Copper-Oxide Rectifiers and Photocells*

L. O. Grondahl

1. Introduction. 2. Discovery of the rectifying action. 3. Structure of the plates of a copper-oxide rectifier. 4. History of solid rectifiers and some general remarks. 5. Method of manufacture and properties of copper-oxide rectifiers. 6. Properties of cuprous oxide. 7. Theory of the copper-oxide rectifier. 8. Copper-oxide photocells.

Introduction

The data presented in the present article have, it would seem, predominantly practical interest. They have, however, been selected on the basis of their theoretical significance, with the aim of giving a thorough survey of experimental material for investigators interested in the physics of the copper-oxide rectifier. A considerable quantity of data is published here for the first time.**

When cuprous oxide is formed on copper under the action of high temperature, the boundary surface between these two substances proves to possess special properties, namely: asymmetric conductivity and comparatively recently discovered photoelectric properties, expressed in the appearance of an electromotive force when this boundary is illuminated.

Both of these phenomena cannot be assigned to the class of contact phenomena, since both the asymmetric conductivity and the photoeffect are distributed quite uniformly over the entire boundary surface. The uniformity is so great that it proves possible to connect in parallel any number of individual elements. Such a high degree of uniformity is due to the method of manufacture, remarkable in that the contacting surfaces of the substances are not subjected to the action of the atmosphere or of other agents that might cause contamination of the surface by foreign substances, so that on one side of the boundary there is copper that is, to the highest degree, pure, and on the other—equally pure cuprous oxide.

These exceptional conditions occur when cuprous oxide is formed on the copper itself under the action of high temperature. When sufficiently pure copper is used, the cuprous oxide proves—

* Rev. of Modern Physics 5, 141, 1933. Translation and editing by N. Khlebnikov.
** These results were obtained in the laboratory of the Union Switch and Signal Company in Pennsylvania.

is so firmly bound to the copper that, for example, when bending a plate with the aim of freeing it from cuprous oxide, the crystals of the latter break more readily than they detach from the base copper.

Besides the cleanliness of the contact surfaces, two circumstances are undoubtedly essential: first, very close contact between the two substances, and, second, uniformity of the conditions under which the cuprous oxide is formed, which determines the uniformity of the bond between the crystals of both substances.

Discovery of the rectifying action

In order to emphasize the difference between the copper—cuprous-oxide combination obtained by the method described and the ordinary contact between these substances, it is interesting to recall the history of the discovery of the rectifying action of the copper-oxide rectifier.

Fig. 1.

At the end of 1920 the author was studying the possibility of using photoelements as relays. According to the requirements of the problem, it was desirable to construct a photoelement capable of giving a considerable current. It was decided to use the internal photoelectric effect. Cuprous oxide was chosen as the light-sensitive layer. Owing to the brittleness of this material, it was decided to obtain the cuprous oxide by partial oxidation of the copper itself, so that the base copper would serve both as the backing for the oxide layer and as one of the poles of the photoelement.

After oxidation of the plate, one of its ends was cleaned of the oxide layer and formed one pole of the photoelement (Fig. 1). The surface of the cuprous oxide was freed from copper oxide by means of an emery wheel, and a lead wire was pressed to it, intended to serve as the second electrode of the photoelement.

Fig. 2.

However, when the resistance of the device obtained (the circuit of which is shown in Fig. 2) was investigated with the aid of a Wheatstone bridge, it turned out that it did not depend on illumination.

As a result of further experiments it was found that this device has, depending on polarity, different resistance (1200 Ω and 400 Ω).

Soon the rectification coefficient was increased to 10:1 and 50:1, which already made it possible to build rectifiers of practical interest. Fig. 3 gives the characteristic of a rectifi-

tal, made from the very first plate (Fig. 1) in 1929, i.e., 9 years after the plate was manufactured.

Structure of the Plates of a Copper-Oxide Rectifier

Before the discovery of the rectifying action of the copper—cuprous-oxide combination, a number of other combinations of solid substances were known that possessed unipolar conductivity. However, in all these cases the asymmetric conductivity was concentrated in a very small region, in a “point” contact between two substances.

Fig. 3.

The considerable active surface obtained in copper-oxide rectifiers is the result of the fact that cuprous oxide is formed at such a high temperature that it can crystallize freely, forming on a surface of arbitrary dimensions a layer of uniform thickness and structure.

Analyses of cuprous oxide show that it does indeed consist chiefly of this compound. Thus, in one case of a very careful analysis it was found that the base copper is 99.949% pure. The copper content in the cuprous oxide was 88.78%. Assuming that the remainder of the material consists of cupric oxide, the cuprous-oxide content is 98.7%.

Microphotographs of etched sections cut perpendicular to the surface of the cuprous oxide show that the crystals

have the form of columns with bases resting on the base copper. This was recently also shown by Torres[^1].

In Fig. 4 (see the insert) a similar microphotograph is given of a cuprous-oxide layer formed by complete oxidation of a copper plate. The line of dark spots running through the middle of the photograph gives the plane where, upon completion of oxidation, the two cuprous-oxide layers met.

If the base copper is polished down in such a way that only a very thin layer of it remains, adjoining the cuprous-oxide copper layer, this layer can easily be separated from the cuprous-oxide layer. After separation of the two substances, on the contacting surfaces of both one can see a kind of network. Fig. 5 shows a microphotograph of such a network on the copper; Fig. 6—a network on the surface of the cuprous oxide. It can be seen that, when superposed, the two networks should coincide.

In Fig. 7 a photograph is given of the same piece of copper at lower magnification. Fig. 8 is the same as Fig. 7, but after etching the surface with nitric acid. From this photograph it is evident that the copper crystals are not dependent on the structure of the network either with respect to size or with respect to shape.

Fig. 9 shows the same surface of cuprous oxide as in Fig. 6, after it had been deeply etched and polished in order to destroy the network, and then etched again so as to reveal the outlines of the cuprous-oxide crystals. It may be said that, in general, the outlines of the crystals remained the same as before.

Fig. 10 shows a part of one of the lines forming the network on the copper, having the appearance of a row of bubbles. Here too one can see the faces of copper crystals.

The series of photographs presented shows that the outlines of the cuprous-oxide crystals do not depend on the form of the copper crystals, and that on the surface of the copper, at the points of contact between cuprous-oxide crystals, some special processes take place. This may be a consequence of freer access of oxygen to these places or of the separation of impurities as a result of crystallization of the cuprous oxide.

