PHOTO-ELECTRIC EMISSION FROM CADMIUM TELLURIDE

Transcription

1 R 428 Philips Res. Repts PHOTO-ELECTRC EMSSON FROM CADMUM TELLURDE 1. ntroduetion by J. J. SCHE:ER and J. van LAAR *) : '241 Summary The photo-electric emission from single crystals as well as from evaporated layers of CdTe has been measured. The resulting photo current versus photon energy curves all show a tail at the long-wavelength side which is ascribed to impurities. 't is concluded 'thát the most reliable value for the work function is obtained from measurements on freshly cleaved single crystals. A possible relationship between the photoemissive properties of CdTe and CdS is discussed. Résumé On a mesuré l'emission photo-électrique de monocristaux et de couches de CdTe déposées dans e vide. Les courbes représentant e courant émis en fonction de 'énergie des photons indiquent pour les ondes longues une émission résiduelle qui est attribuée à la présence d'impuretés On en conclut que la valeur du travail d'extraction la plus correcte se déduit de mesures effectuées sur des monocristaux fraichement clivés On discute une relation possible entre les propriétés photoélectriques du CdTe et du CdS. Zusammenfassung Die lichtelektrische Emission wurde sowohl an Einkristallen als auch an aufgedampften Schichten von CdTe gemessen. Es ergibt sich dab die Kurven die die Abhängigkeit des Photostroms von der Photonenergie darstellen alle in zwei Teile aufzuspalten sind wobei der langwellige Teil Verunreinigungen zugeschrieben wird. Es wird geschlossen dab -der beste Wert der Austrittsarbeit bei Messungen an frisch gespaltenen Einkristallen erhalten wird. Eine mögliche Beziehung zwischen den Photoemissionseigenschaften von CdTe und denen von CdS wird diskutiert. n general the photo-emissivity of a semiconductor will be influenced by surface contamination which makes it necessary to study photo-electric effects on clean surfaces. To obain such a clean surface the following methods are in use: (a) preparation of vacuum-evaporated layers (b) ion bombardment of the surface of a single crystal and subsequent heating in vacuum (c) cleaning of a single crystal surface by heating the crystal in vacuum (d) cleaving or breaking of sins;le crystals in vacuum. To avoid any possible influence of atoms adsorbed from the ambient atmosphere it is desirable to carry out the photo-electric measurements in as high a vacuum as possible immediately after the cleaning process. For achieving a clean surface of a compound semiconductor the method (d) seems to be *) Laboratorium voor Kristalchemie Rijksuniversiteit Utrecht.

2 324 J. J. SCHEER and J:van LAAR the most appropriate as it avoids a possible change of composition in the surface region which is inherent to the other methods. Moreover one reduces the time between the production of the clean surface and the measurements to a few minutes the whole process being carried out at room temperature. For these reasons we have chosen the cleaving method for the study of CdTe. For comparison the studies were extended to measurements of the photoemission from evaporated layers. 2. Results and Discussion A cadmium-tel1uride crystal can be cleaved very easily along the (11) 17 6 SO '>l 3 x / :2 J t ~ '" x / f 7 "</ 6 x 5 x v ~ 4 Cl vi 3 v c 2 ei D. ~ ' / 1 v E t. u v ill v V NV vi! / ' ' ' '4 -Photon energy'inev 6168 Fig. 1. photo-emission from CdTe: On-type single crystal freshly cleaved in vacuum D the same crystal after aging in vacuum for 7 hours X the same crystal after 2 hours heating at 4 C in vacuum <l vacuum-evaporated layer (brought to the same scale as the other curves the photo-electric yield at the high-energy side being about a factor 2 smaller in this case). f / / / f D

3 14 2 la o-n c-p f. 3 ~ if d a' o ~ o!! ' Photon energy in ev 6169 Fig. 2. Comparison of photo-emission from n- and p-type CdTe respectively with 1 16 and charge carriers/cmë the scales of the two curves are adjusted at hv = 6 34 ev. plane by pressing a knife edge on it in the appropriate direction. To perform the cleaving process in vacuum the knife could be moved by means of a stainless-steel bellow.a complete description of the vacuum apparatus will be given elsewhere. The pressure in the vacuum system was lower than 1-1 torr no increase in pressure being observed during the cleavage process. CdTe layers were prepared by evaporation from a single crystal in a vacuum of 1-7 torr. Measurements have been accomplished on a number of different surfaces: (a) the surface of an n-type singlecrystal freshly cleaved in vacuum (figs 1and 2) (b) the same surface after aging at room temperature in a vacuum of 1-1 torr for 7 hours (fig. 1) ~ (c) the same surface after 2 hours heating in vacuum at 4 C (fig. ) (d) as (a) but now with a p-type crystal (fig. 2) (e) a vacuum evaporated layer (fig. ). From the results it is evident that there is practically no difference in the spectral distribution of the photo current between (a) and (b) so the influence of the ambient atmosphere at this pressure turns out to be negligible. n the case

