DFU051F1F1DFU051F1Fig. 1.(a) SEM image of p-n GaN. (b) Comparison of line profile across the p-n junction (broken line) and a calculated potential distribution (solid line). AcknowledgementWe thank professor H. Amano (Nagoya University) for providing the samples.

Mapping of minority carrier lifetime distributions in multicrystalline silicon using transient electron-beam-induced current.

We have used transient electron-beam-induced current (EBIC) to map minority carrier lifetime distributions in multicrystalline Silicon (mc-Si). In this technique, the electron beam from a scanning transmission electron microscope was on-off modulated while the sample was scanned. The resulting transient EBIC was analyzed to form a lifetime map. An analytical function was introduced as part of the analysis in determining this map. We have verified this approach using numerical simulations and have reproduced a lifetime map for an mc-Si wafer.