History of solid rectifiers and some general remarks

The first solid rectifiers were contacts between two dissimilar substances, used almost exclusively as detectors.

The earliest data on the question of rectifiers of this kind are found in the works of Braun[^1] (1874), who investigated the properties of point contacts of various natural crystals and discovered their asymmetrical conductivity. Later analogous results were obtained by Schuster[^2] on copper wires with an oxidized surface or a surface exposed to the action of sulfur.

These experiments established that almost any “point” contact between a conductor and a semiconductor possesses asymmetric conductivity. This is true both in the case where the semiconductor is an entire piece of crystal, and when it is simply the surface of a metal altered by oxidation or by some other reaction. The rectifying action may have different directions even in the case of one and the same piece of metal. Sometimes this direction changes depending on the magnitude of the applied voltage.

All the experiments described were carried out with “point” contacts. There were attempts to apply contact detectors for rectifying stronger currents, but “point” contacts were always retained in doing so[^8]. An increase in current strength was achieved by connecting several rectifiers in parallel.

Increasing the contact surface leads to the same results as connecting several contacts in parallel, i.e., to the resistance of the entire system becoming equal to some average value of the resistances of the individual contacts. In some cases, when one of the points turns out to have a very low resistance, the result is, as it were, a short circuit of the contacts with high resistance.

As a result of all this, as the contact surface is increased, the resistance of the rectifier decreases and the rectifying action falls. Contacts with large surfaces therefore usually have values of forward and reverse resistance that differ comparatively little from one another; at the same time, it is extremely rare for both resistances to be exactly the same.

If a crystal is provided with two contacts, one of very small and the other of considerable area, the latter constitutes a small and practically symmetrical resistance. The resistance of the entire system is determined by the small-area contact, which presents a large resistance. If, in addition, this resistance is asymmetric, the system constitutes a rectifier. Such systems are crystal detectors. But even if the contact with the large surface has a rectification coefficient comparable in magnitude to, or even equal to, that of the contact with the small resistance, the rectifying action of the entire system will be determined by the second of the contacts, as may be seen from the following example.

Suppose that the contact with the large surface has, in direction \(A\), a resistance of \(10\ \Omega\), while in direction \(B\) its resistance is \(100\ \Omega\). Let the contact with the small surface have, in direction \(A\), a resistance of \(100\,000\ \Omega\), and in direction \(B\), \(10\,000\ \Omega\). If the rectifying action does not depend on the current density, the direction with the smaller resistance in such a system will be direction \(B\) (the resistance is equal to \(10\,000\ \Omega\)), i.e., the same as for the contact with the small area. Thus we see that the rectifying action of the system is deter-

is determined by the contact with the greater resistance. The opposite could occur only in the case where the rectification coefficient of the contact with the large surface proved to be many times greater than the coefficient of the other contact.

If a contact of very small surface area (such as is used in detectors) is formed on the free side of the cuprous-oxide layer in a copper-oxide rectifier, then, owing to its high resistance, this contact determines the rectifying action of the system. In this case the direction of rectification of such a system is almost always opposite to the direction of a normal rectifier. The predominance of the rectifying action of the point contact is further intensified here by the fact that, at small current densities, the rectification coefficient of the boundary between copper and cuprous oxide is very small. But the rectification coefficient of point contacts with cuprous oxide is not large. Therefore, as soon as the surface of the point contact increases, the rectification coefficient at the boundary between cuprous oxide and copper increases, becomes much greater than the coefficient of the point contact, and the system is transformed into an ordinary copper-oxide rectifier.

What has been set forth is an illustration of the example given above and may serve to explain certain cases of rectification in the reverse direction described in the literature.

Another type of solid rectifier, which existed before the appearance of the copper-oxide rectifier, is the electrolytic rectifier, known in the form of the two systems of Garretson^7 and Pavlovsky^5. We shall not dwell on them in detail. The action of these rectifiers is based on the presence of a layer of solid electrolyte. In order for the rectifier to begin to operate, electrolytic formation of the rectifying layer is necessary. Rectification occurs only in separate areas of the surface. As operation proceeds, the active sites are destroyed, turning into sites with high resistance; the rectifying action passes to new points, and this continues until the entire surface proves to possess high resistance, so that the strength of the rectified current falls to negligible values.

Method of Manufacture and Properties of Copper-Oxide Rectifiers

It has already been pointed out that the rectifying action of copper-oxide rectifiers is the result of the method of obtaining the contact between copper and cuprous oxide, i.e., oxidation at high temperature.

Oxidation of plates may be carried out in two ways. If one wishes to obtain a plate covered with oxide on both sides, it is necessary that oxygen have free access to both sides. In manufacturing plates with a one-sided coating, access of oxygen to one of the sides is hindered. For this purpose, two plates placed together are oxidized simultaneously. The rectifier-

… properties of the plates are determined by the thermal treatment to which they are subjected after oxidation. There are two kinds of thermal treatment, differing from one another in the rate of cooling of the oxidized plates.

One of these methods consists in rapidly cooling the plates by immersing them in water immediately after oxidation. The other consists in slowly cooling the plates in air.

Plates made by the second method have high resistance. As a result, they can withstand large reverse voltages (up to 30 V), but they permit lower densities of rectified current than rapidly cooled plates. The latter make it possible to obtain much higher current densities, but, having lower resistance, they allow considerably smaller reverse voltages. As a result, the rectified power per plate is approximately the same for both types. As for the rectification coefficient, it proves to be higher for plates with rapid cooling.

By varying the rate of cooling, it is possible to obtain plates with intermediate properties.

In Fig. 11 are presented curves expressing the dependence of the resistance in one and the other direction, as well as the rectification coefficients, on the applied voltage for one of the plates obtained with slow cooling. In Fig. 12 analogous curves are given for a plate cooled in water.

In Fig. 13 is shown the dependence of the forward and reverse resistances and of the rectification coefficient on temperature. Analogous data can be found in many articles8.

Fig. 11.

Top graph: Slow cooling in air.
Vertical axis: Ratio = Current, copper − / Current, copper +.
Horizontal axis: Volts.

Middle graph: The same, but large area.
Vertical axis: Resistance (ohms).
Horizontal axis: Volts.
Labels: copper +; copper −.

