Heedon Hwang

Shape change of InAs self-assembled quantum dots induced by As/P exchange reaction

Abstract Self-assembled InAs quantum dots (SAQDs) were grown on InP and on lattice-matched InGaAs buffer layers by metalorganic chemical vapor deposition (MOCVD). The aspect ratio of the SAQD grown on InP increased with temperature and V/III ratio, but it reduced substantially when grown on InGaAs buffer layers. Moreover, the dots formed on InGaAs were faceted, whereas those on InP were dome-shaped, strongly suggesting that the As/P exchange reaction at the surface played an important role in the kinetics of SAQD formation. The shape of InAs SAQDs on InGaAs buffer layers was a truncated pyramid with four {136} facets and their base edges were parallel to 〈130〉 directions.

Analysis of P adsorption and desorption on the (001) InP surface using surface photoabsorption

We present an in situ study of P desorption and adsorption on the (001) InP surface using surface photoabsorption (SPA). The SPA spectra show three peaks at 2.1, 2.9, and 3.1 eV. Here, based on previous studies, we have identified them as a P dimer related and two surface In dimer related peaks, respectively. The initial sharp drop and the slow recovery of the SPA signal at 430 nm after PH3 is switched on implies that at least two steps are involved in the P desorption process. We also observed a structure at 3.1 eV in the spectrum of the In-stabilized surface. We have determined from temperature dependent desorption measurements that the activation energy for P desorption from this surface is 3.36 eV.

Electrical characterization of InAs/InP self-assembled quantum dots by deep level transient spectroscopy

In this paper, electrical characterization of InAs/InP self-assembled quantum dots is studied. The PL signals shown a peak at 0.67 eV with a full-width at half-maximum of about 50 meV.

In situ analysis of surface photoabsorption spectra during InP ALE in metal organic chemical vapor deposition

Abstract In situ monitoring of InP atomic layer epitaxy (ALE) by surface photoabsorption (SPA) was performed in low-pressure metal organic chemical vapor deposition. Self-limiting adsorption condition of In species was studied for various TMIn injection times. A grazing incidence X-ray study on the thickness of the film grown by ALE, however, showed that the growth rate did not reach one monolayer (ML)/cycle. The SPA signal trace measured during the InP ALE indicated incomplete PH 3 decomposition on the methyl-terminated In surface, and this observation was attributed to the cause of submonolayer growth. We also report a spectroscopic SPA study on the surface state during a cycle of ALE process. The SPA spectrum of the methyl-terminated In surface showed a different line shape at around 1.9 eV (In-dimer region) from that of an In-stabilized surface. But, during H 2 purge at 390°C, this portion of the spectrum was observed to evolve to that of an In-stabilized surface by the desorption of methyl radical.

Growth and in situ analysis of InAs/InP quantum dot stack and its far infrared absorption properties

InAs/InP self-assembled quantum dots (SAQDs) are promising active layers for optical devices for fiber-optic communication. Furthermore, they may be used for the fabrication of uncooled mid and far infrared detectors. InAs/InP SAQDs were grown by low pressure-metalorganic chemical vapor deposition, where As/P exchange reaction and growth interruption step play an important role. The InAs quantum dot (QD) stacks were successfully grown on (001) InP substrate and their optical properties were characterized. Far-infrared absorption peaks were observed at 819 cm-1 (12.20 μm) and 518 cm-1 (19.35 μm) at room temperature by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Raman analysis showed that the peak at 819 cm-1 was attributed to a plasmon related peak in the n-type InP substrate. On the other hand, the absorption peak at 518 cm-1 was regarded as a peak related with intersubband transition in the InAs QDs, suggesting that room temperature operating quantum dot infrared photodetectors (QDIPs) can be fabricated. In situ monitoring of the QD evolution and stacking sequences were also discussed.

Possibility of Two-Step As-Desorption from (001) InP Using Surface Photoabsorption

We present an in situ investigation of As-desorption from the (001) InP surface using surface photoabsorption (SPA). At 470°C, we observed that the SPA signals along [110] remained unchanged even after the AsH3 supply was turned off, while the SPA signal along [110] showed a sharp increase as reported previously. We also measured SPA spectra at several stable states and found that the surface after AsH3 was turned off could not reach that of an In-stabilized surface. We interpret these results in terms of As-desorption occurring possibily as a two-step process, in contrast to P-desorption of a one step process.

Analysis of Surface Photoabsorption Spectra of (001) InP Surfaces

We present an in situ surface photoabsorption (SPA) study on surface structures of (001) InP surfaces. Conventional subtraction spectra of SPA on (001) InP showed two peaks at 430 nm and 600 nm, which we assigned as P dimer and In dimer peaks, respectively. By modifying the conventional subtraction equation to separate contributions from In- and P-stabilized surfaces, we could observe another structure at 400 nm which we interpret as related to In-stabilized surface, showing possibility of complementary role of this modified equation. We also observed evolution of surface states from P- to In-stabilized surfaces at 600°C by the change of PH3 partial pressure.

Two-step As-desorption from (001) InP observed by surface photoabsorption

The investigation of As desorption from the (001) InP during metalorganic chemical vapor deposition was performed using surface photoabsorption (SPA). The behavior of the monochromatic SPA signal showed a clear existence of a metastable state after the AsH3 was turned off. SPA spectra at each stable surface were taken to confirm the interpretation. This result indicates that the previous criterion to measure the amount of As/P exchange reaction is not necessarily correct and that the As-desorption process should be understood as a two-step process.

Effects of thin GaAs cap layer on optical properties of InAs/InGaAs/InP quantum dot

We have studied the influence of thin GaAs capping layer (5-10 monolayers) on optical properties of InAs QDs. After the growth of thin GaAs capping layer, the 70 nm thick top InGaAs capping layer was deposited at 570 /sup o/C or 610 /sup o/C. The insertion of thin GaAs cap layer on QDs led to blue shift of peak of photoluminescence (PL). In addition, full width at half maximum (FWHM) of the PL peak decreases from 35 mev to 20 mev by inserting 10 ML GaAs layer.

In-situ observation of As/P exchange reaction and As carryover in InAs/InP quantum well structures by surface photoabsorption

InAs/InP quantum well structures were grown by low pressure metal organic chemical vapor deposition. During the growth, the As/P exchange reaction and As carryover were monitored in-situ by surface photoabsorption. We simulated the measured SPA signal using a multilayer model and effective medium theory to obtain the As compositional profile which was carried over into the InP layer.