月1絲氏 41 (課程博士(英文))Date of submission (month day, yeal')

January 8小 2021

Department of電気電子佶報工学専

SludcntlD Numbcr

攻1〕plicanl's name

Wakahafa Akihiro

D179201
SUPOTvisoTS

SekiguchiHiroto

Piedfa Lorenzana Jose Nberto

Abstract (Dodo")
Title ofThesis"PI)osphide based type'11Stacked quantum dot arrays enhancing Cωlier
Spatial separation for n1いlti・junction Ⅱ1・V/siphotovoHaic application"

Approx.80o words

Gap based Ⅱ1'V'N materials have atu'acted attention as absorber layer in solar ce11Sfor their lar宮e tunable band 曾ap which cover l.5‑2.o ev under lattice n)atching
Conditions to silicon. Despite this advantageS Π1・V・N 部,owth on si femains as aComplicated task to overcolne due to the genefation of structural defects, such as
thfeading dislocations, stacking faults and antiphase domains. optimization ofgrowth
Conditions and und田'standin牙 of Gap epitaxy is essentialfor the integration of111・V・N
materials and si subS仇'ates by Gap/si hetefoepitaxy. Along with the crystal defects
Onginated from the inta'face Cafrler recombination due point defects andN、composition inhom0牙eneity result in poof convefsion efficiency
Accolnplish hi曹h 仇'ystal quality epitaxy of Gap on si subsu,ate has been achieved
throU曾h great eff0此. Diffusion length of adatom on the 部Own S曝イace is one of thedecisive fact0鵄 to unde玲tand the growth mechanism since for techniques such as
MBE it oP田'ates outside the thetmodynamic equilibhum and the inta,action between
the adatoms, for 宮1'owth temperature of 550゜c and 600゜C 北 Seems that the diffいSionmechanisln is affected by the interaction ofthe Ga adatom and the dime玲 ofthe (2×4)
飢U'face reconstruction.＼入7e condude that the Gap (0OD exhibits similar diffusion

mechanism as the GaAS (0OD for some growths cond北ions This behavior is also
important in the fofmation of nanostTuctures such as quantum weⅡS and quantumdots.

The 牙oal of this work is to contribute to the achievement of efficient 111・V・N
Photovoltaics on si substrates. FⅡ'st, we focus on the diffusion n〕echanism of Gap

holnoeP北axy 牙τOwth by the estimation of the dif61Sion leno'ths of two principal axes
and its relation to the 今fowth temperature. Directional anis0仇、opy was observed alongthe <110> and <11‑0> d辻ections. This anisotropy is hi牙hly consistent with the surface
mの'ph010牙y, this that if islandS 雛'e formed elon今ation of the 牙rowth island alon牙<11刀> is also consistent with larger diffusion len宮th alon牙 the <11刀> d辻ection. Thisanisotropy is usefL11 for the self・annihilation of the antiphase domains in the Gap/si
heteroepitaxy.1n our results for low growth temperature there is not strong anisotropy

Of diffusion lengths, so the seH・annihilation of antiphase dolnains is lesS Ⅱkely toOccur

The next part for the in〕.provement of 111・V・N solar ce11S efficiency, we proposed thetype、n band alignment QDs in a lnp/GaASPN stacked structure foT the improvement
Of the cmlier tec011ection. Type'11 band alignment ptoduce the ca11ier spatial
Separation, by this we expectto rec0ⅡectCωli田's with the QDS, and with a stacked QD
StYuctuTe been able to extract them from the GaASPN solar ceⅡ. FⅡ'st we focus on thefonnation of self'assen)bled lnp Quantum dots (QD)s on Gap. sevefal growth
Conditions were studied. For most of the studied conditions two types of QDs wereObserved charactaized by their 牙eometry lt was found that for low growth fate andVn11ratio more uniform and sma11er QDs can be produced, a transition in QD size wasObsa'ved for specific cond北ions. since the nucleation of lar曾er QDs tends to gen仇'ate

仇'ystal defects, these conditions should be avoided. The lnp QD growth conditions
Used in the present work weTe far 丘'oln typical ones foT lnp homoepitaxy,i.e., very low
牙rowth rate and very low p overpressure. consid田'ing these S北Uations, tl)e effect ofthe v/1Π ratio and/or 部'owth rate on lnp QD size can be dominated by the Tate of pdesorption 丘'om the va虹Ous facet surfaces that fon〕〕 tl)e lnp island

Capping process and stacking of QDs was studied. GTowth interfuption pfocess durin今the capping of the QDs was used to improve the holnogeneity of tl)e stacked QDS

Optilnized growth condition of o.025ML/s and v/1Π ratio of20 togetl〕eT W北h a two・step宮τOwtl〕 interruption consisted of a relnoval of excess ln atomS 丘oln tl)e top ofthe QD
by the 宮τOwthinterruption ofthe partialcapped QD, and the posterioY aattening ofthe

Gap cappin牙 layer by a second 部'owth interTuption process.1＼ハ,o samples werePrepared for the two、step growth interruption evaluation, one lncluded the growthinterruption process and the second was grown without that process. For the sample
Without the 今rowth interruption, the size of stacked QD in仇'eased and defectsappeared in route of stacking.1n con捻ast, salnple with the proposed method, clear
Stacldng ofthe lnp QDs through the layers was present.1n this case, the heigl〕t ofthe
QDs was slightly inの'eased hom 3 t04 nm 丘oln the b0犹om to the top layers proved toCon仇'olthe QDs size and unifonnity Achievin牙 a good quality 30 period lnp/Gap
Structure withoutthe ilnplen)entation of any strain compensation layers
These results can be applied in the f0110wing GaASPN material. And may help indesigning mの'e efficient Gap'based 1Π一V‑N solar ce11S on sisubstrates.