Bottom graph: Copper cooled in air.
Vertical axis: Resistance (ohms).
Horizontal axis: Volts.
Labels: copper +; copper −.

After the plate has been made, the surface of the copper oxide always proves to be covered with a layer of cuprous oxide. This layer is removed in order to reduce the forward resistance (i.e., resist-

tion in the direction of the rectified current) of the plate. The best results are obtained by dissolving copper oxide in a solution of sodium cyanide. Other methods are treatment of the surface with emery, sand, or nitric acid.

Fig. 12.

The next important point in the process of manufacturing a rectifier is the arrangement of contact with the free surface of the cuprous oxide.

The most widely used are contacts made of lead plates coated with a layer of tin, in order to avoid chemical

Fig. 13.

interaction between the lead and the cuprous oxide. In this case, in order to ensure the best possible contact, the surface of the cuprous oxide is rubbed with graphite powder.

Other methods of forming the contact consist in coating the surface of the cuprous oxide with a metallic film, by reducing the cuprous oxide itself, by electrolysis, etc. The advantage of contacts

of this kind is that, when assembling the rectifier, there is no need to resort to strong compression of the plates.

Cuprous-oxide rectifiers can be used under any conditions. Suffice it to say that, on the one hand, they have been used for detecting radio signals, and, on the other, for obtaining a direct voltage of 100,000 V for electrical dust precipitators and for charging storage batteries with power up to 15 kW.

The efficiency of these rectifiers reaches 65%; under laboratory conditions an efficiency of as much as 85% has even been observed.

Fig. 14.

The rectifying plate, consisting of a layer of cuprous oxide, represents a kind of capacitor, the capacitance of which has been determined differently by different investigators.

In our laboratory Dowling and Pleiss found that the capacitance is about \(0.006\ \mu\mathrm{F}\) per \(1\ \mathrm{cm}^2\) and does not depend on frequency up to \(10^6\) cycles per second, nor on the amplitude of the applied voltage within the range of 10 V. In these investigations the capacitance of two plates connected in series and opposed to one another was measured by means of a Wheatstone bridge without the application of a direct voltage. The calculation of the capacitance was based on the idea that the described combination of plates represents a capacitance shunted by a resistance.

Fig. 15.

Schottky and Deutschmann\(^{42}\) measured the capacitance of a single plate by means of a bridge with an equivalent circuit consisting of a capacitance and two resistances, one connected in parallel and the other in series with it. In addition to the alternating voltage of constant amplitude (40 mV), a direct voltage was applied to the plates. The investigations were carried out in the interval from 800 to 2500 cycles. Some results of these experiments are shown in Figs. 14, 15, and 16.

The effect of the capacitance consists in shunting large resistances, which leads to destruction of the rectifying action if only the resistance of the plate in the direction of greater conductivity is insufficiently small in comparison with the capacitive resistance. At ordinary frequencies this condition is satisfied, and therefore the influence of the capacitance on the rectification coefficient and \(\cos\varphi\) may be neglected.

In order for this condition to be satisfied also at high frequencies, it is necessary to operate at high current densities, because

that, while the capacitive resistance is inversely proportional to the frequency and almost independent of the current density, the resistance of the rectifier in the direction of greater conductivity is inversely proportional to the current density and does not depend on the frequency. In those cases when, at high frequencies, it is impossible to work with sufficiently large current densities, the operating conditions of the rectifier can be improved by connecting a reactor in parallel with it34.

The current density permissible in the rectifier depends on how well it is cooled. If no special measures are taken in this direction, the greatest permissible current density will be about \(0.07\ \mathrm{A/cm^2}\); with good cooling it may be brought up to \(0.6\ \mathrm{A/cm^2}\). As for the permissible heating temperature of the rectifier, it must not exceed \(80^\circ\mathrm{C}\) in any case. To avoid rapid wear, \(40^\circ\mathrm{C}\) should not be exceeded.

Fig. 16.

The form of the curve of the rectified current delivered by a cuprous-oxide rectifier is almost not distorted at all. Some change in form nevertheless exists because the resistance of the rectifier in the direction of greater conductivity is somewhat higher at low voltages than at high ones14, 84.

The service life of cuprous-oxide rectifiers is very long and has not yet been determined with accuracy. In them, however, phenomena of wear are observed, expressed in a decrease of the rectification coefficient and of the efficiency. These changes have an exponential character and are especially strong during the first months of operation. The decrease in efficiency is insignificant. In one case, for example, it amounted to \(6\%\) over the course of 4 years.

The cause of these phenomena has not been clarified. As experiments consisting in measuring, by means of a tube voltmeter, the distribution of the potential drop along the rectifier element have shown, the matter lies chiefly in an increase of the resistance at the contact with the free surface of the oxide. The course of the wear phenomena depends strongly on the load conditions and the temperature in which the rectifier has been. The asymptotic character of the change permits one to think that we are dealing with recrystallization phenomena. On the other hand, it is possible that the matter lies in some chemical changes.

WHERE RECTIFICATION OCCURS

It was originally supposed that the rectifying action of cuprous-oxide rectifiers is due to the asymmetric conductivity of the cuprous oxide itself. This asymmetry, however, was found to be impossible to detect,

could not be achieved. Since it had been established that the material of the contact with the free surface of the copper has no influence on the action of the rectifier, and that this action does not depend on the thickness of the oxide layer, it remained to suppose that the rectifying action is concentrated at the boundary between the base copper and the copper oxide. This was proved by direct experiments (consisting in the study of the distribution of the potential drop with the aid of a vacuum-tube voltmeter), the scheme of which is shown in Fig. 17, carried out by Schottky and his collaborators, and also in our laboratory. The measurements, typical results of which are shown in Fig. 18, showed that the rectifying action takes place at the boundary between the copper oxide and the copper, or in any case extremely close to it.

There is other evidence for this assumption as well. If the plate is bent slightly (and then straightened again), thereby disturbing the special connection between the copper and the copper oxide which exists in a copper-oxide rectifier (the bending must not even be so strong as to cause cracking of the oxide layer), the rectification coefficient decreases owing to a reduction in the difference between the resistances

Fig. 17.

TABLE 1

Effect of bending on the plate of a copper-oxide rectifier

in both directions. As can be seen from Table 1, where the results of such experiments are given, as the bending is repeated the plate from the rectifier turns into an ordinary plate of copper oxide

copper with two contacts, possessing a very low resistance in both directions.

