CA1137604A - Shallow homojunction solar cells - Google Patents

Abstract

SHALLOW-HOMOJUNCTION SOLAR CELLS

Abstract Improvements in shallow-homojunction solar cells based upon a plurality of layers of a direct gap semiconductor material such as GaAs, as well as their fabrication, are disclosed. The shallow-homojunction solar cells have a n+/p/p+ structure in which the n+ top layer is limted to a thickness which permits significant carrier generation to occur in a lower semiconductor layer. An anodic antireflection coating is applied over the n+
top layer, and a particularly preferred method for applying the antireflection coating is by anodization. These solar cells can be grown on relatively inexpensive substrates, if desired, such as silicon or germanium.

Description

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Back~round Art Solar cells have been de~e.loped ~or g~nera~ing electrical energy directly from sunllght. Xn g~neral, these cells can be classified as either heterojunction devices, which depend upon junctions such as thos~ ~ormed between two different semi-conductor materials or between a metal and a semi-conductor or from a metal~insulator~semiconductor . sandwich, an~ homojunotion devices which depend only upon junctions formed between layers of the same ~emiconductor material doped to different impurity le~s to provide different electrical propert.ies.

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~ 3eretofore, homojunction cells using direct-g~p semiconductor materials have generally exhibited dis-appointing efficiencies. One reason for the relatively low efficiencies in homojunction solar cells is be-5 lieved to be the high absorption coefficient whichis inherent in direct gap semiconductor materials such as gallium arsenide. For example, approximately half of the carriers due to AM 1 radiation are gen-erated within 0.2 ~m of the surface of gallium ar-10 senide. Therefore, for materials such as GaAs, whichalso has a high surface recombination velocity, most of the carriers generated by solar radiation recombine before they reach the junction causing a significant decrease in conversion efficiency.
One approach which has been used to overcome this problem has been the use of a thin window layer of gallium aluminum arsenide (Gal_xAlxAs) grown over the GaAs wafer by liquid phase epitaxy.
Such cells may be referred to as heteroface cells.
20 Because the recombination velocity is much less at a Gal_xAl ~s/GaAs interface than at a GaAs sur-face, higher conversion efficiencies have been achieved. Thus, Hovel and Woodall report conversion efficiencies of up to 22~ for Gal_xAlxAs/GaAs hetero-25 face solar cells but only up to 14~ for GaAshomojunction solar cells for air mass i (AM 1~
radiation. See Hovel and Woodall, J. M., 12th IEEE
Photovoltaic Specialists Conf., 1976 (Institute of Electrical and Electronic Engineers, New York, 1976), 30 p. 945.
Nevertheless, aluminum is so reactive in the vapor phase that it is difficult to prepare hiyh quality Gal xAlxAs layers by conventional chemi-cal vapox deposition, which is a highly preferred ~l3~

fabrication method. Because of this, it has been necessary to grow Gal_xAlxAsx layers by metal-organic chemical vapor deposition. See Dupuis, R. D., Dapkus, P. D., Yingling, R. D. and Moody, 5 L. A., Appl. Phys. Lett., 31, 201 (1977). This method can be both more expensive and more time consuming than conventional chemical vapor depo-sition.

Disclosure of the Invention This invention relates to improved shallow-homojunction photovoltaic devices and methods for their fabrication. These shallow-homojunction de-vices are based upon a plurality of layers of direct gap semiconductor materials suitably doped to pro-15 vide an n /p/p structurè. An antireflection coat-ing is applied over the n top layer and the nt top semiconductor layer is also limited to a thick-ness within the range which permits, upon light irradiation, significant carrier generation to 20 occur in the p layer below the n+ top layer.
Thus, the top junction is referred to as a shallow-junction. p~ef~ r'ec~
In a particularly ~r~ fabrication method, the antireflection layer deposited on the n+ top 25 semiconductor layer is formed by anodization. The anodic layer forms an excellent antireflection coating and requires no ~acuum processing. Addi-tionally, the anodization process serves to reduce the thickness of the top semiconductor layer without 30resulting in significant surface degradation.
Thus, homojunction solar cells can be provided which have conversion efficiencies approaching those obtainable with heteroface cells. Fur-thermore, the use of a Gal_xAlxAs layer is avoided which eliminates a relatively complicated and ex-pensive step in the overall fabrication of a solar cell.
Still another significant advantage of the photovoltaic devices described herein is the ease with which ohmic contacts can be applied to them since they have high concentrations of dopants in their outer layers. This is particularly important 10 because shallow homojunctions can have their junction qualities destroyed by attempts to form ohmic con-tacts at elevated temperatures. With this device, ohmic contacts can be applied by directly plating a metal layer on the semiconductor surfaces without 15 elevated temperatures so that the junction quality is preserved.
These shallow homojunction solar cells addi-tionally possess vastly superior resistancé to degradation by electron bombardment than either 20 Gal xAlxAs heterojunction cells or shallow homojunc-tion GaAs cells having a p top layer. Because of this, the shallow homojunction cells described herein have great potential for use in space applications.
The heavily doped p+ layer additionally enables 25 excellent solar cells to be grown on substrate mat-erials other than the host semiconductor materials, i.e., the semiconductor material which absorbs sun-light and generates electrical current.
Gallium arsenide solar cells, according to this 30 invention, have been grown on gallium arsenide and germanium substrates with equally outstanding effi~
ciencies o~ over 20% at AM 1.

