as explained above in relation to gaas - based devices , in terms of the window performance , it is desirable to use a window layer having high aluminium fraction , but this is not generally advisable for practical reasons concerning oxidation / hydrolysis . the present inventors have devised devices and methods of fabrication in which it is possible to suppress exposure of the window layer to oxidizing species . in some preferred embodiments , this is done by ensuring that a protection layer ( e . g . gaas , ( al x ga 1 - x ) 0 . 51 in 0 . 49 p , al x ga 1 - x as ) is maintained to cover the window layer during and after the removal of the contact layer ( also referred to herein as “ cap layer ”). in one embodiment , the protection layer itself can be employed as the etch - stop layer in the selective etching process of the cap layer . alternatively , in another embodiment , a dedicated etch - stop layer is introduced ( between the protection layer and cap layer ( contact layer )). this etch - stop layer assists in the selective etching process of the cap layer . after the etch - stop layer has served its purpose , it may be removed using another selective etching process , before further device processing ( e . g . the subsequent deposition of an arc ). alternatively , it may be left in place , with / without further modification ( e . g . densification / dehydration by thermal annealing ), before further device processing ( e . g . the subsequent deposition of an arc ). three design options as set out in tables 1 , 2 and 3 below . these design options are further illustrated in fig6 . the structure and function of the window protection layer and of the etch - stop layer are described below . to suppress oxidation / hydrolysis of the window layer , a window protection layer is introduced . the window protection layer covers the window layer . the window protection layer is composed of a semiconductor material that has a substantially lower propensity to oxidise / hydrolyse than the window layer . for example , candidate materials include al x ga 1 - x as with 0 & lt ; x & lt ; 0 . 8 , and ( al x ga 1 - x ) 0 . 51 in 0 . 49 p with 0 & lt ; x & lt ; 1 . 0 . in general , the lower the aluminium content of a semiconductor , the more resistant it is against oxidation / hydrolysis in a gaas - based system . note that an al x ga 1 - x as with x = 0 ( i . e . gaas ) surface is also susceptible to atmospheric oxidation . however , in this case , the oxide forms a dense , unbroken layer with a thickness of about 3 nm after 8 days [ reference 20 ]. the diffusion - limited growth rate is highly parabolic with d ( nm )= 0 . 5969 + 0 . 5929 log [ t ( min )]. soaking gaas in h 2 o 2 forms a stable native oxide that is about 1 . 4 - 1 . 7 nm thick , and the logarithmic growth rate is considered to be slow enough to be effectively a self - limiting ( diffusion - limited ) process [ reference 21 ]. since the window protection layer generally has a relatively low aluminium fraction , it also has a lower bandgap energy than the window layer . hence , in order to avoid substantial absorption of useful photons , the window protection layer should not be thicker than is necessary to prevent or significantly reduce oxidation of the window layer . the minimum window protection layer thickness required depends on the layer composition , device design and device processing , but it is in the range of approximately 1 - 60 ml ( ml is monolayer ) [ reference 22 ]. the maximum thickness that is advisable depends on the bandgap energy of the protection layer and the energy range of useful photons . fig7 illustrates the absorptance of a 1 nm thick window protection layer of gaas ( based on bulk absorption coefficients , ignoring any quantum confinement effects ), showing that the layer thickness should be kept to a minimum . using an al x ga 1 - x as or ( al x ga 1 - x ) 0 . 51 in 0 . 49 p window protection layer , the absorptance decreases with increasing x . it is expected that , when the thickness of the window protection layer is very small , the absorption coefficient may be different than that for bulk layers , due to quantum confinement effects . high doping levels may also modify the absorption coefficients , due to bandgap narrowing and the band - filling effect known as the burstein - moss shift [ reference 23 ]. in principle , as long as the composition of the window protection layer is not identical to that of the cap layer , there is potential for the protection layer to also function as an etch - stop layer during the selective etching process that removes the cap layer . for example , this is possible in the case of the epitaxial layer structure tabulated in table 4 . the gaas cap layer can be etched selectively over al 0 . 