A very large value of the reverse resistance at the boundary copper oxide—copper, in comparison with the contact resistance at the free surface of the oxide, creates at the contact with the free surface of the oxide conditions owing to which this contact also, as it were, becomes a rectifier. As has already been noted, the resistance in contacts between metals and semiconductors is very great at small voltages and falls as the voltage increases. Therefore, since when current flows in the direction of greater resistance almost all the voltage drop occurs at the boundary of cuprous oxide with copper, the contact with the free surface proves in this case to have a greater resistance than when

[Figure labels: high-resistance direction; low-resistance direction; copper to oxide; oxide to lead; total; voltage; high resistance (ohms); low resistance (ohms); 14.5 V at 1.6 V; 130 V at 1.8 V.]

Fig. 18.

current flows in the other direction, when the distribution of the voltage drop between the two boundaries changes in the sense that a much larger part of the total voltage falls on the share of the contact with the free surface. This can readily be seen from Fig. 18.

The general property of the contact between a semiconductor and a metal noted here is illustrated by Fig. 19, whose curves refer to the large and small contacts of a crystal detector with a cuprous-oxide crystal from a rectifier.

Properties of cuprous oxide

In connection with the discovery of the cuprous-oxide rectifier, a large number of works appeared devoted to the study of the properties of cuprous oxide. It has already been mentioned that both in our laboratory and by other investigators \(^{34,42}\) complete symmetry of the conductivity of cuprous oxide by itself was found. It was also established that,

COPPER-OXIDE RECTIFIERS AND PHOTOCELLS

that its resistance does not depend, within very wide limits, on the potential gradient.

Very interesting results were obtained by O. von Auwers, who found that the resistance of cuprous oxide depends on the pressure of the surrounding gas. When the pressure is lowered, in the case of air or hydrogen, the resistance decreases; in the case of oxygen, on the contrary, it increases. This investigator also observed that at temperatures of about 56° C cuprous oxide becomes opaque. Other investigators[^196] did not find this latter fact.

Fig. 19.

Von Auwers also points out that at 56° C the coefficient of expansion of cuprous oxide undergoes a sharp change. Whereas below this temperature the coefficient has a positive sign and is very small, above 56° C it becomes negative.

The conductivity of cuprous oxide apparently has a purely electronic character. As experiments by various investigators[^135] [^132] [^125] show, its resistance in the degassed state is considerably higher than in the presence of oxygen. It may therefore be thought that the conductivity of cuprous oxide is a secondary phenomenon caused by the presence in it of free oxygen.

Dobar[^112] carried out chemical and microscopic investigations of Cu₂O in states of high and low conductiv-

ity. Chemical analysis gives identical results in both cases.

Microscopic studies have revealed the presence, in the bulk of the well-conducting cuprous oxide, of opaque crystallites. The cuprous oxide in the low-conductivity state does not contain these crystallites, but small cavities appear in it. X-ray analysis reveals no difference in the crystalline structure of the two kinds of cuprous oxide.

Donwald and Wagner \(^{139}\) investigated the conductivity of cuprous oxide at temperatures from 800 to \(1000^\circ\) C and found a definite relation between the conductivity and the oxygen pressure: conductivity \(=\) constant \(\times\) (oxygen pressure)\(^{1/7}\). Fott investigated the Hall effect in cuprous oxide in the temperature interval from \(-70^\circ\) C to \(+75^\circ\) C and found that the electron concentration varies in accordance with the equation:

\[ n = n_0 e^{-\frac{a}{kT}} . \]

It should be noted that there is one more work by von Auwers, giving the following summary of the results: the Hall effect of cuprous oxide is very large. The influence of illumination on the Hall effect is very small. The change in the resistance of cuprous oxide in a magnetic field is very small. The influence of illumination on the change in resistance in a magnetic field is very large. It follows from this that cuprous oxide is a semiconductor with a significant internal photoelectric effect.

Theory of the Cuprous-Oxide Rectifier

A satisfactory theory of the cuprous-oxide rectifier must explain the following fundamental facts:

1. The resistance of the plates in the direction of lower conductivity is very large.

2. As the voltage decreases, the difference between the resistances in the two directions decreases, but never reaches zero, so that the rectifying action exists even at the very lowest voltages.

3. The resistance in the direction of lower conductivity has a sharp maximum lying between 0.75 and 1.5 V.

4. After this maximum, this resistance falls linearly with increasing voltage.

5. As the voltage increases, the resistance in the direction of greater conductivity falls very rapidly, almost according to an exponential law.

6. The rectifier plates possess a very considerable electrostatic capacitance, due to the properties of the surface separating the cuprous oxide and the copper.

7. With the exception of this capacitance, the rectifier presents an ohmic resistance to alternating current in both directions.

1. The rectifying action is inertia-free.

2. When the surface of the interface between copper and cuprous oxide is illuminated, a potential difference arises at the terminals of the rectifier.

The starting points of the theories proposed to explain the rectifying action of the cuprous-oxide rectifier are the most diverse phenomena and conceptions concerning the structure of the plate, namely:

1. Thermoelectricity.
2. Electrolysis.
3. Peculiarities of the crystalline structure of the plates, producing an “action of point and plane.”
4. The presence in the crystal lattice of atoms with an insufficient number of electrons.
5. A difference in the magnitude of the energy required by an electron to cross the boundary between copper and cuprous oxide in one direction and in the other.
6. The presence of a nonconducting layer at the boundary between cuprous oxide and the base copper.
7. The spatial separation of copper from cuprous oxide in connection with the cold emission of electrons.
8. The presence of an electron atmosphere inside the cuprous oxide.

9. Various combinations of the assumptions of the last five points.

10. According to the thermoelectric theories, the rectifying action is a consequence of a thermoelectromotive force arising as a result of heating of the interface surface between copper and cuprous oxide. This e.m.f. facilitates the passage of current in the direction of greater conductivity and hinders its passage in the reverse direction. The electromotive force of the pair cuprous oxide—copper is very large, but, unfortunately for the thermoelectric theories, has a direction opposite to that which it ought to have. Moreover, experiments have shown that the existence and magnitude of the thermoelectromotive force are independent of the rectifying action. For example, plates subjected to bending, the result of which was the destruction of the rectifying action, showed no change whatever in their thermoelectric properties.

