DESCRIPTION Title of the invention: MULTI-JUNCTION SOLAR CELL, COMPOUND SEMICONDUCTOR DEVICE, PHOTOELECTRIC CONVERSION DEVICE, AND COMPOUND-SEMICONDUCTOR-LAYER LAMINATION STRUCTURE TECHNICAL FIELD [OOOI] The present disclosure relates to a multi-junction (also called a tandein-type, a stack-type, or a la~nination-type) solar cell, a cotnpound se~niconductor device, a photoelectric conversion device, atid a compound-semiconductor-layer lamination structure that use a compound semicotiductor. BACKGROUND ART [0002] As a co~npound semiconductor configared of two or Inore types of elenietlts, man)r types exist depending on a conlbination of the eletnents. Also, by laminating a lot of compound senlicotiductor layers made of different tnaterials, a conlpourtd senliconductor device having various fntnctions and various physical properties are achievable. As an exanlple thereof, a solar cell lnay be mentioned. Here, as a solar cell, a silicon-based solar cell that uses silico~ia s a semiconductor, a compound seniiconductor solar cell that uses a compound semicondc~ctor, an organic solar cell that uses an organic material, etc. liiay be mentioned. In particular, the conipoand senliconductor solar cell has been developed aiming further improvement in energy conversion efficiency. I [0003] As a means for inlprovitig energy conversion efficiency of tlie conlpound semicondnctor solar cell, there are provided a niethod in \vliich a plurality of sub-cells each configured of a thin-film solar cell that is configured of a plurality of co~iipo~uisde miconductor layers are laniinated to form a multi-junction solar cell, a method in which an effective con~binatioti of cotnpound semico~iductor liiaterials configaring the conipo~und seniiconductor layers are searched, etc. Each of cotnpound seniiconductors such as GaAs and InP has a lunique band gap, and a ~vavelengtlio f liglit to be absorbed is different depending 011 this difference in band gap. Therefore, by laminating a plurality of types of sub-cells, efficiency of absorption of solar light that has a \vide \~v~velengtrhan ge is improved. In laniination, a combination of lattice constants and physical property values (such as band gaps) of crystal structures of the cotnpound semiconductors configuring the respective sub-cells is important. [0004] By tlie way, most of the multi-junction solar cells under current consideration are classified into a lattice-matched type and a lattice-mismatched type. 111 the lattice-matched type, conipound semiconductor layers are laminated that are made of compound semiconductors having lattice constants that are alniost the same with one another. In the lattice-mismatclied type, compound semico~iductor layers are laniinated that are made of compound seniiconductors having lattice constants that are different from one another with the use of metaniorpl~ic growth accompanied by dislocation. However, in the tiietatnorpliic growtl~ method, undesirable lattice mismatch inevitably occurs, and tl~erefore,t here is an issue that quality of the conipound semiconductors is significantly 2 lowered. [0005] 011 the other liand, in recent years, there has been proposed a method of nianufacturing a multi-junction solar cell that utilizes a substrate bonding teclinique in junction of compound sen~iconductor layers, and a four-junction solar cell that has a strc~ctore of Ino 4aGao5 ~P/GaAs/InGaAsP/In5o3 Ga0.47Ah~a s been reported. [0006] This substrate bonding technique is a technique to form hon~ojunction or lieterojonctio~~be tween the conlpound semiconductor layers to be joined, and niay be classified, for example, into a direct bonding scheme in \\.hich different compo~und semiconductor layers are bonded directly to one another (for example, see Non-patent Literature 1: "Wafer Bonding and Layer Transfer Processes for High Efficiency Solar Cells", NCPV and Solar Progl.am Review Meeting 2003), and a sclieme in \\~hiclic on~pounds e~niconductorla yers are joined with a connection layer in bet\veen. The substrate bonding technique lias an advantage tliat