3 ga 0 . 7 as [ reference 24 ] such that the al 0 . 3 ga 0 . 7 as layer functions as an effective etch - stop layer . for the selective etching of gaas over al 0 . 3 ga 0 . 7 as , reference 25 reported s = 116 using a c 6 h 8 o 7 : h 2 o 2 - based etch , and for selective etching of gaas over al 0 . 2 ga 0 . 8 as , reference 26 reported selectivity as high as s = 256 using a c 6 h 8 o 7 : h 2 o : h 2 o 2 - based etch . in addition , the al 0 . 3 ga 0 . 7 as layer serves to protect the window layer from the selective etch solution and any exposure to water / air with a precisely known thickness of al 0 . 3 ga 0 . 7 as material that is resistant to oxidation / hydrolysis . fig8 shows plots of the transmission of light , t ( λ ), calculated for normal incidence when travelling from air to gaas media , through an anti - reflection coating ( 108 nm mgf 2 / 62 nm zns ), a gaas window protection layer of 1 . 0 nm or 2 . 0 nm , and al 0 . 85 ga 0 . 15 as window layer . for reference , also shown are plots for as - grown and part - oxidised layers with the anti - reflection coating . the plots were calculated as for fig5 . in another embodiment ( see table 2 and fig6 ), a second layer is introduced , between the window protection layer and the cap layer . this layer is dedicated to functioning as an etch - stop layer in the selective etching process of the cap layer . the etch - stop layer is composed of a semiconductor that provides etch selectivity during the process of removing these areas of the cap layer ( s ) by an appropriate wet / dry selective etching process . allows more freedom in choice of the protection layer and etch - stop layer composition and thickness , and in the choice of the selective etch process suppresses exposure of the window layer to oxidising / hydrolysing species during the selective etching process , and thereafter permits cleaning ( e . g . deoxidation ) of the semiconductor surface prior to arc deposition without damaging the window layer for example , this it is possible to take advantage of these features in the case of the epitaxial layer structure shown in table 5 . the gaas cap layer can be etched with very high selectively over alas [ references 13 , 15 ]. then , if desired , the thin alas layer ( and any native oxides ) can be etched away with high selectively over al 0 . 3 ga 0 . 7 as [ reference 35 ] leaving a clean al 0 . 3 ga 0 . 7 as window protection layer in place , ready for the arc layer deposition . alternatively , the etch - stop can be left in place after removing the cap layer , with / without further modification ( e . g . densification / dehydration by thermal annealing ), before further device processing ( e . g . the subsequent deposition of an arc ). if the etch - stop layer itself is not to be etched from the device during processing , the used etch - stop layer may be treated in some other way . for example , it may be thermally annealed prior to the arc deposition in order to modify its composition and change any hydroxide phases to denser , more stable oxide phases , and / or to deplete elemental arsenic and arsenic - based compounds ( e . g . as , as 2 o 3 ). when densified , the native oxide layer can provide an additional barrier against oxidation / hydrolysis of the underlying layers . selective etching may be performed using a wet etch chemistry [ reference 27 ]. the selectivity depends on the materials wet etch solution used . for example , selective wet solutions include : c 6 h 8 o 7 ( citric acid ): h 2 o 2 - based [ references 24 , 28 , 29 ] ( cooling this selective wet etching solution can provide an anisotropic etching profile [ reference 30 ]) c 6 h 8 o 7 ( citric acid ): k 3 c 6 h s o 7 ( potassium citrate ): h 2 o 2 - based [ reference 31 ] c 6 h 8 o 7 ( citric acid ): nh 4 oh : h 2 o 2 - based [ reference 32 ] c 4 h 6 o 4 ( succinic acid ): h 2 o 2 - based , ph - adjusted ( e . g . using nh 4 oh ) [ reference 29 ] c 4 h 6 o 6 ( tartaric acid )- based [ reference 33 ] c 2 h 2 o 4 ( oxalic acid )- based [ reference 29 ] nh 4 oh : h 2 o 2 - based , ph - adjusted [ reference 29 ] hf -, hcl -, h 2 so 4 -, h 3 po 4 -, hno 3 -, hi -, h 3 po 2 -, or nh 4 oh - based solutions , etc . [ references 24 , 34 , 35 ] selective etching can also be performed using a dry etch chemistry ( reactive ion etching ). for example , a dry etch chemistry