11. The electrolytic theory of the cuprous-oxide rectifier belongs to Pelabon. Some propositions of this theory rest on very shaky foundations. First of all, Pelabon considers that cupric oxide has a considerably lower resistance than cuprous oxide. This, in any case, contradicts what is observed with respect to cupric oxide in rectifier plates. As is known, on top of the cuprous oxide of freshly prepared plates there is a layer of cupric oxide. The resistance of such plates is very great, and to reduce it the removal of the oxide layer is necessary. The only confirmations of the view set forth can be the experiments of Bedeker¹².

Further, Pelabon assumes that cupric oxide is located between the crystals of cuprous oxide, and that the conductivity of the cuprous-oxide layer is due almost exclusively to this circumstance. This con-

seems improbable, because the cuprous oxide layer has a very pure red color in transmitted light. Moreover, analysis shows that the cuprous oxide is 98.7% pure. The amount of copper oxide is thus so small that, in order to explain the conductivity of the layer, it would be necessary to assume that it is in a superconducting state.

Finally, Pelabon admits that the conductivity of copper oxide has an electrolytic character, so that when current flows through it in one direction oxygen is liberated from it, producing a large resistance, whereas when the current flows in the opposite direction well-conducting copper is liberated. Such an electrolytic action, however, would have to be accompanied by the phenomenon known as “forming,” which, as is known, is not observed in copper–cuprous-oxide rectifiers. In view of all this, the electrolytic theory, based on the assumption of the conductivity of copper oxide, must be rejected as unsatisfactory.

1. Theories based on the “action of point and plane” should in general be abandoned, since microphotographic investigations reveal nothing that would justify such an assumption.

Some resemblance to these theories is borne by one of the theories proposed by Schottky and his collaborators68, based on several other views. During experiments on contacts with a free surface of cuprous oxide, they found that rectification occurs better with very small contact surfaces, or when the contact is divided into a whole series of contacts of very small surface area. On this basis, they explained the action of the rectifier by the fact that at the boundary between copper and cuprous oxide these two substances are separated from one another, except at individual points. However, further experiments by Schottky and Weibel82, devoted to the study of the thermal conductivity of the boundary, led them to the conclusion that contact must take place along the entire surface of separation, since only in this way can the large value of the thermal conductivity observed experimentally be explained.

1. In the theory proposed by K. Wagner71, it is assumed that the passage of current from cuprous oxide to copper (i.e., in the direction of lower conductivity) is caused by electrons knocked out of non-ionized copper atoms present as “defects” in the cuprous-oxide lattice. But in that case one would expect, for current in this direction, the existence of a certain limit—a saturation current. Since no such limit is observed, the proposed explanation must be regarded as unsatisfactory.

2. One of the first theories was based on the theory of detector action proposed by Schottky. If it is assumed that two substances have different work functions, or that the potential humps on passing through the surface have different forms, it can be shown that the contact between such substances will be ob—

For the article by L. O. Grondal’

Fig. 4.                Fig. 5.

Fig. 6.                Fig. 7.

to possess different conductivities in different directions. This is a very tempting route for explaining the rectifying action, but it is hardly suitable, because the work functions for copper and cuprous oxide differ too little from one another.

1. Theories postulating the existence of an insulating layer between the copper and the cuprous oxide may be illustrated by the theory proposed by Slepian[^27]. Slepian assigns thermionic, solid-state, and even electrolytic rectifiers to a single class of “thin-film rectifiers.” By a “thin film” he does not necessarily mean geometrically thin layers, but any layers containing an insignificant number of atoms of the substance. In the case of the copper–cuprous-oxide rectifier it is assumed that, near the boundary between the copper and the cuprous oxide, the crystals of the latter are deformed as a result of their bonding with the copper. This is based on the erroneous assumption that the crystal form at the boundary is the same for both substances. It is further supposed that, as a result of the stresses caused by this deformation, an extremely thin layer with a very high resistance is formed at the boundary, and that electrons can escape more readily from the copper than from the cuprous oxide. As a consequence, the copper becomes the cathode.

On the basis of the most recent investigations this theory appears to correspond more closely to reality than one might previously have thought. As has already been indicated, it was found that cuprous oxide, freed from an excess of oxygen, becomes almost a nonconductor. It is easy to imagine that near the surface of the copper the concentration of free oxygen is extremely low, owing to its strong affinity for copper. Oxygen from the layers nearest to the copper combines with it, thus forming a layer of oxide free from extraneous oxygen and, consequently, possessing a very high resistance. These ideas, developed by Schottky and Waibel, seem to the author to be the first plausible explanation of the existence of an insulating layer that may be the cause of the rectifying action. However, even granting all that has been stated, it is difficult to imagine how such conditions can exist throughout the entire lifetime of the rectifier, unless one assumes that the copper at the boundary continues to absorb oxygen even at room temperature.

The authors just mentioned carried out several highly interesting experiments which showed that contact between cuprous oxide and a gold or silver film obtained by evaporation possesses asymmetric conductivity only in the case where the surface of the oxide had previously been subjected to bombardment by atoms or electrons, or had undergone electrolytic action. As a result of such treatment, rectification ratios of up to 1:20 (at 2 V) were observed. Bombardment apparently creates a rectifying layer, which can be removed by dissolution and restored again by repeating the treatment. Table 2 shows the influence of the gradual destruction of the rectifying layer on the rectification ratio.

The rectifying layer may be a layer of cuprous oxide deprived of oxygen as a result of bombardment and may thus be similar to the oxide layer near the basic copper.

1. Frenkel^45, Frenkel and Ioffe^75, and also van Geel explained the rectifying action by cold emission of electrons in the presence of a spatial separation between cuprous oxide and copper.

If their assumptions are accepted, the theory does not seem hopeless, but it is extremely difficult to believe that the cuprous oxide is in fact separated from the copper over a considerable part of the boundary surface. This is contradicted by the facts noted in connection with the description of the structure of rectifying plates, which show that the contact at the boundary is very close, and also by Schottky’s experiments on the thermal conductivity of the boundary layer. Another difficulty consists in the fact that the difference in contact potentials is too small for rectification to be explained by it. Finally, from the standpoint of cold emission it is apparently difficult to explain the presence of a resistance maximum in the direction of lower conductivity, existing at voltages of about 1 V.