it is not acconlpanied by an increase in threading dislocation. Existence of the threading dislocation leads to a not-preferable effect on electron perfor~nance of the compound se~niconductor layers. In particular, tlie existence of the threading dislocation provides an easy diffusion path in the compound semiconductor layers as with a dopant and a recombination center, and causes a decrease in carrier demsity of tlie conipol~nd se~nicotiductorl ayers. Also, the substrate bonding technique resolves the issue of lattice mismatch, and further avoids epitaxial growth caused by tlie lattice mismatch. Therefore, threading dislocation density tliat degrades the performance of the solar cell is largely reduced. In this substrate 3 boridiiig technique, a covalent bonding is formed in an interface between different substances, in particular, in a hetero interface. At this time, it is important to perfort11 a substrate botidiiig process at a temperatnre by which thermal variation does not exceed a dynamic barrier necessary for progression in threading dislocation. [0007] In junction by the direct bonding scheme, se~iiicotiductor-sel~iiconductobr onding is perfor~iied ia a anclear scale. Therefore, transparency, heat conductivity, heat resistance, and reliability of the jutiction portion are superior than those in a case where junction is for~iiedw ith the use of nietal paste, a glass raw illaterial (frit), etc. In this direct bonding sche~iie, at1 integrated-type or two-terminal compound semiconductor device is allowed to be integrated to a {nodule with simplicity equivalent to that in a solar cell coilfigured of a single-junction device, specifically, only by alloyiiig the respective coiiipouiid semiconductor layers to be lamitlated. CITATION LIST NON-PATENT LITERATURE [OOOS] Non-patent Literature 1: Wafer Bonding atid Layer Transfer Processes for High Efficieiicy Solar Cells, NCPV and Solar Program Review Meeting 2003 SUMMARY OF THE INVENTION [0009] However, in any of the above-described inulti-junction solar cells, contact resistatice of the jutiction portion is relatively high, and large improvement in energy coiiversioii efficiency has not been obtained at present. 4 [OOIO] Therefore, it is desirable to provide a ~iiulti-j~~rictsiolna r cell, a compound se~lliconductord evice, a photoelectric conversion device, and a compound-se~iiicoctductor-layerl aminati011 structure that reduce tlie contact resistance of tlie junction portion and are capable of performing energy collversioli with high efficiency. [0011] A innlti-junction solar cell of an embodiment of the present disclosure includes a plurality of sub-cells that are laminated, the plurality of sub-cells each being configured of a plurality of compound setniconductor layers that are laminated. An amorplions co~inection layer made of an electrically-conductive niaterial is provided in at least one place between tlie sub-cells adjacent to each other. [0012] A compo~uid seiiiiconductor device of an embodinlent of the present disclosure includes a plurality of a plurality of colnpoulid se~nicoliductorl ayers that are laminated. An amorphous collliectio~il ayer ~iiadeo f an electrically-conductive lnaterial is provided in at least one place between tlie compound semiconductor layers adjacent to each other. [00 131 A photoelectric conversioli device of at1 elnbodi~ne~lotf the present disclosure includes a plurality of a plurality of coml~ound semiconductor layers that are laminated. An alnorphous connection layer made of an electrically-cond~~ctivmea terial is provided in at least one place between the compoutld semicot~ductorl ayers adjacent to each other. [00 1 41 A compound-semiconductor-layer lamination structure of an elnboditnent of tlie present disclosure includes a plurality of a pluraIity of co~npound semiconductor layers that are laminated. An amorphous connection layer made of an electrically-co~iductivem aterial is provided in 5 at least one place bet\veen the compound se~niconductor layers adjacent to each other. [0015] In the multi-junction solar