Semiconductor Electrodes XI. Behavior of n- and p-Type Single Crystal Semconductors Covered with Thin n-TiO Films

Single crys ta l semiconductors (n-Si, p-Si , n -GaAs , p -GaAs , n -GaP, n-In.P, and n -CdS) were coated wi th n t y p e TiO~ by a chemical vapor deposi t ion technique, and the e lec t ron and hole t r ans fe r proper t ies across the he te ro junct ion so produced were invest igated. The qua l i ty of the deposi ted TiO~ film depended upon severa l factors inc luding t empe ra tu r e and subs t ra te mater ia l . When high qual i ty c rackf ree TiO2 coats were obta ined on n t y p e substrates , the subs t ra te was s tabi l ized with no dissolut ion dur ing the pho to-ox ida t ion of water . However , the oxida t ion was due o ~ y to the photoexci ta t ion of the TiO~, and any holes produced in the subs t ra te were not t r ans fe r red th rough the TiO~ to the solution. The use of p t y p e subst ra tes coated wi th TiO2 as photocathodes was l imi ted by band-bend ing requ i rements at the p n he te rojunct ion and the TiO~-solution interface. The ut i l iza t ion of small bandgap (E s) semiconductor electrodes, i.e., Eg < 2.5 eV, such as CdS, Si, and GaAs, for the efficient conversion of solar energy to e lectr ical or chemical energy in e lect rochemical cells is l imi ted by the dissolut ion or decomposi t ion of the semiconduct o r i tself (1). Photoeffects and the e lec t rochemical be havior of semiconductor electrodes have been descr ibed e lsewhere [see, e.g., (2-4) and references there in] . Various a t tempts have been made to s tabi l ize the sur face of smal l bandgap semiconductors wi thout changing the i r e lect rochemical p roper t ies for hole or elect ron t ransfe r th rough the e lec t rode-so lu t ion interface. For example , thin meta l films have been deposi ted on the surface of the semiconductor (5) or different redox couples (e.g., Na2S/S wi th CdS) have been added to the solut ion to a l low a p re fe ren t ia l e lectrode react ion r a the r than the dissolut ion of the semiconductor (6-9). N t y p e TiO2, SnO2, and SrTiO~ show sufficient s tab i l i ty so as not to decompose dur ing the pho to -ox ida t ion of wa te r (10-12). However , these have bandgaps ~ 3.0 eV so that they ut i l ize only a ve ry smal l percentage of the solar spec t ru r~ There has recen t ly been some in teres t in using chemica l ly vapor deposi ted coatings on semiconductor e lectrodes. Fo r example , CVD TiO2 coats on n -CdS have been repor ted (13) to increase its efficiency as a photoanode and decrease the dissolution of the CdS. Other work in this a rea (14) indicates tha t n G a P wi th TiO2 coatings s tabi l ize the GaP agains t photodissolut ion at potent ia ls pos i t ive of the flatband. The he te ro junc t ion produced at the in ter face of the two d iss imi lar semiconductors creates an energy ba r r i e r to the t rans fe r of car r ie rs across the junct ions wi th a magn i tude de te r mined by the re la t ive posi t ion of the conduct ion and valence bands, as shown in the discussion following. This ba r r i e r would be expected to h inder the flow of pho togenera ted carr iers f rom the subs t ra te th rough the pro tec t ive coating. However , ene rgy levels and surface states m a y exis t in the bandgap region and be ut i l ized for e lec t ron t rans fe r reactions, e.g., an in te rmedia te level has been proposed for TiO2 (6). I f these levels ex tend into the semiconductor and can be considered "bandl ike," the poss ib i l i ty exists for the t ransfe r of electrons and holes th rough the coat ing via these in t e r media te levels. In this work, var ious smal l bandgap semiconductors (n-Si, p-Si , n-CdS, n-GaAs, p -GaAs, n -GaP, and n I n P ) * Electrochemical Society Student Member 9 * * Electrochemical Society Active Member 9 K~y words: photoelectrochemistry, h e t e r o j u n c t i o n s , s e m i c o n d u c t o r s , photovoltaic cells. were coated wi th po lycrys ta l l ine TiO2 by chemical vapor deposi t ion (CVD) to examine the resul t ing elect r ical p roper t ies both wi th and wi thout a solut ion interface. The posi t ion of the valence and conductions bands vs. vacuum for the var ious semiconductors is shown in Fig. 1 (15, 16). The procedure for obtaining a good TiOz film on smal l bandgap single crys ta l semiconductors, a modification of the one prev ious ly de scr ibed (17), the e lect rode proper t ies , and the conditions by which photoexc i ted electrons or holes are t rans fe r red from the subs t ra te to the solut ion via the large bandgap TiO~ film on the surface are repor ted .