In CdS crystals with an iV-shaped negative differential conductivity range, stationary high-field domains adjacent to the electrodes are observed. With increasing applied voltage these steplike domains increase in width, staying attached to the cathode until they fill the entire crystal; then a still higher-field domain forms at the anode and increases in width. These domains can be explained within an earlier published theory, and allow the determination of electron densities at the cathode-CdS boundary and in the fieldquenched region. The analysis of these stationary domains presents a new tool for work-function (metalsemiconductor) investigations.

It is shown that, in n-type material with negative differential conductivit,y due to field quenching, st'ationary cathodeor anode-adjacent high-field domains occur dependent on electron density a t the cathode and applied voltage. Cathode-adjacent high-field domains can be observed only with a blocking cathode. Anode-adjacent high-field domains occur with a slightly blocking cathode a t applied voltages above the range a t which cathodeadjacent domains are observed or as the only domain type possible with an injecting cathode. With different cathode metals evaporated onto the same CdS :Ag, A1 crystal the electron density in the conduction band n,(E) for space charge free conditions has been determined as a function of the actual field between 0 and 240 kV/cm.Es wurde gezeigt, da13 in n-leitendem Material mit negativ differentiellem Widerstand, der durch Feldtilgungseffekte verursacht ist, stationare Hochfelddomanen an der Kathode oder Anode in Abhangigkeit von der Elcktronenkonzentration an der Kathode und von der angelegten Spannung auftret,en. Hochfelddomanen an der Kathode konnen nur mit blockierender Kathode beobachtet werden. Hochfelddomanen an der Anode treten bei einer schwach blockierenden Kathode auf, wenn die angelegte Spannung oberhalb des Bereiches der Hochfelddomanen an der Kathode liegt, oder sie treten als einzig moglicher Domanentyp bei injizierender Kathode auf. Mit verschiedenen Kathodenmaterialien, die auf denselben (CdS :Ag, A1)-Kristall aufgedampft waren, wurde die Elektronenkonzentration im Leitungsband n,(E) fur den raumladungsfreien Fall als Funktion der wirkenden Feldstarke zivischen 0 und 240 kV/cm bestimmt.Supported by the U.S. Office of Naval Research, Washington, D.C.2, A discussion about the transition between stationary and moving domains is given by Kroll (submitted to phys. stat. sol.). 23'

No abstract

In CdS, unstable moving domains are observed, which exist in a current-voltage range between the range for stationary electrode-adjacent domains and the range for transitions to undeformed moving domains. These current -voltage ranges have been investigated in the same crystal by changing the electron concentration a t the boundary of a pseudocathode. Several CdS crystals, differently doped with Ag, Al, and &, have been investigated. An electro-optical method using the Franz-Keldysh effect was employed for domain observation. The influence of the distance between the electrodes on domain formation is discussed.I n CdS werden nichtstabile, wandernde Domiinen beobachtet, die in einem StromSpannungs-Bereich existieren, der zwischen dem Bereich der stationiiren, an der Elektrode anliegenden Domiinen und dem Ubergangsbereich zu undeformierten, wandernden Domiinen liegt. Diese Strom-Spannungs-Bereiche wurden in demselben Kristall durch hnderung der Elektronenkonzentration an der Grenzschicht einer Pseudokathode untersucht. Verschiedene CdS-Kristalle, .die unterschiedlich mit Ag, Al und Cu dotiert waren, wurden untersucht. Eine elektro-optische Methode, die den Franz-Keldysh-Effekt ausnutzt, wurde fur die Beobachtung der Domiinen verwendet. Der EinfluD des Elektrodenabstandes auf die Domiinenausbildung wird diskutiert.

It is shown that stationary cathode-adjacent high-field domains, which occur in a range of negative differential conductivity, can be used to determine quite accurately the minimum electron density close to the cathode. From this density, the work function between cathode metal and photoconductor can be obtained for finite currents if hole currents and tunneling are negligible. Although the minimum electron density close to the cathode is at least two orders of magnitude below the bulk electron concentration (blocking cathode), gain factors well in excess of 100 have been observed, thus justifying the neglect of holes in the transport equation. For slightly doped CdS, as used in these investigations, it is concluded that tunneling can also be neglected. Using the method of stationary high-field domains, it is observed that, for a CdS-gold contact, the work function decreases markedly with increasing current densities. Values of about 0.4 eV for a current density of 10~* A/cm 2 , measured at 200°K, are obtained.