cell, the compound semiconductor device, the photoelectric conversion device, and the cotnpound-senliconductor-law la~nillation structure of an e~nbodiment of the present disclosure, the amorphous connection layer made of the electrically-conductive material is provided. Therefore, contact resistance of a junction interface of the sub-cells or the compound semiconductor layers is reduced, and energy conversion efficiency is improved. BRIEF DESCRIPTION OF THE DRAWINGS [0016] [FIG. I ] (A) ant1 (B) of FIG. 1 are conceptual diagrams of compound semiconductor layers atid the like for explaining a method of manufacturing a multi-junction solar cell, a co~npo~uidse miconductor device, a photoelectric conversion device, or a compound-semicol~d~ictor-layelra mination structure in Example 1. [FIG. 21 (A) and (B) of FIG. 2 are conceptl~al diagrams of the compound semiconductor layers and the like for explaining the ~nethod of ~nannfacturing the multi-junction solar cell, the compound semiconductor device, the photoelectric conversion device, or the compound-semicondtictor-layer lalnination structare in Example 1, followii~g( B) of FIG. 1. [FIG. 31 FIG. 3 is a conceptual diagram of compo~untls e~nicondc~ctolary ers and the like for explaining the method of i~lanufacturingt he mnlti-junction solar cell, the compound se~niconductor device, the photoelectric conversion device, or the colnpoluld-semiconductor-layer lamination 6 structure in Example 1, following (B) of FIG. 2. [FIG. 41 (A) and (B) of FIG. 4 are conceptual diagrams of multi-junction solar cells, compound semiconductor devices, photoelectric conversioa devices, or co~npound-semiconductor-law lamination structures in Example 2 and Example 3, respectively. [FIG. 51 (A) and (B) of FIG. 5 are conceptual diagra~us of co~npou~ld semiconductor layers and the like for explaining a method of ~ n a ~ ~ u f a c t ~ ~ r i n g a multi-j1111ction solar cell, a co~npound semiconductor device, a photoelectric conversion device, or a compound-semicotlductor-layer La~~linatiosntr ucture in Example 4. [FIG. 61 (A) and (B) of FIG. 6 are conceptual diagran~s of the compountl seniiconductor layers and the like for explaining the tilethod of inanufacturing the multi-junction solar cell, the compound semiconductor device, the photoelectric collversion device, or the compound-semiconductor-layer lamination slructure in Example 4, followillg (B) of FIG. 5. [FIG. 71 FIG. 7 is a conceptual diagranl of the compound se~llico~~ductor layers and the like for explaining the neth hod of ~iianufacturing the molti-junction solar cell, the compound semiconductor device, the photoelectric conversion device, or the compound-semiconductor-lam l a ~ n i ~ l a tsitoru~c~tu re in Exalllple 4, following (B) of FIG. 6. [FIG. 81 (A) and (B) of FIG. 8 are conceptoal diagrams of compound setnicondoctor layers and the like for explaining a ~lletllodo f manufacturing a multi-junction solar cell, a conlpound semiconductor device, a photoelectric collversio~l device, or a compo~ul~d-semicot~ductor-la~.er 7 lamination structure in Example 5 . [FIG. 91 (A) and (B) of FIG. 9 are conceptual diagrams of the compound se~niconductor layers and the like for explaining the method of ~nanufacturing the multi-junction solar cell, the compound semiconductor device, the photoelectric conversion device, or the compound-se~uiconductor-layer lamination structure in Exanlple 5, follo\\~ing( B) of FIG. 8. [FIG. 101 (A) and (B) of FIG. 10 are conceptual diagratiis of the cotnpound seniicondnctor layers and the like for explaining tlie method of manufacturing the multi-junction solar cell, tlie compound semiconductor device, the photoelectric conversion device, or the compoutld-semiconductor-layer lalnination structure in Example 5, following (B) of FIG. 9. [FIG. 111 FIG. 11 is a schelnatic cross-sectional view of a compound semiconductor device, a photoelectric conversion device, and a compound-sei~iicoaductor-layer lamination structure in Example 6. [FIG. 121 FIG. 12 is a schenlatic cross-sectional view of a conlpound