integration of iii  v semiconductor devices on silicon substrates would allow the fabrication of high efficiency optoelectronic and photonic devices using the well - established complementary metal oxide semiconductor ( cmos ) technology . although of the great technological interest , results in this field are far from being optimized . some issues like lattice mismatch between gaas and si and formation of anti - phase domain in gaas are not yet fully solved . one attractive approach to minimize these issues is the fabrication of local artificial substrates ( las ) . the concept of las lies in the possibility to prepare a suitable surface for a subsequent epitaxial growth not on the whole surface of the sample , but locally , using as base some structures fabricated , for example , by a nano - patterning technique . we propose a self - assembling growth procedure based on droplet epitaxy [ 4 - 6 ] for the fabrication of gaas islands on si substrate with high uniformity , selectable size , and density . these islands should act as las for the integration of iii  v high quality nanostructures on si . in our experiments , we used si(001 ) substrates cleaned with standard rca treatment and finally dipped into hf solution to get an h - terminated surface . substrates were loaded in a conventional gen ii molecular beam epitaxy ( mbe ) system equipped with an arsenic valved cell . substrate temperature was raised to 780c to desorb hydrogen , as confirmed with the change in surface reconstruction observed with reflection high energy electron diffraction ( rheed ) from ( 1  1 ) to a mixed ( 2  1 )   ( 1  2 ) . the substrate temperature was decreased to the value reported in table 1 for the ga deposition . a ga molecular beam flux was supplied with a deposition rate of 0.075 ml / s . ga droplets were finally crystallized at 150c by exposure to an as flux of 5  10 torr for 5 min . during the arsenic irradiation , the surface morphology of the five samples was investigated ex situ with a transmission electron microscope ( tem ) and an atomic force microscope ( afm ) . ga deposition temperature , ga coverage , average diameter and density of the islands island density was estimated by large area tem and afm scan ( 2  2 m or more ) . left panel of fig . 1 shows a tem image of the surface of sample e , right panel shows the surface of sample a. the value of island density  reported in table 1 evidences a dependence on the substrate temperature during ga deposition . it is thus possible to use these values to finely tune island density through a proper choice of the ga deposition temperature . varying the amount of deposited ga thus makes possible to control the mean island volume . large area tem scans on surface of sample e ( left panel ) and a ( right panel ) by means of afm measurements , it is also possible to calculate the base diameter , the height , and the size dispersion of the islands . afm measurements also make possible to calculate the aspect ratio ( ratio between height and base diameter ) . this value increases with island size and reaches 0.5 for the larger islands , i.e. when height and radius have the same value . the calculated standard deviation on base size distribution is less than 10% for all the samples . this value is similar to the best values reported for sige islands grown on si patterned substrate . transmission electron microscope measurements confirmed the values obtained by afm for the island mean base size . diffraction pattern obtained from tem measurements allows also to estimate the crystalline quality of the gaas islands . images from a single island ( right panel in fig . 2 for sample a ) show two different clear bragg spots for si and gaas , demonstrating the formation of single crystal gaas islands . diffraction images obtained with diffraction vector   ( left panel in fig . 2 for sample e ) evidence moir fringes . the spacing between fringes gives a good estimation for distances between planes   in si and gaas , and the values we obtained are in agreement with the ones expected for bulk material , thus demonstrating fully relaxation of gaas islands . left panel : tem image of a single island on sample e obtained with diffraction vector . right panel : ( 220 ) bragg spot on diffraction pattern from a single island on sample a figure 3 reports the typical profile of randomly chosen islands on samples b ( left panels ) , d ( central panels , magnified by a factor of 2 ) , and e ( right panels magnified by a factor of 3 ) along   ( upper panels ) and [ 110 ] ( lower panels ) . the slope exposed by the islands is mostly compatible with the plane { 111 } near the si surface and with the plane { 113 } in the upper part of the islands . a similar behavior with the presence of these planes however , by using the droplet , epitaxy technique is possible to achieve a clear improvement in size distribution . from the profiles obtained , we could correlate the increment in the aspect ratio from 0.3 up to 0.5 with the exposed planes on the profiles . the bigger islands expose more portions of surface with { 111 } slope and reduce { 113}. the formation of different surface planes plays an important role for the subsequent fabrication of small quantum dots on the top of local substrates . a stronger affinity for the nucleation of inas small quantum dots on high index surface like { 113 } or { 115 } than on { 111 } is reported . we expect that the control on exposed surfaces could allow the selection of a particular area of the las as a preferential nucleation site for inas quantum dots . afm profiles along   ( upper panels ) and [ 110 ] ( lower panels ) for islands on samples b ( two left panels ) , d ( two center panels ) and c ( two right panels ) we have presented a fabrication procedure for the self - assembly of gaas islands on si substrate via droplet epitaxy to be used as las . the islands are made by a single relaxed crystal and are tunable in size and density changing substrate temperature and amount of supplied ga . the islands expose large portions of surface with a slope compatible with { 111 } and { 113 } , and the aspect ratio reaches 0.5 for larger islands . the control on size , density , and exposed surfaces , the well - defined shape and the low thermal budget requirements make these islands good candidates for the fabrication of las and the integration of iii  v nanostructures on si substrate . this work was supported by the cariplo foundation under the project quadis2 ( contract no . 2008 - 3186 ) and by the italian prin - miur under the project goccia ( contract no . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .