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Surface states of Gap
SURFACE SCIENCE 7 (1967) 486489 0 North-Holland Publishing Co., Amsterdam
SURFACE
STATES
OF GaP
Received 3 January 1967 Prior to the surface state experiments, conductivity and Hall coefficient measurements were made on GaP with silver paste, silver epoxy and phosphor bronze contacts. Results agreed with those reported elsewherel). However, these contacts were not ohmic and difficulty was experienced in getting reproducible results. Aluminum and indium were tried, and up to - 600 “K indium was found to be the most satisfactory and dependable. A rectangular block of polycrystalline p-type GaP of dimensions 6.0 mm x 3.3 mm x 1.O mm was etched in boiling nitric acid for two minutes; indium contacts were then evaporated on to two opposite faces of the specimen. Conductivity-temperature plots gave an energy gap of 2.2 eV and the position of the impurity level as AE,=O.O35 eV. Hall effect measurements indicated that the specimen was p-type with a low carrier concentration, - 10” carriers cm- 3 at room temperature, and a mobility of about 20 cm2/V sec. A severe bending of the conductivity curve at high temperatures indicates greater scattering effects than previously reportedl) and that the mobility no longer follows the expected T-* dependence at the higher temperatures, but decreases more rapidly. The specimen was fitted with insulated plates on the two large faces for dc field effect measurements. A dc bridge method2) was used for examining the surface states for various ambients. Plots of the total induced charge on the surface ZT against the change in conductance AG are shown in fig. 1. (Dry oxygen and dry air gave similar results.) Only a small density of fast states was apparently present in the dry nitrogen atmospheres). In wet oxygen, slow states were present, but since they to a thin disappear above - 100 “C and below 0 “C they were attributed layer of moisture on the surface of the crystal4) or to unstable compounds formed on the surface. For dry air and dry oxygen, ‘oxide’ layers formed giving similar densities of surface states, inversion layers forming on the surface (fig. la). The fact that a maximum occurs for negative induced charges and that there is a lack of AG when the plate potential was switched off, indicates that present theory does not apply to the portion of the curve in fig. la representing .Z,< -200 (arb. units), and that it is not normal oxide 486
SURFACE
Fig. 1.
Change in conductance
STATES
OF GBP
481
with total induced charge for (a) dry air, (b) wet oxygen.
that forms in the presence of dry atmosphere (air or 0,) but a more complex surface layer. As with germanium, the relaxation is non-exponential and depends upon the temperature and the ambient atmosphere. It is possible to use Morrisson’ss) model in which the shape of the decay curves can be explained in terms of a uniform surface with carrier exchange over the surface barrier acting as the rate-limiting step. After the surface states are disturbed from equilibrium, the rate of charge transfer appears to be proportional to the excess carrier concentration. For a wet oxygen atmosphere (fig. lb) more usual results were obtained including the distribution of time constants for the slow states. As with ger-
Fig. 2.
Field effect mobility against total induced charge for (a) dry air, (b) wet oxygen.
488
E.J.M. KENDALL
manium and silicon, the fast states appear to be located at the surface of the semiconductor, whilst the slow states are located at the surface of, and in, the ‘oxide’ films on the surface of the CaP. It may be seen that in fig. 1 the conductance decreases and goes through a minimum as the surface conductance tends to become inverted. This is due to the hole density near the surface decreasing to a small value before the electron density has increased any extent. For dry air, there is the reverse process occurring after this minimum and the conductance goes through a maximum. This is probably due to holes being released from shallow traps. Fig. 2 shows the variation of the field effect mobility (pr,n.) with induced surface charge Z,. When estimating the change in conductance of a semiconductor due to surface space-charge layers, besides obvious changes in the electron and hole concentration in the surface region there is the change in carrier mobility due to surface scattering to consider. This reduces /~r.n below L(~(bulk mobility). For a wet oxygen ambient, the variation of /I~.~. with C, is similar to the theoretical result for germanium~), however there appears be to an exponential variation of ~~.n. on either side of the maximum and minimum values (fig. 2), which indicates that the variation of ~r.~. is essentially determined by the changing number of electrons and holes in the space-charge layers, and that there is very little change in the effective mobility of the carriers already present. This means that the surface scattering is small and that ,uF,n.is closer to pclsthan expected. For a dry air ambient, a more complex variation is seen with a minimum, but apparently no maximum. The portion of the curve corresponding to -C, contains a possible exponential section, but the rest of the curve indicates more complex mechanisms in which surface scattering certainly plays an important role. Previous observations 7) concerning surface states in GaP Schottky-Barrier diodes indicate that it is possible to explain the capacity variation with reverse bias of such diodes without using surface states; but the author then proceeds to vindicate models using a relatively high density of surface states. Assuming that slow states have deliberately been excluded in such diodes, fast surface states certainly exist. The densities of these states are relatively low. The partially ionic nature of GaP obviously influences any theoretical interpretation of the surface states, and modifications to present theories 3*5, are required. Lakehead University, Port Arthur, Ontario, Canada
Physics Department,
E.J. M. KENDALL
SURFACE
STATES
OF GaP
489
References 1) G. F. Alfrey and C. S. Wiggins, 2. Naturforsch. 15a (1960) 267. 2) M. Lasser, C. Wysocki and B. Bernstein, in: Semiconductor Surface Physics, Ed. R. H. Kingston (Univ. of Pennsylvania Press, 1957) p. 197. 3) J. D. Levine and P. Mark, Phys. Rev. 144 (1966) 751. 4) H. Statz, G. A. de Mars, L. Davis, Jr. and A. Adams, in: Semiconductor Surface Physics, Ed. R. H. Kingston (Univ. of Pennsylvania Press, 1957) p. 139. 5) S. R. Morrison, in: Semiconductor Surfuce Phy.sics, Ed, R. H. Kingston (Univ. of Pennsylvania Press, 1957) p. 169. 6) J. R. Schrieffer, Phys. Rev. 97 (1954) 64i. 7) A. M. Cowley, J. Appl. Phys. 37 (1966) 3024.
SURFACE
STATES
OF GaP
Received 3 January 1967 Prior to the surface state experiments, conductivity and Hall coefficient measurements were made on GaP with silver paste, silver epoxy and phosphor bronze contacts. Results agreed with those reported elsewherel). However, these contacts were not ohmic and difficulty was experienced in getting reproducible results. Aluminum and indium were tried, and up to - 600 “K indium was found to be the most satisfactory and dependable. A rectangular block of polycrystalline p-type GaP of dimensions 6.0 mm x 3.3 mm x 1.O mm was etched in boiling nitric acid for two minutes; indium contacts were then evaporated on to two opposite faces of the specimen. Conductivity-temperature plots gave an energy gap of 2.2 eV and the position of the impurity level as AE,=O.O35 eV. Hall effect measurements indicated that the specimen was p-type with a low carrier concentration, - 10” carriers cm- 3 at room temperature, and a mobility of about 20 cm2/V sec. A severe bending of the conductivity curve at high temperatures indicates greater scattering effects than previously reportedl) and that the mobility no longer follows the expected T-* dependence at the higher temperatures, but decreases more rapidly. The specimen was fitted with insulated plates on the two large faces for dc field effect measurements. A dc bridge method2) was used for examining the surface states for various ambients. Plots of the total induced charge on the surface ZT against the change in conductance AG are shown in fig. 1. (Dry oxygen and dry air gave similar results.) Only a small density of fast states was apparently present in the dry nitrogen atmospheres). In wet oxygen, slow states were present, but since they to a thin disappear above - 100 “C and below 0 “C they were attributed layer of moisture on the surface of the crystal4) or to unstable compounds formed on the surface. For dry air and dry oxygen, ‘oxide’ layers formed giving similar densities of surface states, inversion layers forming on the surface (fig. la). The fact that a maximum occurs for negative induced charges and that there is a lack of AG when the plate potential was switched off, indicates that present theory does not apply to the portion of the curve in fig. la representing .Z,< -200 (arb. units), and that it is not normal oxide 486
SURFACE
Fig. 1.