semicondactor device, a photoelectric conversion device, and a componnd-seilliconductor-layer lamination structure in Example 7. [FIG. 131 (A) and (B) of FIG. 13 are conceptual diagranls of a cotnpound semiconductor device, a photoelectric conversion device, or a compound-semiconductor-layer lamination stracture in Esatiiple 8. [FIG. 141 FIG. 14 is a concepti~al diagram of a modification of the multi-junction solar cell, the compound se~niconductor device, the photoelectric conversion device, or tlie colnpound-semico~Iductor-layer 8 laniination structure in Esaniple 1. [FIG. 151 FIG. 15 is a characteristic diagrarii illustrating film-forming characteristics of metal atoms. [FIG. 161 FIG. 16 is a characteristic diagram illustrating a relationship between a thickness of a Ti layer and light tra~~s~nittance. [FIG. 171 (A) and (13) of FIG. 17 are photographs showing a result of an infrared niicroscopic transmission experiment. [FIG. 181 FIG. 18 is a graph showing a relationship bet\veen pl~otone nergy and an absorption coefficient in each concentration of a p-type dopant in a p-type GaAs layer. [FIG. 191 FIG. 19 is a grapli showing a relationship between a thickness of the p-type GaAs layer at p-type dopant concentration of 3 x 1 0 ' ~ and transmittance of solar light at niasimum wavelength of 2.5 ptn. [FIG. 201 FIG. 20 is a photograph of a bright-field image, of an interface of a junction of an TnP substrate and a GaAs substrate, obtained by a scanning transniission electron microscope. [FIG. 211 FIG. 21 is a grapli showing variation in tlie thickness of the Ti layer and the light trans~iiittance over titiie. [FIG. 221 FIG. 22 is a graph showing variation in the tliickncss of tlie Ti layer and the light transmittance over time. [FIG. 231 FIG. 23 is a grapli slio\ving a result of quantitative analysis of concentration of each atoni in each distance in a lamination direction of tlie niulti-junction solar cell in Esaniple 1 based on energy dispersive X-ray spectrometry. [FIG. 241 FIG. 24 is a photograph of a cross-section of a bonding junction 9 interface obtained by a trans~nissiorie lectron microscope. MODES FOR CARRYING OUT THE INVENTION [0017] The present disclosure will be described below based on Exaliiples referring to the drawings. I-lowever, the present disclosure is not liniited to the ~xamples,a nd various numerical values, materials, etc. in the Exa~nplesa re examples. It is to be noted that the description will be given in the following order. I . Description related to general nlatters of a molti-junction solar cell, a compound semicondactor device, a photoelectric conversioli device, and a colnpound-se~iiico~idtictor-1a)rer laniination structure of the present disclosure 2. Example 1 (the multi-junction solar cell, the conlpo~ind seniiconductor device, the photoelectric conversion device, and the colnpound-semiconductor-layer lamination structure of the present disclosure) 3. Exalnple 2 (a lliodification of Esaniple 1) 4. Example 3 (another ~liodificatio~ofi Exaniple 1) 5. Example 4 (another niodificatio~l of Example 1) 6. Exatiiple 5 (a modification of Example 4) 7. Exa~liple 6 (still another inodification of Exa~nple 1) 8. Example 7 (a modification of Example 6) 9. Exatilple 8 (another modification of Example 6) atid others [0018] [Description related to general matters of a multi-junction solar cell, a compound se~niconductor device, a photoelectric conversion device, and a co~npound-semiconductor-layer lanlinatio~i structure of the present 10 disclosure] In a multi-junction solar cell, a compound semiconductor device, a photoelectric conversion device, or a compound-seniiconductor-layer lainination structure of the present disclosure (hereinafter, inay be collectively and simply referred to as "multi-junction solar cell and the like of tlie present disclos~~rein" some cases), depending on lattice constants of co~npou~tsde miconductors that configure adjacent sub-cells or adjacent colllpoulld semiconductor layers (hereinafter, the sub-cells or the co~npound se~nico~lductolary ers may be collectively