Structural and photoluminescence properties of In-0.9(Ga/Al)(0.1)As self-assembled quantum dots on InP substrate

Self-assembled In0.9Ga0.1As, In0.9Al0.1As, and InAs quantum dots (QD) were fabricated in an InAlAs matrix lattice-matched to an InP substrate by molecular beam epitaxy. Preliminary characterizations were performed using transmission electron microscopy, photoluminescence, and reflection high-energy electron diffraction. Experimental results reveal clear differences in QD formation, size distribution, and luminescence between the InAs and In-0.9(Ga/Al)(0.1)As samples, which show the potential of introducing ternary compositions to adjust the structural and optical properties of QDs on an InP substrate. (C) 2000 American Institute of Physics. [S0021-8979(00)10213-0].

A study on strong room temperature photoluminescence of a-SiNx : H films

Photoluminescence measurements have been performed in Si-rich a-SiNx:H (x less than or equal to 1.3) alloys prepared by glow discharge. It is observed that the blue shift of the peak of room temperature luminescence spectrum with increasing N content parallels increasing intensity. Two distinct luminescence mechanisms are proposed in a-SiNx:H with the threshold near x = 0.8. For low x, the samples show typical luminescence properties of a-Si:H, while for high x, the normalized luminescence bands are independent of temperature. Combining percolation theory, the luminescence origins are discussed on the basis of Brodsky's quantum well model. (C) 2000 Elsevier Science B.V. All rights reserved.

Photoluminescence studies of type-II self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots grown on (311)A GaAs substrate

We investigated the photoluminescence (PL) of self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots (QDs) grown on (311)A GaAs substrate. The PL peak at 10 K shifts to lower energy by about 30 meV when the excitation power decreases by two orders of magnitude. It has a red-shift under pressure, that is the character of X-like transition. Moreover, its peak energy is smaller than the indirect gap of bulk Al0.5Ga0.5As and In0.55Al0.45As. We then attribute that peak to the type-II transition between electrons in X valley of Al0.5Ga0.5As and heavy holes in In0.55Al0.45As QDs. A new peak appears at the higher energy when temperature is increased above 70 K. It shifts to higher energy with increasing pressure, corresponding to the transition from conduction Gamma band to valence band in QDs. The measurements demonstrate that our In0.55Al0.45As/Al0.5Ga0.5As quantum dots are type-II QDs with X-like conduction-band minimum. To interpret the second X-related peak emerged under pressure, we discuss the X-valley split in QDs briefly. (C) 2000 American Institute of Physics. [S0003-6951(00)04622-2].

Effect of In-mole-fraction in InGaAs overgrowth layer on self-assembled InAs/GaAs quantum dots

We have studied the optical and structural properties of InAs/GaAs QDs covered by InxGa1-xAs (0 less than or equal to x less than or equal to 0.3) layer using transmission electron microscopy, photoluminescence (PL) spectra and atomic force microscopy. We find that the strain reduces in the growth direction of InAs islands covered by InGaAs instead of GaAs layer. Significant redshift of PL peak energy and narrowing of PL linewidth are observed for the InAs QDs covered by 3 nm thick InGaAs layer. In addition, atomic force microscopy measurements indicate that the InGaAs islands will nucleate on top of InAs quantum dots, when 3 nm In0.3Ga0.7As overgrowth layer is deposited. This result can well explain the PL intensify degradation and linewidth increment of quantum dots with a higher In-mole-fraction InGaAs layer. The energy gap change of InAs QDs covered by InGaAs may be explained in terms of reducing strain, suppressing compositional mixing and increasing island height. (C) 2000 Elsevier Science B.V. All rights reserved.

Interband luminescence and absorption of GaNAs/GaAs single-quantum-well structures

We have investigated the interband electron transitions in a GaNAs/GaAs single quantum well (QW) by photoluminescence and absorption spectra. The experimental results show that the dominant photoluminescence at low temperature and high excitation intensity originates from transitions within the GaNAs layer. The interband transition energy for QWs with different well widths can be well fitted if a type-II band line up of GaNAs/GaAs QWs is assumed. (C) 2000 American Institute of Physics. [S0003-6951(00)03220-4].