TABLE 2

1. The author of the present article has also attempted to explain the rectifying action of the cuprous-oxide rectifier. In this theory it is first of all assumed that a very close contact exists between copper and cuprous oxide. The author further considers that the copper atoms nearest the boundary may be in such a position that they can be regarded either as belonging to the cuprous-oxide crystals or to the copper crystals. Assuming such a character of the contact between copper and cuprous oxide, one may consider that the work expended by an electron in crossing the boundary is equal to the difference of the work functions of the two substances. According to the measurements of Barton and Goetz, cited by Schottky and Deutschmann, the work functions for cuprous oxide and copper are 4.8 V and 4.4 V, respectively. The difference of these quantities is sufficiently small that one may expect a considerable number of transitions even at room temperature.

Further, we may assume that the number of free electrons in copper is many times greater than in cuprous oxide. Taking the number of electrons per unit volume to be proportional to the conductivity and assuming the same mobility of electrons in both substances, we obtain that in \(1\ \mathrm{cm}^3\) of copper there are \(10^7\) times more electrons than in \(1\ \mathrm{cm}^3\) of cuprous oxide. Since a considerable percentage of electrons in copper has a kinetic energy greater than is needed for crossing the boundary (their number is further increased by the circumstance,

that the electrons in copper are in a degenerate state), an intensified diffusion of electrons must arise, leading to the establishment of a dynamic equilibrium between copper and cuprous oxide, in which the number of electrons crossing the boundary in one direction and in the other proves to be equal. In the presence of such an equilibrium, in the cuprous oxide near the boundary there must be formed a large concentration of electrons, decreasing with distance from the boundary.

The emergence of such conditions, which determine (as will be seen from what follows) the asymmetric conductivity of the boundary between cuprous oxide and copper, is possible only thanks to the exceptional uniformity of the structure of this boundary. If this uniformity did not exist, the passage of electrons of the described character could be compensated by reverse passages through parts of the surface with a different work of transition. Such, in all probability, are all “good electrical contacts” that do not possess asymmetric conductivity. Such also are contacts with a free surface of cuprous oxide.

In order to clarify what role this electronic atmosphere plays in the occurrence of the asymmetry of conductivity, let us recall in what this asymmetry is expressed. The resistance in the direction of greater conductivity decreases almost exponentially with increasing voltage. As the voltage approaches zero, this resistance approaches a certain value, the same value to which, decreasing as the voltage decreases, the resistance in the direction of lesser conductivity approaches. When the voltage is increased in the direction of lesser conductivity, the resistance first rises to a maximum lying between 0.75 V and 1.5 V, and with further increase of the voltage falls linearly.

For the theory of the cuprous-oxide rectifier, the essential point is chiefly to clarify the mechanism by which the very high resistance in the direction of lesser conductivity arises.

Basing ourselves on the idea of the formation of an electronic atmosphere inside the cuprous oxide, let us turn to Figs. 11 and 12 and first consider the case of current flowing in the forward direction. In the state of equilibrium, of which we spoke above, only a very small number of electrons in the copper have a kinetic energy sufficient to overcome the potential hump created by the electrons located inside the oxide and the copper near the boundary. Therefore, when a voltage is applied which drives the electrons in this direction, so long as the voltage is small, only a few electrons are given the increase in kinetic energy sufficient for crossing the boundary; this means a large resistance at low voltages. As the voltage is increased, the number of electrons capable of surmounting the potential hump becomes larger and larger. And an increase in the number of such electrons means a decrease in resistance.

When considering the flow of current in the reverse direction, at

at small voltages we encounter the same situation, i.e. there exist only very few electrons capable of crossing the potential barrier. As the voltage is increased, the electrons are driven toward the copper, and the outer slope of the barrier becomes steeper, which makes it difficult for the electrons to pass through it. It is also possible that the height of the barrier increases on account of electrons arriving from the side of the free surface of the cuprous oxide. All this leads to an increase of the resistance as the voltage is increased, up to a certain maximum value, obtained experimentally at voltages between 0.75 and 1.5 V. But the applied voltage, in addition, forces the electrons to pass into the copper and thus lowers the height of the barrier. This latter effect becomes predominant at higher voltages, and therefore the resistance in the direction of lower conductivity, with further increase of the voltage, begins to fall.

The most weighty argument in favor of the presence of a nonconducting layer between the copper and the cuprous oxide is the existence of electrostatic capacitance in rectifying plates. The proposed theory can, apparently, explain the presence of capacitance without resorting to the notion of the existence of an insulating layer. It appears that the conditions just described can be regarded as a pseudocapacitance. When current flows in the direction of greater conductivity, part of the electrons forming the electronic atmosphere is carried away. When the direction of the current is reversed, these electrons return and the electronic atmosphere is restored.

We thus have a storage of electric charges, depending on the voltage, and which therefore, in any case, from this point of view, resembles a capacitance. By calculation it can be shown that, to explain the observed capacitance, it is sufficient to assume a very small change in the concentration of electrons.

With the aid of the equations describing electronic emission, derived by Bartlett^58, we constructed the curve for the current in the direction of greater conductivity. This curve, in its general form, is very close to that obtained experimentally. Applying the same formula to the current in the direction of lower conductivity proved unsuccessful. Dzhentri^13 suggested that, if the electronic atmosphere really plays a role, the current should vary with voltage according to the power law \(3/2\). The results obtained by substituting the published data into the formula are satisfactory, but are unlikely to be of decisive significance.

1. In all probability, the real explanation will prove to be some combination of the last five theories presented. The presence of capacitance in the plates makes very tempting the assumption of the existence of an insulating layer. At the same time, it is extremely difficult to imagine this nonconducting layer even in the form of a layer of cuprous oxide possessing high resistance (not containing free oxygen), since with time this resistance increases. This increase may

are, of course, the result of secondary phenomena; however, if this is indeed so, the state of affairs proves to be precisely the opposite of what might have been expected, unless one assumes that the free oxygen present in cuprous oxide is bound to the crystal lattice in such a way that it is deprived of the possibility of diffusing. The latter assumption, however, does not seem plausible.

The assumption of an electronic atmosphere is very promising, since it easily explains the presence of capacitance and the approximate equality of the resistances in both directions at low voltages. If the existence of an insulating layer were found experimentally, it could be interpreted as the result of the action of space charge.