referred to as "sub-cells and the like" in some cases), the adjacent sub-cells and the like inay be of a lattice-matched type or a lattice-~~iis~iiatclt~ypced. IIo\\~evcr,a s a wholc, these lattice-matched type/ lattice-~i~isliiatclietdy pe exist in a inixed manner. Mere, in the multi-junction solar cell and the like of tlie present disclosure, a con~iectio~lla yer may be preferably provided between these adjacent sub-cells and the like wIie11 the adjacent sub-cells and the like are of the lattice-mismatched type. In the multi-junction solar cell and the like of the present disclosure, the lattice-~iiismatclied type refers to a type in \vhich Illisfit dislocation occurs wlie~i a compound semiconductor layer is epitaxially grown on a certain compound semiconductor, and when the epitasially-gro\\.l1-rot compound senliconductor layer has a thickness over a critical film thickness. It is to be ~lotetl that, in the multi-junction solar cell and the like of tlie present disclosure, \vhere a lattice constant of a compound semiconductor that configures one sub-cell and the like that arc adjacent to the connection layer is LC,, and a lattice constant of a cotnpound se~iliconductor that configures tlie other sub-cell and the like that are 11 adjacent to the connection layer is Lc2, being the lattice-mismatcl~ed type means, for example, being a type that satisfies: ( L C ~ - L C ~ ) Ix L1o C-~~~ ~ (A) or ( L C ~ - L C ~ ) I L C1 0~-3~ - ~ (B) It is to be noted that, when a value of (Lcl-Lc2)ILcl is out of the above-described range, that is, when the value satisfies: - ~ x l o - ~ ~ ( ~ c l - ~ c ~ ) / ~ c l < l x(Cl)o, - ~ it is tlic lattice-matched type. However, Expressioti (A), Expression (B), and Expression (C) are only exainples. Further, in the tuulti-junction solar cell aiid the like of the present disclosure including such a preferable mode, it may be preferable to acllieve a for111 in \vhicli a tuntiel junction layer is provided in a place between the adjacent sub-cells and the like in whic11 the connection layer is not provided. [0019] Here, as described above, the connection layer is a laycr having a~norphous characteristics, and is configured of nletal or alloy. Specifically, as a illaterial configuring the connection layer, it may be preferable to use a material having oh~nicc haracteristics with respect to the coinpoulld seti~iconductorla yer to be connected, more specifically, ~iletalo r alloy having a work function that is smaller than a Fermi level of an 11-type semiconductor or is larger than a Fermi lcvcl of a p-type semiconductor. Accordingly, contact resistance is sigiiificatitly reduced, and favorable ohmic connectio~i is achievable. It is to be noted that "amorphous connection laper" or "connection layer having a~i~orphoucsl~ aracteristics" lllealis a state that does not have long-tern1 orderly characteristics as in 12 crystal and that does not allow a lattice image to be observed in an image taken by a transmission electron tnicroscope as shown in FIG. 24. [0020] A tnetal thin fill11 (having a thickness of several nanometers or less, for example) is typically formed based on a PVD method such as a vacuum evaporation method and a sputtering method. IIomever, at that time, the tlletal thin film is often formed in an island-like shape and is rarely formed in a layer-like shape. Further, when the metal thitl film is formed in an island-like shape, it is difficult to control the fill11 thickness wit11 high accuracy. In forination of a nletal thin fill11 based on the vacuunl evaporation method, often, an island is fornled after atotns and molecules absorbed on a base have uttdergone processes sucl~ as surface diffusion, collision arid aggregation, and desorption, and the island grows to be combined with an adjacent island, thereby forming a continuous thin film. At that time, formation of an island, transitioii from being amorphous to being a crystal layer, variation in crystal aligntnent, etc. occur. [0021] Moreover, when evaporation is performed in a case \\lhere the thickness of the metal thin film is set to about that of a monoatomic layer, it is considered that whether metal atoius have a two-diinensional Iaper structure or a three-dimensional island structure depends on interaction of binding energy between inetal atolns in an uppermost face and tnetal atotns existing therebelow, and binding energy between the tnetal atoins and the base. When the tnetal atolns are more stable to be combined to the nietal atoms, the tnetal atoms have the three-ditnensiot~al island structure. 