containing chlorine and fluorine e . g . ccl 2 f 2 - plasma , sicl 4 : cf 3 - plasma or a dry chemistry containing methane and hydrogen , e . g . ch 4 : h 2 plasma . it is preferred that the window layer and cap layer are doped to be the same conductivity type as the device layer ( e . g . emitter ) beneath it . for instance , for a p - n heteroface solar cell configuration ( with an n - gaas substrate , n - gaas base , and p - gaas emitter ) in which the window layer sits directly upon a p - type ( about 2 × 10 18 cm − 3 ) emitter layer , the window layer and cap layer should both be p - type doped ( about 5 × 10 18 cm − 3 and about 5 × 10 19 cm − 3 , respectively ). similarly , the protection and etch - stop layers should be doped to be the same conductivity type ( or nominally undoped ). the window protection , etch - stop and cap layers are preferably doped . a variety of different semiconductor materials may be used in the window , window protection , and cap layer , including one or more of : gaas , alas , inas , gainas , algaas , alinas , algainas , gap , alp , inp , alinp , algainp , gainp , algaasp , gainpas , alinpas , algainpas , gasb , insb , alsb , gaassb , alassb , alinsb , gainsb , gaalassb , algainsb , aln , gan , inn , ga 1 - n n , algainn , gainnas , algainnas , znsse . the window , window protection , etch - stop and / or cap layers may be composed of lattice - mismatched ( to substrate and / or layer beneath the window layer ) semiconductor ( s ). such an arrangement is set out , for example , in u . s . pat . no . 7 , 119 , 271 [ corresponding to reference 36 ]. the light - emitting / light - absorbing layers of the device may contain arrangements of quantum - wells and / or quantum dots . at least one of the layers in the device may be composed of digital alloys . at least one of the layers may be composed of a series of semiconductor materials , or be graded ( continuous , stepped or digitally ), in composition . the window layer may be graded ( continuously , stepped or digitally ) in composition such that the bandgap increases towards the illuminated side . this encourages any minority carriers that are generated in the window layer to migrate to the emitter . it can also help reduce drops in electrical potential across the window layer . the window protection layer may be graded in composition such that the bandgap decreases towards the illuminated side , such as to minimize absorption in the protection layer while maintaining its stability against oxidation / hydrolysis . a passivation layer ( e . g . silicon nitride , sin x , which forms a good diffusion layer against oxidizing species ) may be deposited prior to the arc formation ( or form part of , or all of , the arc layer ). the schemes proposed here do not preclude the use of epitaxial lift - off ( elo )/ substrate transfer . indeed , the layer sequence can be grown in reverse sequence , and a suitable epitaxial lift - off technique used to remove the substrate and expose the cap layer for processing as described . additional etch - stop layers can be added above the cap layer ( i . e . grown before the cap layer ) to assist in elo . low bandgap semiconductor materials ( e . g . inas , in x ga 1 - x as grown on metamorphic buffer layers ) can be used to further reduce the specific contact resistance between the metal contact and the semiconductor layers . growth of a very highly doped cap layer ( s ) of semiconductor ( for example , gaas , in x ga 1 - x as 0 & lt ; x & lt ; 1 ) reduces the specific contact resistance between the metal contact and the semiconductor device layer . typically , a doping density within the 1 × 10 18 to 1 × 10 20 cm − 3 range is used in the cap layer . p ++ - gaas ( c ) or p ++ - ingaas ( c ) may be used to provide low specific contact resistance . typically , the cap layer thickness is about 200 - 600 nm . increasing thickness may increase the series resistance of the device . decreasing the thickness may result in elements from the ohmic contact metallization ( and defects associated with the ohmic contact formation ) diffusing into the active regions of the device , with detrimental effects on performance . delta - doping may be in the semiconductor cap layer to decrease the specific contact resistance between the semiconductor cap layer and the metal contact . cr / au , ti / pd / au , pd / ti / pd / au , ti / pt / au , pd / ti / pt / au , zn - containing alloys ( for example , auzn ), or be - containing alloys ( for example , aube ) may be used for forming electrical contacts to a p - type semiconductor used as the semiconductor cap layer or to a p - type semiconductor used as the semiconductor substrate . cr / au , ti / pd / au , pd / ti / pd / au , ti / pt / au , pd / ti / pt / au , au / ge / au / pd / au , pd / ge / au / pd / au , au / ge / au / ni / au , pd / ge / au / ni / au or ge - containing alloys may be used for forming electrical contacts to n - type semiconductor , where n - type semiconductor is used as the semiconductor cap layer or to a n - type semiconductor used as the semiconductor substrate . pre - treatment of the semiconductor surface may be carried out using oxygen plasma ( asking ) prior to ohmic contact deposition . this step is typically used to remove resist / carbon residues . pre - treatment of semiconductor surface may be carried out using de - oxidising / passivating wet chemical solutions prior to ohmic contact deposition . for example , such solution may be based on hcl , h 2 so 4 , nh 4 oh , ( nh 4 ) 2 s x . pre - treatment of the semiconductor surface may be carried out using dry plasma - based chemistries prior to ohmic contact deposition . for example , nitrogen - based or argon - based plasmas may be used . an anti - reflective coating may be designed and implemented using a single layer or multiple layers of dielectric material ( s ) of the appropriate optical thickness , the design of which is known to those skilled in the art . a preferred single layer system is a layer of sin x of the appropriate refractive index and thickness . other systems include a dual layer arc of zns / mgf 2 , tio 2 / m g f 2 or ta 2 o 3 / mgf 2 . anti - reflective coatings may include sub - layers of many different materials , some of which are as follows : al 2 o 3 , zro 3 , mgf 2 , sio 2 , cryolite , lif , thf 4 cef 3 , pbf 2 , zns , znse , si , ge , te , pbte , mgo , y 2 o 3 , sc 2 o 3 , sio , hfo 2 , zro 2 , ceo 2 , nb 2 o 3 , ta 2 o 5 , and tio 2 [ reference 1 ]. arc protection may be provided using a hydrophobic over - layer . typically , the hydrophobic layer is composed of an organosilane / fluorinated hydrocarbon . the hydrophobic layer may be applied in a layer that is as little as several nm in thickness . the hydrophobic layer may be applied by dipping the anti - reflective layer into a liquid bath of the hydrophobic polymer , or through vapour deposition or by other suitable methods . various hydrophobic materials may be utilized that are well known to those skilled in the art [ reference 2 ]. the semiconductor layer thicknesses and compositions are most preferably included in the optimisation of the arc performance . fig8 shows calculated plots of the transmission performance of solar cell structures having mgf 2 / zns antireflective coating formed on 1 . 0 nm or 2 . 0 nm gaas window protection layer , formed in turn on a 30 nm al 0 . 85 ga 0 . 15 as window on a gaas emitter , in comparison to a solar cell structure having no gaas window protection layer . as shown , the transmission of useful photons through to the light - absorbing regions of the solar cell structure is considerably improved in comparison with the situation where the 30 nm al 0 . 85 ga 0 . 15 as window is part oxidised . fig9 shows the results of measurements carried out to determine the etch selectivity of an etching system designed to etch first the semiconductor cap layer at an appreciable rate , “ stop ” at an etch stop layer , and then the removal of the etch stop layer by a second stage of the etching system , this second stage then “ stopping ” once the etch - stop layer is removed , due to the window protection layer being formed directly beneath the etch stop layer . in fig9 , each sample was etched for a given time in citric acid solution / h 2 o 2 etch 5 : 1 , rinsed in deionised water , then etched for a 120 second fixed buffered hf solution 5 : 1 etch , all at room temperature . the citric acid solution was prepared by dissolving 500 g c 6 h 8 o 7 . h 2 o in 500 ml deionised water . the use of alas ( or , more generally , al x ga 1 - x as ) as a window layer is compatible with both single - junction and multi - junction solar cells . the material is simple to grow . the protection of the window layer from oxidation allows the avoidance of uncertainties and non - uniformities , which in turn allows the more straightforward implementation of an optimised anti - reflection coating . the use of the window protection layer lifts the generally - accepted restriction on the al x ga 1 - x as aluminium fraction , so that the wide bandgap that al x ga 1 - x as offers ( about 3 . 