Van Geel[^61] proposed a combination of Schottky’s detector theory and the assumption of the existence of a non-conducting layer with very large potential gradients in it. He also takes into account the action of space charge. The principal feature of the theory is cold emission of electrons, and the author concludes that the difference between the substances forming the contact, even if it is expressed in the existence of an “internal” work function, is sufficient to explain the rectifying action. One of the consequences of this theory consists in the assertion that the influence of temperature should manifest itself only through a change in the resistance of the cuprous oxide itself. This, however, is scarcely plausible, since the indicated resistance is too small to appreciably affect the flow of current in the direction of lower conductivity. And it is precisely in this direction that the influence of temperature is manifested most strongly.

None of the theories proposed so far is entirely satisfactory. All that may be regarded as definitely established is that the phenomenon is electronic in character and that it is due to a difference in the ease with which electrons penetrate through the boundary between cuprous oxide and base copper in the one direction and in the other.

Cuprous-Oxide Photoelements

The author of the present article, during experiments with cuprous-oxide rectifiers, established that the plate exhibits a photoelectric effect which reduces the efficiency of the rectifier. Further investigations in collaboration with Geiger showed that the reduction of the efficiency of the rectifier is caused by the appearance, under the action of illumination, of an electromotive force that impedes the flow of current in the direction of greater conductivity. Sörensen’s investigations proved that the photoelectric action occurs at the boundary between the cuprous oxide and the base copper.

Later B. Lange reported on a cuprous-oxide photoelement in which the photoeffect also took place on the surface of contact-

of contact between cuprous oxide and the thin metallic film deposited on it. We shall call the first type of photoelements rear-wall, and the second—front-wall, in accordance with German terminology.

Schottky and Lange, together with their co-workers, carried out extensive work on the study of the properties of these photoelements.

In photoelements of the first type, it is necessary, as far as possible, for light to pass freely through the oxide to the boundary between the cuprous oxide and the copper. Therefore contact with the free surface of the oxide was arranged in the form of a mesh85, 86. Initially the mesh was made in the form of a spiral of lead wire. Later they began to resort to coating the free surface of the oxide with copper (as well as with other metals), by cathode sputtering or by reducing the cuprous oxide. Reduction can be carried out by electrolysis, and also by immersing plates just removed from the oxidizing furnace in oil or in an alcohol solution*. After obtaining a metallic layer on the surface of the oxide, part of it is removed with the intention that the coating should constitute a mesh**.

The magnitudes of the photocurrent obtained from the first photoelements, which were ordinary rectifying plates under illumination of 2000 Lx and with a measuring instrument resistance of 150 Ω, were of the order of 15–20 μA. Lange88 reported a sensitivity of 50 μA/Lm for rear-wall and 500 μA for front-wall photoelements. Wilson124 recently reported obtaining a power of 1 W from 1 m² of the surface of a rear-wall photoelement. Another photoelement by the same author, with a surface area of 6.45 cm², had a sensitivity of \(10^{-4}\) A/Lm. According to Lange’s data107, rear-wall photoelements give a potential difference of \(25 \cdot 10^{-6}\) V/Lm, and front-wall ones—\(15 \cdot 10^{-6}\) V/Lm.

According to Dume’s measurements100, cuprous-oxide photoelements do not exhibit any significant inertia even at frequencies of 6000 cycles.

The distribution of sensitivity over the spectrum of these photoelements depends on the method of manufacture. Rear-wall photoelements have a short-wave sensitivity limit of about 570 mμ; at 630 mμ there is a selective maximum, after which the curve gradually falls to the long-wave sensitivity limit, lying at 1.4 μ. The short-wave sensitivity limit shifts with changes in temperature in accordance with the change in transparency of cuprous oxide. Data of this kind are given in Fig. 20, borrowed from Lange103.

* In the USSR (VEI), to obtain a metallic film on the surface, reduction in an aqueous glycerin solution is used.

** When the metallic film is formed by cathode sputtering, the mesh can be obtained directly, without resorting to mechanical treatment of the surface after deposition. For this it is sufficient to screen the surface of the oxide with a mica plate or a piece of silk with corresponding cutouts. The method belongs to VEI. Translator’s note.

The curve of the distribution of sensitivity over the spectrum of front-wall photocells extends considerably farther into the visible region. Its shape also depends on the transparency of the layer traversed by the light, which in this case is a film of copper, silver, or gold.

Lange and others investigated the effect of temperature on the sensitivity of cuprous-oxide photocells. According to Lange, back-wall photocells give a maximum potential difference at \(-140^\circ\text{C}\) and a maximum current strength at \(-60^\circ\text{C}\). The results obtained by Lange are presented in Fig. 21. From the same article Fig. 22 is borrowed, showing the effect of temperature on front-wall photocells.

Fig. 20.

In our laboratory results were obtained that contradict these data of Lange. Our results, relating to the illumination of a back-wall photocell by the undecomposed light of a 15-watt lamp, are presented in Table 3.

The influence of an extraneous e.m.f. on the photoeffect was investigated by Schottky, whose results are illustrated in Fig. 23. Both curves were taken at \(-183^\circ\text{C}\). The solid curve refers to a well-rectifying contact, the dashed curve to a contact with a low rectification coefficient.

Fig. 21.

Fig. 22.

Considering the illuminated part of the photocell surface as a source of e.m.f., and the remaining surface as a shunting resistance, Schottky showed that the source of electrons in a back-wall photocell is the boundary between copper and cuprous oxide. The same was proved by Sorensen’s experiments in our

TABLE 3

laboratories, consisting in the successive elimination of all other possible sources of electrons.

Schottky89, 99, 106, Dember90, 100, and others established the following facts.

1. The short-circuit current is directly proportional to the intensity of illumination. (Perucca and Deaglio105 showed that under certain conditions the proposition of direct proportionality between voltage and intensity of illumination is valid.)

Fig. 23.

1. The flow of electrons across the boundary between cuprous oxide and copper always takes place in the direction from cuprous oxide to copper, i.e. in the direction of lower conductivity.

The theory of cuprous-oxide photocells is in a more satisfactory state than the theory of rectifying action.