Change in conductance
STATES
OF GBP
481
with total induced charge for (a) dry air, (b) wet oxygen.
that forms in the presence of dry atmosphere (air or 0,) but a more complex surface layer. As with germanium, the relaxation is non-exponential and depends upon the temperature and the ambient atmosphere. It is possible to use Morrisson’ss) model in which the shape of the decay curves can be explained in terms of a uniform surface with carrier exchange over the surface barrier acting as the rate-limiting step. After the surface states are disturbed from equilibrium, the rate of charge transfer appears to be proportional to the excess carrier concentration. For a wet oxygen atmosphere (fig. lb) more usual results were obtained including the distribution of time constants for the slow states. As with ger-
Fig. 2.
Field effect mobility against total induced charge for (a) dry air, (b) wet oxygen.
488
E.J.M. KENDALL
manium and silicon, the fast states appear to be located at the surface of the semiconductor, whilst the slow states are located at the surface of, and in, the ‘oxide’ films on the surface of the CaP. It may be seen that in fig. 1 the conductance decreases and goes through a minimum as the surface conductance tends to become inverted. This is due to the hole density near the surface decreasing to a small value before the electron density has increased any extent. For dry air, there is the reverse process occurring after this minimum and the conductance goes through a maximum. This is probably due to holes being released from shallow traps. Fig. 2 shows the variation of the field effect mobility (pr,n.) with induced surface charge Z,. When estimating the change in conductance of a semiconductor due to surface space-charge layers, besides obvious changes in the electron and hole concentration in the surface region there is the change in carrier mobility due to surface scattering to consider. This reduces /~r.n below L(~(bulk mobility). For a wet oxygen ambient, the variation of /I~.~. with C, is similar to the theoretical result for germanium~), however there appears be to an exponential variation of ~~.n. on either side of the maximum and minimum values (fig. 2), which indicates that the variation of ~r.~. is essentially determined by the changing number of electrons and holes in the space-charge layers, and that there is very little change in the effective mobility of the carriers already present. This means that the surface scattering is small and that ,uF,n.is closer to pclsthan expected. For a dry air ambient, a more complex variation is seen with a minimum, but apparently no maximum. The portion of the curve corresponding to -C, contains a possible exponential section, but the rest of the curve indicates more complex mechanisms in which surface scattering certainly plays an important role. Previous observations 7) concerning surface states in GaP Schottky-Barrier diodes indicate that it is possible to explain the capacity variation with reverse bias of such diodes without using surface states; but the author then proceeds to vindicate models using a relatively high density of surface states. Assuming that slow states have deliberately been excluded in such diodes, fast surface states certainly exist. The densities of these states are relatively low. The partially ionic nature of GaP obviously influences any theoretical interpretation of the surface states, and modifications to present theories 3*5, are required. Lakehead University, Port Arthur, Ontario, Canada
Physics Department,
E.J. M. KENDALL
SURFACE
STATES
OF GaP
489
References 1) G. F. Alfrey and C. S. Wiggins, 2. Naturforsch. 15a (1960) 267. 2) M. Lasser, C. Wysocki and B. Bernstein, in: Semiconductor Surface Physics, Ed. R. H. Kingston (Univ. of Pennsylvania Press, 1957) p. 197. 3) J. D. Levine and P. Mark, Phys. Rev. 144 (1966) 751. 4) H. Statz, G. A. de Mars, L. Davis, Jr. and A. Adams, in: Semiconductor Surface Physics, Ed. R. H. Kingston (Univ. of Pennsylvania Press, 1957) p. 139. 5) S. R. Morrison, in: Semiconductor Surfuce Phy.sics, Ed, R. H. Kingston (Univ. of Pennsylvania Press, 1957) p. 169. 6) J. R. Schrieffer, Phys. Rev. 97 (1954) 64i. 7) A. M. Cowley, J. Appl. Phys. 37 (1966) 3024.