0 ev ) can be more fully exploited . this allows superior window transmission and superior minority carrier confinement . the anti - reflection coating performance is also boosted , through the use of low loss materials ( including the al x ga 1 - x as window ), since the anti - reflection coating can be made with higher optical quality . in effect , a flat , clean surface is presented to the ar coating , substantially free of oxide / hydroxide . this is the ideal surface for the deposition of zns and mgf 2 materials . reflection and scattering that would otherwise be expected from oxidised / hydrolysed al x ga 1 - x as is therefore avoided . known devices tend to degrade over time due to ongoing oxidation of the window layer . the use of the window protection layer substantially reduces such oxidation in service , thereby significantly extending the service life of the device . known window materials such as ( al x ga 1 - x ) 0 . 51 in 0 . 49 p have a relatively low aluminium fraction , and so are relatively robust against oxidation . however , they have inferior bandgap energy ( about 1 . 9 & lt ; e γ & lt ; 2 . 6 ev ) to the preferred window materials used herein , resulting in increased absorption of useful photons in the window and in decreased minority carrier confinement . furthermore , these materials are complex to grow (( al x ga 1 - x ) 0 . 51 in 0 . 49 p is a quaternary system ), and may result in poorer interfaces and increased risk of surface recombination . in order that the skilled person may even more readily understand the effectiveness of the embodiments of the present invention , it is instructive to compare the technical properties of devices with and without a window protection layer . in the following discussion , the layers over the p - type gaas layer in a solar cell device according to an embodiment of the invention were as follows ( moving upwards through the device from the p - type gaas towards the contact layer ): 30 nm p - al 0 . 9 ga 0 . 1 as ( window layer ); 2 . 5 - 5 . 0 nm be + - gaas ( window protection layer ); 2 . 0 nm be + — alas ( etch stop layer ); 2 ml un - gaas ; 300 nm be ++ — gaas ( contact layer ). as has been described above , the gaas protective layer reduces or inhibits algaas oxidation , allowing the use of high - al content window layers to reduce absorbance of useful photons . the etch stop layer allows the layers above the p - type gaas layer to have precisely known thickness after etching using wet chemistry techniques . this allows for predictable performance from the dual - layer antireflective coating ( arc ) system . fig1 shows reflection spectra for a device with an algaas window layer but with no window protection layer , the spectra measured over a period of 20 days . also shown is the day 20 spectrum for a corresponding device with an anti - reflection coating . as can be seen , the reflectance spectra change markedly between days 0 and 20 . this is considered to be due to the formation of an oxide layer on the al 0 . 9 ga 0 . 1 as window layer and the subsequent unpredictable change in surface optical qualities and poor arc performance . fig1 shows the results from an analysis of the reflection spectra measured over an 8 - day period for an unprotected al 0 . 9 ga 0 . 1 as window layer . the lines show fitting using a multi - layer model . the significant change in behaviour is considered to be due to the formation of an inhomogenous oxide layer on the al 0 . 9 ga 0 . 1 as window layer with a porous surface . fig1 and 13 show spectroscopic ellipsometry measurements over an 8 - day period for an unprotected al 0 . 9 ga 0 . 1 as window layer . the lines show fitting using a multi - layer model . fig1 and 15 show the results of a multi - layer model fit of the evolution of degradation of an unprotected al 0 . 9 ga 0 . 1 as window layer . a rough oxide - like inhomogeneous layer grows , consuming algaas . optical scattering due to roughness is apparent to the naked eye by day 4 . a roughness parameter ( see fig1 ) is required in the fit . fig1 shows the variation of refractive index ( n ) and extinction coefficient ( k ) at a wavelength of 400 nm for an unprotected al 0 . 9 ga 0 . 1 as window layer . the fitted optical constants of the layer indicate a transition from semiconductor - like to oxide - like refractive index and extinction coefficient . fig1 shows the results of a multi - layer model fit on the stability of the layer thickness for a protected al 0 . 9 ga 0 . 1 as window layer according to this embodiment of the invention . this shows the stability against air - exposure of the device . fig1 shows reflection spectra for a device with a protected al 0 . 9 ga 0 . 1 as window layer according to this embodiment of the invention . the spectra were measured over a period of 20 days . also shown is the day 20 spectrum for a corresponding device with an anti - reflection coating . as can be seen , there was very little variation in the reflection spectra with time . after deposition of the arc ( zns / mgf 2 ), the device showed low reflectivity . the objectives of this evaluation were to demonstrate the utility and performance of etch stop layers in embodiments of the present invention , and particularly to evaluate : ( i ) whether the etch - stop with ‘ protected window ’ results in a predictable etch depth and in a smooth semiconductor surface , ( ii ) whether the etch - stop process provides a wide processing window , and ( iii ) whether undercut of the mask is prohibitive . the epitaxial layer structures were as shown in tables 7 and 8 . five samples were taken from wafer growth a2217 and patterned ( solar cell grid pattern ) with photoresist by standard photolithography techniques . each piece was etched using the following procedure : ( i ) native oxide removal in dilute hcl acid , followed by a rinse in de - ionised water , ( ii ) selective etching of the gaas cap layer in citric acid : h 2 o 2 ( 5 : 1 ) solution , ( iii ) selective etching of the etch - stop layer in dilute hcl acid , where the time spent in citric acid : h 2 o 2 ( 5 : 1 ) solution was varied . the photoresist was removed in acetone and each sample was then measured using an atomic force microscope ( afm ) to determine the height of the etched step and the surface roughness of the etched material . as a reference , a sample from a2214 with no window protection and etch stop layers was processed and measured . the results of the investigation are set out in table 9 . sample a was etched for a time less than that required to fully remove the gaas layer . the roughness of the etched surface will be the equivalent of having a fixed time etch through a gaas layer . sample b was etched to clear the gaas layer with no significant over etch . in this case it is seen that the etch has stopped on the alas layer as expected with a roughness better than a timed gaas etch ( a ). sample c was etched for a time which allowed clearing of the gaas layer plus some over etch . again it is seen that the etch had stopped on the alas etch - stop layer and that roughness is better than ( a ). sample d was etched for 10 minutes with the intention of finding out when the etch - stop is compromised . in this case it appears that the etch stop was breached during the citric acid : h 2 o 2 etching and that the gaas protective layer was removed before the final dilute hcl etch was performed ( which attacks the algaas window layer ). roughness is slightly worse that that seen where the etch stop remained intact . sample e was etched for 20 minutes . again , it is clear that the etch stop was breached during the citric acid : h 2 o 2 etching and that the gaas protective layer was removed before the final dilute hcl etch was performed . as the gaas buffer layer acted as an etch stop during the final dilute hcl etch , the surface roughness is comparable to c . two samples from piece a2214 were etched as a reference . this sample had no dedicated etch stop layer ( nor protection layer of any kind ) and it was expected that the selective citric acid : h 2 o 2 etch would ‘ stop ’ somewhere on / in the algaas window layer . it can be seen that the results are comparable with sample e . the results above show that the etch stop layer functions as expected and provides a means of forming a protection layer over the window layer with low surface roughness . the etch stop is capable of resisting over - etching , providing a wide processing window . selective etching of the cap layer was carried out using an ohmic contact as an etch mask . a piece of a2217 was processed using a lift - off process to form a patterned p - ohmic metal ( ti — pd — au - based ) etch mask . this metal pattern was annealed using the rta at 360 ° c . and subjected to the same etch process as used for the etch tests above , but with a fixed citric acid : h 2 o 2 etch time of 120 seconds . the sem micrograph of fig1 shows an isolated five micron metal line after etching . in the sem image , the dark line formed by the algaas layer can be seen , and the cap etch terminated above the algaas layer at the etch - stop . it is seen that there was no undercut of the metal finger . thus , the etch stop layer works effectively and provides sufficient process latitude to be used in manufacturing . the embodiments above have been described by way of example . on reading this disclosure , modifications of these embodiments , further embodiments and modifications thereof will be apparent to the skilled person and as such are within the scope of the present invention . 