Photoluminescence in Si and Be directly doped self-organized InAs/GaAs quantum dots

We report on the photoluminescence in directly Si- and Be-doped self-organized InAs/GaAs quantum dots (QDs). When the doping level is low, a decrease in linewidth is observed. However, it will decrease the uniformity and photoluminescence peak intensity of QDs when the doping level is high. We relate this phenomenon to a model that takes the Si or Be atoms as the nucleation centers for the formation of QDs. (C) 2000 Elsevier Science B.V. All rights reserved.

Annealing effect on the surface morphology and photoluminescence of InGaAs/GaAs quantum dots grown by molecular beam epitaxy

Postgrowth rapid thermal annealing was performed on InGaAs/GaAs quantum dots grown by molecular beam epitaxy. The blue shift of the emission peak and the narrowing of the luminescence line width are observed at lower annealing temperature. However, when the annealing temperature is increased to 850 degrees C, the emission line width becomes larger. The TEM image of this sample shows that the surface becomes rough, and some large clusters are formed, which is due to the interdiffusion of In, Ga atoms at the InGaAs/GaAs interface and to the strain relaxation. The material is found to degrade dramatically when the annealing temperature is further increased to 900 degrees C, while emission from quantum dots can still be detected, along with the appearance of the emission from excited state. (C) 2000 Elsevier Science B.V. All rights reserved.

The structural and photoluminescence properties of self-organized quantum dots in InAs/In0.53Ga0.47As multilayer on InP substrate

Self-organized InAs/In0.53Ga0.47As quantum dot (QD) multilayers were grown on InP substrate by molecular beam epitaxy. The structural and optical properties were characterized by using cross-sectional transmission electron microscopy (TEM) and photoluminescence (PL), respectively. Vertically aligned InAs quantum dots multilayer on InP substrate is demonstrated for the first time. Photoluminescence with a line width of similar to 26 meV was observed from the QDs multilayer. (C) 2000 Elsevier Science B.V. All rights reserved.

Pressure behaviour of self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots with multi-modal distribution in size

The pressure behaviour of In0.55Al0.45As/Al0.5Ga0.5As self-assembled quantum dots (QDs) has been studied at 15 K in the pressure range of 0-1.3 GPa. The atomic force microscopy image shows that the QDs have a multi-modal distribution in size. Three emission peaks were observed in the photoluminescence (PL) spectra, corresponding to the different QD families. The measured pressure coefficients are 82, 93 and 98 meV GPa(-1) for QDs with average lateral size of 26, 52 and 62 nm, respectively. The pressure coefficient of small QDs is about 17% smaller than that of bulk In0.55Al0.45As An envelope-function calculation was used to analyse the effect of pressure-induced change of barrier height, effective mass and dot size on the pressure coefficients of QDs. The Gamma-X state mixing was also included in the evaluation of the reduction of the pressure coefficients. The results indicate that both the pressure-induced increase of effective mass and Gamma-X mixing respond to the decrease of pressure coefficients, and the Gamma-X mixing is more important for small dots. The calculated Gamma-X interaction potentials are 15 and 10 meV for QDs with lateral size of 26 and 52 nm, respectively. A type-II alignment for the X conduction band is suggested according to the pressure dependence of the PL intensities. The valence-band offset was then estimated as 0.15 +/- 0.02.

Photoluminescence study of multilayer In0.55Al0.45As/Al0.5Ga0.5As quantum dot at various temperature

The photoluminescence of self-assembled multilayer In0.55Al0.45As/Al0.5Ga0.5As quantum dot (QD) was measured at various temperatures. Strong photoluminescence of wetting layer (WL) and quantum dots were observed at the same time. Furthermore, direct excitons thermal transfer process between the wetting layer and quantum dots was observed. In the study of temperature dependence of PL intensity it was found that the PL peak of wetting layer contains two quenching processes: at low temperature, excitons are thermally activated from localized states to extended two-dimensional states and then trapped by QDs; at high temperature excitons quench through the X valley of barriers. Using rate equation excitons thermal transfer and quenching processes were analyzed quantitatively.

