Self-organized semiconductor structures with reduced dimensionality are currently attracting strong interest due to the potential device advantages.The size

distribution and density of quantum dots(QDs)influence on the optical and electrical properties of QDs device.The QDs with bimodal size distribution have been investigated widely due to its some distinctive features compared to the single Gaussian distribution QDs.Thus

it is very important to understand the effects of the growth parameters and mechanisms on the growth of bimodal size distribution QDs.The InAs QDs are grown on the(100)semi-insulating GaAs wafers in Riber 32 compact 21 solid source molecular beam epitaxy(MBE)system equipped with a valved cracker cell for arsenic and reflection high-energy electron diffraction(RHEED)for

in situ

monitoring of the growth process.Growth temperature is calibrated by infrared pyrometer.First

a GaAs buffer layer with thickness of 100 nm was deposited

and then a single layer of quantum dots was grown by depositing InAs with thickness of 2.7ML at different growth temperature(520℃ or 480℃)

finaly a 15 nm GaAs cap layer was deposited.The RHEED pattern shows a transformation from streaky to well developed diffraction spots

typical of a three dimensional(3D)growth mode.The same structures without cap layer were grown for atomic force microscopy(AFM)topographic investigation.The measurements of the distribution and topographic images of uncapped dots were taken ex situ by Nanoscope Ⅲ AFM.The photoluminescence(PL)measurements were performed in closed-cycle N

2

cryostat under the excitation of 514.5 nm line of an Ar

+

laser.Its luminescence spectra were detected with a Fourier transform infrared spectrometer operating with an InGaAs photodetector.Effect of growth temperature on the evolution of bimodal QDs is investigated by combining AFM and PL.The QDs density is significantly reduced from 8.0×10

10

cm

-2

to 5.0×10

9

cm

-2

as the growth temperature increases from 480℃ to 520℃

while the average QDs diameter and height becomes larger.The formation of bimodal QDs depends on the growth temperature:at 480℃ growth temperature

large QDs result from the small QDs coalition;at 520℃ growth temperature

indium desorption and InAs segregation result in the formation of small QDs.