1 i . vurgaftma , meyer , j . r ., ram - mohan , l . r ., “ band parameters for iii - v compound semiconductors and their alloys ”, j . appl . phys ., 89 ( 11 ), 2001 . 2 bode , m . h ., ourmazd , a ., ‘ interfaces in gaas / alas — perfection and applications ’, journal of vacuum science & amp ; technology b , 10 ( 4 ), pp . 1787 - 1792 , 1992 . 3 bimberg , d ., heinrichsdorff , f ., bauer , r . k ., ‘ binary alas / gaas versus ternary gaalas / gaas interfaces — a dramatic difference of perfection ’, journal of vacuum science & amp ; technology b , 10 ( 4 ), pp . 1793 - 1798 , 1992 . 4 bocchi , c ., ferrari , c ., franzosi , p ., bosacchi , a ., franchi , s ., ‘ accurate determination of lattice mismatch in the epitaxial alas / gaas system by high - resolution x - ray diffraction ’, journal of crystal growth , 132 ( 3 - 4 ), pp . 427 - 434 , 1993 . 5 handbook of optical constants of solids ii , e . d . palik , ed , academic press , orlando london , 1991 . 6 dallesasse , j . m ., holonyak , n ., jr ., sugg , a . r ., richard , t . a ., el - zein , n ., ‘ hydrolyzation oxidation of al x ga 1 - x as — alas — gaas quantum well heterostructures and superlattices ’, appl . phys . lett ., 57 ( 26 ), pp . 2844 - 2846 , 1990 . 7j . m . dallesasse , p . gavrilovic , n . holonyak , jr ., r . w . kaliski , d . w . nam , e . j . vesely , and r . d . burnham , ‘ stability of alas in al x ga 1 - x as — alas — gaas quantum well heterostructures ’, appl . phys . lett ., 56 ( 24 ), p . 2436 - 2438 , 1990 . 8 j . m . dallesasse , n . el - zein , n . holonyak , jr ., k . c . hsieh , r . d . burnham , and r . d . dupuis , ‘ environmental degradation of al x ga 1 - x as — gaas quantum - well heterostructures ’, j . appl . phys ., 68 ( 5 ), pp . 2235 - 2238 , 1990 . 9 c . algora del valle , c . alcaraz , m . f ., ‘ performance of antireflecting coating - a1gaas window layer coupling for terrestrial concentrator gaas solar cells ’, ieee trans . electron . dev ., 44 ( 9 ), pp . 1499 - 1506 , 1997 . 10 rey - stolle , i ., algora , c ., ‘ optimum antireflection coatings for heteroface a1gaas / gaas solar cells — part i : the influence of window layer oxidation ’, journal of electronic materials , 29 ( 7 ), pp . 984 - 991 , 2000 . 11 rey - stolle , i ., algora , c ., ‘ optimum antireflection coatings for heteroface algaas / gaas solar cells : part ii : the influence of uncertainties in the parameters of window and antireflection coatings ’, journal of electronic materials , 29 ( 7 ), pp . 992 - 999 , 2000 . 12 van riesen , s ., bett , a . w ., ‘ degradation study of iii - v solar cells for concentrator applications ’, progress in photovoltaics : research and applications , 13 ( 5 ), pp . 369 - 380 , 2005 . 13 m . tong , d . g . ballegeer , a . ketterson , e . j . roan , k . y . cheng , and i . adesida , “ a comparative - study of wet and dry selective etching processes for gaas algaas ingaas pseudomorphic modfets ”, j . electron . mater . 21 ( 9 ), 1992 . 14 carter - coman c ., bicknell - tassius r ., benz r . g ., brown a . s ., jokerst n . m ., “ analysis of gaas substrate removal etching with citric acid : h 2 o 2 and nh 4 oh : h 2 o 2 for application to compliant substrates ”, journal of the electrochemical society , 144 ( 2 ): l29 - l31 , feb 1997 . 15 chang e y , lai y l , lee y s , chen s h , “ a gaas / alas wet selective etch process for the gate recess of gaas power metal - semiconductor field - effect transistors ”, journal of the electrochemical society , 148 ( 1 ), pp g4 - g9 , 2001 . 16 grundbacher r , chang h , hannan m , adesida i , “ fabrication of parallel quantum wires in gaas / algaas heterostructures using alas etch - stop layers ”, journal of vacuum science & amp ; technology b , 11 ( 6 ), pp . 2254 - 2257 , 1993 . 17 broekaert , t . p . e ., fonstad , c . g ., ‘ alas etch - stop layers for ingaalas / inp heterostructure devices and circuits ’, ieee transactions on electron devices , 39 ( 3 ), pp . 533 - 536 , 1992 . 18 sanfacon , m . m ., tobin , s . p ., ‘ analysis of algaas / gaas solar cell structures by optical reflectance spectroscopy ’, ieee transactions on electron devices , 37 ( 2 ), pp . 