All the proposed theories connect the e.m.f. arising under illumination with the internal photoeffect of cuprous oxide, described by Pohl126. The photoeffect of the blocking layer resembles the external photoeffect in that here too there occurs emis-

out of the substance (cuprous oxide). Like other kinds of photoelectric effect, the photoeffect of the blocking layer may be characterized by the amount of energy necessary to tear electrons out of cuprous oxide. The long-wavelength limit for the internal photoeffect of cuprous oxide is equal to \(4.1\,\mu\). The long-wavelength limit of the photoeffect of the blocking layer is \(1.4\,\mu\). The difference in the work functions in the two cases, equal to \(0.58\ \mathrm{V}\), may represent the work necessary for transferring an electron from cuprous oxide into copper.

Schottky pointed out that the relation between the current strength and the amount of absorbed light energy depends on several factors. For a photocurrent to arise, the absorption of light must take place near the boundary of the cuprous oxide, so that the liberated electrons can penetrate into the metal. The current for a given illumination thus depends on what percentage of the light is absorbed sufficiently close to the boundary. In addition, it will be determined by the ratio between the number of absorbed quanta and the liberated electrons. In Waibel’s experiments with a front-wall photocell, whose front electrode consisted of a gold film obtained by cathodic sputtering, at a wavelength of \(500\,m\mu\), corresponding to the maximum of the spectral sensitivity distribution curve, the number of liberated electrons amounted to \(25\%\) of the number of absorbed quanta. In other cases this number corresponded to \(50\%\), which gives about \(1\) coul./cal.

In photocells of this kind, the efficiency of light of a given wavelength is the greater the more strongly it is absorbed, since the stronger the absorption, the greater the number of electrons that will be liberated near the boundary. It has been established that electrons released at a distance of \(1\,\mu\) from the boundary can still take part in forming the photocurrent. This means that they must undergo a considerable number of collisions and yet retain a reserve of energy sufficient to cross the boundary. From this Schottky concludes that these are the very same electrons that determine the internal photoeffect. Schottky\(^{123}\) and his co-workers established that cuprous oxide in a well-conducting state almost does not exhibit an internal photoeffect. They also found that the photoeffect of the blocking layer is absent in plates that do not possess rectifying action. If the electrons that determine the internal photoeffect are identical with the electrons of the photoeffect of the blocking layer, this may serve as confirmation of the assumption that there exists an insulating layer at the boundary between copper and cuprous oxide.

Rupp\(^{118}\) investigated the influence of a magnetic field on the photoeffect of the blocking layer. He found that the photocurrent decreases proportionally to the square of the magnetic-field strength, independently of the wavelength of the incident light. He also showed that this decrease corresponds, to an accuracy of \(10\%\), to an increase in the resistance of cuprous oxide under the action of the magnetic field. From this Rupp concludes that photoelectrons and conduction electrons in cuprous oxide move

in one direction, and that the role of light is reduced merely to the liberation of electrons.

In this connection, the new experiments of Schottky123 are of great interest; by applying an external voltage he obtained photocurrents 1000 times greater than without voltage.

Whatever the final explanation of the rectifying action of the cuprous-oxide rectifier and of the accompanying action of the photoelectric effect may be, in all probability it will be connected with the concept of a layer possessing high resistance. The existence of such a layer apparently finds experimental confirmation. But even in this case the details of the theories of both phenomena still await development.

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II. Photoelements

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5. E. Rupp, Change of resistance in photocells with a blocking layer under the action of a magnetic field, ZS. Phys. 76, 597, 1932.

6. K. Schaff and O. Weinbaum, Study of the structure of copper-oxide rectifiers, Phys. ZS. 33, 386, 1932.

7. W. Schottky, New phenomena in contacts between substances of different conductivity, Forschung u. Fortschr. 8, 22, Aug. 1932.

8. C. Sinelnikow and A. Walther, Experimental methods for studying photocells with a blocking layer, Phys. ZS. d. Sow. 1, 6, 1932.

9. F. Waibel, The Becquerel effect as the photoeffect of a blocking layer, ZS. Phys. 76, 281, 1932.

10. F. Waibel and W. Schottky, New data on the photoeffect in a blocking layer, Phys. ZS. 33, 15, 583, 1932.

11. E. O. Wilson, Photocells made from rectifier plates, Electronics, 5, 4, 312, 1932.

III. Properties of cuprous oxide

1. K. Badeker, Electrical conductivity and thermoelectromotive force of certain metallic compounds, Ann. d. Phys. 22, 749, 1907.

2. A. H. Pfund, Photosensitivity of cuprous oxide, Phys. Rev. 3, 289, 1916.

3. R. Hiecke, Electrical conductivity of solid insulators, Electrot. und Mschbau. 44, 537, 1926.

4. O. von Auwers, Change of magnetic resistance and the Hall effect in cuprous oxide, Wiss. Veröff. a. d. Siemens Konzern 9, 294, 1930.

5. B. Gudden, Electrical conductivity of semiconductors, Erlanger Berichte 62, 289, 1930.

6. W. Vogt, Electrical measurements on cuprous oxide, Ann. d. Phys. 7, 183, 1930.

7. O. von Auwers, Some properties of cuprous oxide, Naturwiss. 6, 133, 1931.

8. M. Le Blanc and H. Sachse, Electronic conductivity of copper oxides, Ann. d. Phys. 11, 727, 1931.

9. M. Le Blanc and H. Sachse, Electronic conductivity of solid oxides, Phys. ZS. 32, 887, 1931.

10. E. Engelhard and B. Gudden, Applicability of Ohm’s law to cuprous oxide, ZS. Phys. 70, 701, 1931.

11. B. Gudden, Conductivity and photoelectrons in insulators and semiconductors, Phys. ZS. 32, 825, 1931.

12. B. Gudden and G. Mönch, Some properties of cuprous oxide, Naturwiss. 17, 361, 1931.

13. M. C. Neuburger, Precise measurements of the lattice constants of copper, ZS. Phys. 67, 845, 1931.

14. M. Randall, R. F. Nielsen and G. H. West, Free energy of certain copper compounds, Ind. and Eng. Chem. 23, 388, 1931.

15. H. Dünwald and C. Wagner, The nature of the electrical conductivity of cuprous oxide, ZS. Phys. Chem. 17, 467, 1932.

16. N. Ostroumow, Some unresolved questions concerning cuprous oxide, Phys. ZS. d. Sow. 1, 7, 1932.

17. A. H. Wilson, Theory of electronic semiconductors II, Proc. Roy. Soc. (London) A 134, 277, 1932.