011 the other hand, when the tnetal atonls are more stable to be con~bincd\ vitl~ the base, the tnetal atoms have the two-dimensional layer structure. FIG. 13 15 illustrates analogized characteristics of binding energy between metal atoms and metal atoms for the respective nletal atorus [binding energy between adato~ns]a nd analogized characteristics of binding energy between nletal atotns and the base (here, GaAs or TnP) [binding energy between adatoni and substrate]. Metal atotns in Group (A) and Group (B) located in an upper region of a dashed line in FIG. 15 have high binding energy with respect to the base, and therefore, it is considered that the metal atoms are allo~ved to have the two-diniensional layer structure. Accordingly, as the inaterial of the connection laper, it may be preferable to use tlie nletal atoms belonging to Group (A) or Group (B). [0022] Therefore, it may be preferable to achieve a Corm in \vhich the connection layer made of an electrically-conductive tnaterial, more specifically, the connection layer configured of metal or alloy includes at least one type of atom (nietal atom) selected from a group consisting of titaniun? (Ti), aluniin~t~(nA l), zirconium (Zr), hafnium (Hf), tungsten (W), tantalun~ (Ta), tnolybdetlu~ii (Mo), tiiobiu~n (Nb), and vanadium (V). It is to be noted that, also when at0111 such as iron (Fe), chromium (Cr), nickel (Ni), and aluminum (Al) is fnrther included in the cotitiectioll layer, the characteristics thereof is riot influenced at all. Further, in this case, a thickness of the amorphous connection layer map be desirably 5 nni or less: and niay be preferably 2 nln or less. FIG. 16 shows a result of measuring a relationship betmeen the thickness of the Ti layer and transniission characteristics of light having wa\~elengtlis within a range fro111 450 nni to 800 nm botli inclusi\~e,f or example, and it can be seen therefrom that light transmittance of about 80% is secured \\.hen tlie thickness is 5 nm or less. Further, by allo~\.i~igth e tl~icknesst o be preferably 2 nln or less, light trans~nittance of 95% or higher is secured. Alternatively, it may bc preferable to achieve a for111 in which the connection layer is configured of a nlaterial selected from a group co~isistingo f aluminutn-oxide-doped zinc oxide [AZO], indio~n-zincc otnposite oxide [IZO], galliu~n-dopedz inc oxide [GZO], indium-gallium coniposite oxide [IGO], In-GaZn04 [IGZO], and i india~n-tinc omposite oxide [lTO], that is, a nlaterial that is transparent and has electric conductivity. Alternatively, it is possible to achieve a form in which the connection laper is made of an amorplious con~pound semiconductor, specifically, portion (to be noted, this is amorphous) of a conlpound semiconductor layer in an interface of a co~npound semiconductor layer and a compound setniconductor layer. In such a for~n, the issue of lattice mismatch is avoidable by providing the portion of the aniorphous compoand se~niconductor layer in between, and occurrence of faults such as dislocation is avoidable. liere, the thickness of the a~norphous connection layer configured of the above-described materials that is transparent and has electric conductivity, or the thickness of the amorphous connectio~l layer configured of an amorphous compound setniconductor may be desirably 1x10.~o r less. It is to be noted that the connection layer may be configured, for exarnple, of the above-described lnetal atoms, and thereby, contact resistance is sufficiently reduced, specifically, the contact resistance is allowed to satisfy p,