4 326 J. J. SCHEER and J. van LAAR (c) a tail appears at the long-wavelength side this tail being more pronounced in the case of an evaporated layer. t will be seen from fig. 1 that the same tail has a tendency to appear also in the curve of the aged surface. From these experiments we conclude that the variable photo-emission at the long-wavelength. side must be associated with surface contamination effects and so evaporation does not seem to be a proper way of producing clean surfaces. Assuming the emission at the long-wavelength side to be due to impurities the starting point for photo-emission from the top of the valence band can be found in all cases by extrapolating the steep part of the -hv curve. Simple linear extrapolation leads to a value of 5 9 ± 5 ev for all experiments and so this value can be considered as the photo-electric work function cp of a clean CdTe (11) surface. From a theoretical point of view this linear extrapolation is quite arbitrary but the so-obtained experimental cp value can not be far wrong because for the freshly cleaved surfaces it is not strongly dependent on the chosen method of extrapolation. Neglecting the tail effects there is no difference in photo-emission from n- and p-type CdTe crystals as is shown in fig. 2. This can be understood from the following considerations. The kinetic energy required for the excited electrons in the conduction band to surmount the surface potential barrier is much greater than the energy necessary for electron-hole pair production. n general this ionization process leads to very-low mean free-path values. So for instance the mean free path for hot electrons in silicon was estimated to be about 2 A 1)2) and we expect the escape depth for photo electrons in CdTe to be of the same order of magnitude. Consequently the photo-emission will hardly be influenced by band bending 3)4) as the surface layer from which 'the photo electrons escape forms only a small fraction of the total space-charge region especially when the crystals are only moderately doped as was the case in our measurements. n this connection the small influence of the ambient atmosphere becomes also clear as the main effect of adsorbed atoms may be a change in the bending of the energy bands. 3. Preliminary investigations on CdS n a tentative experiment we measured the photo-emission from a CdS crystal broken in air but without making any further provisions for cleaning. For comparison measurements on CdTe were performed under identical circumstances. The resulting -hv curves shown in fig. 3 are of the same shape. At the long-wavelength side the CdTe curve exhibits a relatively large tail as compared with the vacuum-cleaved crystal but again the starting point for emission of valence-band electrons can be obtained by extrapolating the steep part of the curve now resulting in a value of about 6 1 ev. The difference from

5 PHOTO-ELECTRC EMSSON FROM CADMUM TELLURDE 327 o 5 J JV fr. V L V! CdS. /...o-e:"'" kt ~ photon energy in ev 617 Fig. 3. Photo-emission from CdTe and CdS single crystals cleaved in air; the two curves are plotted on the same scale. the above-mentioned.. value of 5 9 ev has to be ascribed to inaccuracies. in. the 'extrapolation caused by the disappearance of the lower part of the band emission into the tail effect. ' t is possible to interpret the CdS data in the same way. Doing this one obtains a cp value of 7 ± 2 ev the tail emission being caused by surface contamination n this connection it is interesting to consider Shuba's results 5) from measurements on CdS evaporated layers in the region ev. From the low quantum efficiency of the photo current Shuba concluded that the photo electrons were not originating from the valence band which is in accordance with our opinion. We may remark that this quantum efficiency is of the same order of magnitude as we found for the tail enission from an evaporated CdTe layer as is shown in fig. 4. However Shuba estimated the starting point for enission from the valence band to be at 5 7 ev which is not in agreement with our value of 7 ev. We note that the difference in photo-electric work function cp for CdS and CdTe obtained from fig. 3 is about 1 ev which is nearly equal-to the difference in bandgap the width of the forbidden zone being 2 43 and 1 45 ev respectively. Consequently CdS and CdTe have approximately the same electron affinity of 4:5 ev. Further experiments with vacuum-cleaved CdS crystals will be carried out to confirm this conclusion.

6 328 J. J. SCHEER and J. van LAAR m r lö~------~----~~------~ ~--... photon energy in ev 6171 Fig. 4. Photo-emission from evaporated layers of CdTe and CdS. The CdS data are taken from Shuba 5). Eindhoven June 1961 REFERENCES.1) P. A. Wolff Phys. Rev ) B. Senitzky Phys. Rev ) W. E. Spicer R.C.A. Rev ) J. J. Scheer Philips Res. Repts ) u. A. Shuba J. tech. Phys. Moscow