Temperature dependence of the optical properties of InAs/GaAs self-organized quantum dots with bimodal size distribution

In this work we report the optical and microscopic properties of self-organized InAs/GaAs quantum dots grown by molecular beam epitaxy on (1 0 0) oriented GaAs substrates. A distinctive double-peak feature of the PL spectra from quantum dots has been observed, and a bimodal distribution of dot sizes has also been confirmed by scanning tunneling microscopy (STM) image for uncapped sample. The power-dependent photoluminescence (PL) study demonstrates that the distinctive PL emission peaks are associated with the ground-state emission of islands in different size branches. The temperature-dependent PL study shows that the PL quenching temperature for different dot families is different. It is shown that the coupling between quantum dots plays a key role in unusual temperature dependence of QD photoluminescence. In addition, we have tuned the emission wavelength of InAs QDs to 1.3 mu m at room temperature. (C) 2000 Elsevier Science B.V. All rights reserved.

Methods to tune the electronic states of self-organized InAs/GaAs quantum dots

After capping InAs islands with a thin enough GaAs layer, growth interruption has been introduced. Ejected energy of self-organized InAs/GaAs quantum dots has been successfully tuned in a controlled manner by changing the thickness of GaAs capping layer and the time of growth interruption and InAs layer thickness. The photoluminescence (PL) spectra showing the shift of the peak position reveals the tuning of the electronic states of the QD system. Enhanced uniformity of Quantum dots is observed judging from the decrease of full width at half maximum of FL. Injection InAs/GaAs quantum dot lasers have been fabricated and performed on various frequencies. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Tunneling behaviors of photogenerated electrons in In0.15Ga0.85As/GaAs quantum well photoelectrodes

The photovoltaic spectral features and the behaviors of photocurrent versus the electrode potential for near surface In0.15Ga0.85As/GaAs quantum well electrodes have been investigated in nonaqueous solutions of ferrocene and acetylferrocene. The photovoltaic spectrum shows a sharp structure that reflects confined state-to-state exciton transition in the quantum well. Deep dips are observed in the photocurrent versus the electrode potential curves in both electrolytes at the different electrode potentials under the illumination of exciton resonance wavelength. These dips are qualitatively explained by considering the interfacial tunneling transfer of photogenerated electron within the quantum well.

Effect of Si overgrowth on the structural and luminescence properties of Ge islands on Si(100)

The effect of Si overgrowth on the structural and luminescence properties of strained Ge layer grown on Si(1 0 0) is studied. Capping Si leads to the dissolution of Ge island apex and reduced island height. The structural changes in island shape, especially in chemical composition during Si overgrowth have a large effect on the PL properties. The integrated PL intensity of Ge layer increases and there are large blue shifts in peak energies after capping Si. The PL spectra from buried Ge layer are consistent with type-II band alignment in SiGe/Si. We show that the PL properties from buried Ge layer may be tailored by modifying the cap layer growth conditions as well as post-growth annealing. (C) 1999 Elsevier Science B.V. All rights reserved.

Improvement of photoluminescence of strained SiGe/Si layers on patterned Si substrate

A strained SiGe/Si superlattice structure has been grown on a patterned Si substrate and its photoluminescence has been studied. The patterned substrate is composed of pyramid-like structures. It is found that there are Ge-rich SiGe quantum wires (QWR) at the crossings of adjacent planes that form the pyramid-like structure. Photoluminescence of strained the SiGe layer grown on a planar substrate and a patterned substrate was compared. The total intensity of photoluminescence from the patterned substrate was 5.2 times larger than that from the planar substrates. The result is discussed and it is believed that this increase in photoluminescence is related to the observed QWRs. (C) 1999 Elsevier Science Ltd. All rights reserved.