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We have studied the optical gain spectra in GalnN/GaN and GaN/AlGaN double heterostructures and quantum wells at room temperature employing the stripe-excitation method. We compare the results with data for other III–V semiconductors. The optical gain is strongly anisotropic, with almost no gain for the TM mode. For quantum wells, the material gain increases with decreasing well width as expected. Whereas for GalnN/GaN structures only a single gain peak is observed, consistent with a free-carrier gain model, measurements on GaN/AlGaN structures reveal two peaks, which are assigned to localized exciton and exciton-LO-phonon gain.

Selective area growth of GaInAs has been performed by low pressure metalorganic vapor phase epitaxy (MOVPE) on SiO 2 -masked InP substrates. The growth rate and composition variations induced by the mask pattern have been evaluated by cathodoluminescence for two In precursors, trimethylindium (TMI) and dimethylaminopropyl-dimethyl-indium (DADI) in combination with triethylgallium (TEG) and AsH 3 , and compared to model calculations taking gas phase diffusion into account. The resulting characteristic parameter D / k could be attributed to the chemical stability of the precursors. We found that the use of DADI results in a less pronounced growth rate and In incorporation enhancement at the mask edges than the use of TMI, but DADI still has to be labeled “chemically less stable” than TEG under MOVPE conditions. Gas phase reactions with the hydrides probably have to be taken into account to understand these findings.

Differential electrotransmission measurements on InGaAs/InP and InGaAs/InGaAsP multi quantum well structures (MQW) with well widths 8,10 and 20 nm with InGaAsP barriers of E g (77 K ) = 1.2 eV are presented. The differential electrotransmission spectra were fitted with a lorentzian lineshape model for the excitonic transition. A Stark shift of up to 18 meV was observed for the 8 nm InGaAs/InGaAsP quantum well structure. The results were compared to a calculation of the Stark effect including the field dependent exciton binding energy. We find good agreement between experiment and theory. The well width dependence of the oscillator strength shows a maximum for L z = 11 nm when changing the field from 1 · 10 4 V/cm to 1 · 10 5 V/cm .

By optical gain spectroscopy we have studied the fundamental laser properties of GaInN/GaN heterostructures grown on sapphire. Utilizing the stripe excitation method we have measured optical gain spectra at room temperature. Due to the low symmetry of the wurtzite structure and the resulting splitting of the uppermost valence bands, we find optical gain only for the TE mode. Our analysis shows that the optical gain is due to direct band‐to‐band transitions in an electron‐hole plasma. For gain amplitudes typically found in lasers, we find carrier densities up to 3×1019 cm−3, which are likely to lead to rather large threshold current densities.

A 7F/sup 2/ DRAM cell and corresponding vertically folded bitline architecture has been fabricated using a 0.175 /spl mu/m CMOS technology. This concept features an advanced 30/spl deg/ tilted array device layout and an area penalty free inter BL twist. The presented scheme minimizes local well noise by maximizing the number of twisting intervals.

GaInAs layers were grown selectively on partially masked InP substrates by metalorganic vapor phase epitaxy using the conventional In precursor trimethyl‐indium (TMI) and the intramolecularly saturated compound dimethylaminopropyl‐dimethyl‐indium (DADI) for comparison. We obtained for both TMI and DADI an excellent morphology of the selectively grown structures. Significant differences between the two In sources were determined for the unwanted nucleation on the masked area and for the homogeneity of the composition of the grown ternary material. We attribute this observation to the higher chemical stability of DADI and a changed decomposition behavior, which leads to longer effective diffusion lengths for DADI and its thermally decomposed fragments in the gas phase.

The authors have studied the combination of the two alternative precursors DADI (dimethylaminopropyl-dimethylindium) and tertiary-butyl-arsine (TBA) in atmospheric pressure metalorganic vapor phase epitaxy of GaInAs lattice matched to InP. They could not detect any problem which could be specifically due to this precursor combination. In particular, no parasitic side reactions have been observed either directly or indirectly. The growth of GaInAs at a very low V/III ratio of 2 could be realized with this precursor combination. By studying selectively grown GaInAs stripes which have been grown at low pressure using either DADI or TMIn, a better selectivity was obtained for the DADI grown structures. The growth rate enhancement near the edge of larger fields as well as the composition changes for different ratios of stripe width and masked area could be lowered by this precursor. >

Using spatially and spectrally resolved low-temperature cathodoluminescence, we have investigated GaInAs/InP quantum wells grown by selective epitaxy. For quantum wells with a nominal thickness of 0.6 nm we observe a splitting of the luminescence into several lines due to the presence of fluctuations in the quantum well thickness. On 1.75/spl times/1.75 mm/sup 2/ sized patterns we observe large monolayer flat terraces up to a size of 90 /spl mu/m. >

We investigated optical properties (cathodoluminescence linescans) of selectively deposited quantum wells. As expected for selective MOVPE, composition and growth rate of bulk layers and quantum wells with thicknesses above 6 nm are determined by gas phase diffusion. Thin quantum wells (L/sub z/

Selectively deposited GaInAs InP quantum wells show extended monolayer terraces which are significantly larger than corresponding monolayer islands observed in the case of conventional non-selective epitaxy. We investigated the relation between the growth conditions and the formation of the monolayer step structure in lateral direction by cathodoluminescence. The influence of the arsine supply during GaInAs growth appeared to be the crucial parameter. A reduced V III ratio during GaInAs growth results in a disappearance of the monolayer step structure. It is possible to control separately the vertical interface abruptness, limited by an As P exchange at the interface, and the lateral interface configuration (terraces).

We have studied the optical gain in nitride-based laser structures. We find evidence for excitonic gain at room temperature. A strong polarization dependence of the gain is observed, in accordance with the band structure. We have used optical gain spectroscopy employing the stripe excitation method in order to elucidate the mechanisms of optical gain in GaInN-GaN and GaN-AlGaN double heterostructures and quantum wells.

Both spontaneous and stimulated emission processes are essential ingredients for constructing a laser from the nitrides. Based on our picosecond time-resolved photoluminescence studies we show that spontaneous radiative recombination is strongly influenced by excitonic effects, both in bulk GaN and in quantum wells. Particularly in quantum wells, localization of excitons plays an important role. We have studied the optical gain spectra in GaInN/GaN and GaN/AlGaN double heterostructures and quantum wells, grown by LP-MOVPE, using the stripe excitation method. Both room temperature and low temperature measurements were performed. Based on our results, we discuss the physical mechanism of optical gain in the nitrides as well as consequences for laser operation. We show that localization or, equivalently, the formation of quantum dot like structures, governs the optical gain mechanism in the nitrides.