450 - 454 , 1990 . 19 the concise macleod , version 8 . 0b , copyright © thin film center inc 1997 - 2000 . thin film center inc ., 2745 e via rotonda , tucson ariz . 85716 . 20 f . lukes , ‘ oxidation of si and gaas in air at room - temperature ’, surface science , 30 ( 1 ), pp . 91 - 100 , 1972 . 21 desalvo g . c ., bozada c . a ., ebel j . l ., look d . c ., barrette j . p ., cerny c . l . a ., dettmer r . w ., gillespie j . k ., havasy o . k ., jenkins t . j ., nakano k ., pettiford c . i ., quach t . k ., sewell j . s ., via g . d ., ‘ wet chemical digital etching of gaas at room temperature ’, journal of the electrochemical society , 143 ( 11 ), pp . 3652 - 3656 , 1996 . 22 note : in a compound semiconductor such as gaas , a monolayer refers to the distance between two planes of ga atoms . for growth on a ( 100 ) gaas surface 1 ml = a 0 / 2 , where a 0 = 0 . 56536 nm is the lattice parameter for gaas . 23 burstein , e ., ‘ anomalous optical absorption limit in insb ’, phys . rev ., 93 , pp . 632 - 633 , 1954 . 24 kim , j ., lim , d . h ., yang , g . m ., ‘ selective etching of algaas / gaas structures using the solutions of citric acid / h 2 o 2 and de - ionized h 2 o / buffered oxide etch ’, j . vac . sci . technol . b , 16 ( 2 ), p . 558 , 1998 . 25 desalvo , g . c ., tseng , w . f ., comas , j ., ‘ etch rates and selectivities of citric acid / hydrogen peroxide on gaas , algaas , ingaas , inalas , and inp ’, journal of the electrochemical society , 139 ( 3 ), pp . 831 - 835 , 1992 . 26 chin - i liao , po - wen sze , mau - phon houng , yeong - her wang , ‘ very high selective etching of gaas / al 0 . 2 ga 0 . 8 as for gate recess process to pseudomorphic high electron mobility transistors ( phemt ) applications using citric buffer solution ’, japanese journal of applied physics , 43 ( 6b ), pp . l800 - l802 , 2004 . 27 clawson , a . r ., ‘ guide to references on iii - v semiconductor chemical etching ’, materials science and engineering , 31 , pp . 1 - 438 , 2001 . 28 chin - i . liao , mau - phon houng , yeong - her wang , ‘ highly selective etching of gaas on al 0 . 2 ga 0 . 8 as using citric acid / h 2 o 2 / h 2 o etching system ’, electrochem . solid - state lett ., 7 ( 11 ), pp . c129 - c132 , 2004 . 29 zhao , r ., lau , w . s ., chong , t . c ., li , m . f ., ‘ a comparison of the selective etching characteristics of conventional and low - temperature - grown gaas over alas by various etching solutions ’, japanese journal of applied physics , 35 ( 1a ), pp . 22 - 25 , 1996 . 30 kasahara , k ., ohkubo , s ., ohno , y ., ‘ layer selective anisotropic wet etching with a cooled citric acid / h 2 o 2 solution for high performance gaas hjfets ’, compound semiconductors 1996 . proceedings of the twenty - third international symposium on compound semiconductors , ( 115 ), pp . 507 - 510 , 1997 . 31 chang , e . y ., lai , y . l ., lee , y . s ., ‘ a gaas / alas wet selective etch process for the gate recess of gaas power metal - semiconductor field - effect transistors ’, journal of the electrochemical society , 148 ( 1 ), pp . g4 - g9 , 2001 32 hue , x ., boudart , b ., crosnier , y ., ‘ gate recessing optimization of gaas / al 0 . 22 ga 0 . 78 as heterojunction field effect transistor using citric acid hydrogen peroxide ammonium hydroxide for power applications ’, journal of vacuum science & amp ; technology b , 16 ( 5 ), pp . 2675 - 2679 , 1998 . 33 shigyo , k ., umemura , s ., kawase , k ., ‘ chemical etching behavior and mechanism of undoped gaas in tartaric acid — hydrogen peroxide solution systems ’, electrochemistry 72 ( 6 ), pp . 466 - 470 , 2004 . 34 pearton , s . j ., ‘ critical issues of iii - v compound semiconductor processing ’, materials science and engineering b , 44 ( 1 - 3 ), pp . 1 - 7 , 1997 . 35 wu , x . s ., coldren , l . a ., merz , j . l ., ‘ selective etching characteristics of hf for al x ga 1 - x as / gaas ’, electronics letters , 21 ( 13 ), pp . 558 - 559 , 1985 . 36 united states patent application publication , us 2003 / 0145884 a1 , ‘ wide - bandgap , lattice - mismatched window layer for solar conversion device ’, aug . 7 , 2003 . 37 milanova m , mintairov a , rumyantsev v , smekalin k ., ‘ spectral characteristic of gaas solar cells grown by lpe ’, journal of electronic materials ; 28 ( 1 ), pp . 35 - 38 , 1999 . 38 van riesen , s ., schubert , u ., bett a . w ., ‘ gaas photovoltaic cells for laser power beaming at high power densities ’ proceedings of the 17 th european photovoltaic solar energy conference , munich , 2001 ; pp . 182 - 185 .