Brief Description of the Drawings FIG. 1 is a cross sectional elevational view of a typical prior art solar cell employing a thin Ga1_x AlxAs window;
FIG. 2 is a cross-sectional elevation view il-lustrating one embodiment of a solar cell according to this invention;
FIG. 3 i~ a cross-sectional elevation view of another embodiment of a solar cell according to this 10 invention;
FIGS. 4(a) and 4(b) are schematic illustrations of the application of gold contacts to a solar cell of this invention;
FIGS. 5(a) and 5(b~ are schematic illustrations 15 of the application of tin contacts to a solar cell of this invention;
FIG. 6 is an exploded cross-sectional view illustrating the area around one contact finger of a solar cell Labricated according to this invention;
FIG. 7 is a plot of data illustrating the quantum efficiency at varying wavelengths for a solar cell fabricated according to this invention;
FIG. 8 is a plot comparing measured reflectivity to theoretical reflectivity for an anodic anti-25 reflection la~er applied to a GaAs shallow-homojunc-tion device according to this invention;
FIG. 9 is a plot of data illustrating the quantum efficiency at varying wavelengths for a solar cell of this invention having an anodic anti-30 reflection coating;
FIG. 10 is a graphical presentation of the impurity profile for a GaAs shallow-homojunction photovoltaic device of this invention grown on a germanium substrate;
FIG. 11 is a plot of data illustrating the ~3~

spectral response of a GaAs solar cell of this invention having an n+ layer thinned by an anodi-zation-strip cycle;
FIG. 12 is a plot of data for the spectral response of a GaAs solar cell of this inventio~, and comparin~ internal and external quantum effici~ncy;
FIG. 13 is a plot of data illustrating th~
power conversion efficiency as a function of n~
layer thickness for four GaAs solar cells of this invention;
FIG. 14 is a plot of data illustrating the decrease in IsC with increasing electron irradi-ation fluences for a GaAs shallow-homojunction solar cell of this ir~vention;
FIG. 15 is a plot of data illustrating the decrease in IsC, VOC, fill factor and power conversion efficiency with increasing electron irradiation fluences for another GaAs shallow-homojunction solar cell of this invention.

20 Best Mode of Carrying Out the Invention The invention will now be further described with particular reference to the Figures.
A prior art homojunction photovoltaic device 10 is illustrated in FIG. 1. The substrate 12 i5 25 formed from an n GaAs wafer. Typically, substrate 12 might be doped to a carrier concentration of 1016-1017 carriers/cm3. Layer 14 is formed over layer 12 and comprises p GaAs. 1ayer 16 is formed from p+ Ga1_xAlxAs and might have a carrier con-30 centration of 1018 carriers/cm3. A typical thicknessfor layer 16 is less than one micrometer. This Gal_xAlxAs window has been used because the re-combination velocity fox carriers generated upon solar illuminhtion i5 much less at the Gal_xAlxAs/

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GaAs interface than it is within GaAs itself. In practice, a device of FIG. 1 can be formed by depositing a thin layer 16 of p+ Gal_xAlxAs over an n GaAs substrate and subsequently diffusing some 5 of the p-dopants from the Gal_xAlxAs layer into the GaAs substrate to form layer 14.
Unfortunately, Gal_xAlxAs coatings are difficult to apply using chemical vapor deposi-tion and are relatively difficult to use in the 10 formation of an ohmic contact. Because of this, ohmic contacts are typically applied to devices such as that shown in FIG. 1 by employing vacuum coating techniques followed by alloying. These techni~ues are relatively expensive and detrimental 15 to shallow-homojunctions.
FIG. 2 illustrates one embodiment of an im-proved photovoltaic device of this invention.
Therein, substrate 22 is formed from a wafer of p+ GaAs. Substrate 22 might be formed, for 20 example, from GaAs suitably doped with p-dopants such as zinc, cadmium, berylium or magnesium to a carrier concentration of at least about 1018 carriers/
cm3. The thickness is not critical for substrate 22, but might be between about 1 and about 500 um. Layer 25 24 is formed on the upper surface of layer 22 and is a layer of GaAs suitably doped with p-dopants to a caxrier concentration of about 1014-101~ carriers/
cm3. The thickness of layer 24 depends upon the minority carrier diffusion length and absorption 30 coefficient, with a typical range for GaAs of from about 1 to about 5 ,um.
Layer 26, formed from n+ GaAs, is epitaxially deposited upon layer 24. Layer 26 may be formed from GaAs suitably doped with n-dopants, such as sulphur, 35 selenium or silicon to a carrier concentration of at ~l~3~

least about lol7 carr ers/cm3. It ls critical to limit the thickness Qf lay~r 26 to one which allows sig-nificant carrier generation within layex ~4. Thus, layer 26 would typically be limited to a 5 maximum thickness of 1500 A, and preferably less.
Great care is necessary to assure that such thin layers are uniform, and not all deposition techniques are suitable. The techniques believed to be suitable include chemical vapor deposition, 10 molecular beam epitaxy, liquid phase epitaxy and ion beam implantation~ In addition, if care is taken to dope the top layer 26 to a sufficiently high concentration, an ohmic contact can be formed on the surface without degrading junction character-15 istics.
It has been found that the n /p/p structurehas significant advantages over previously employed structures. The p+ layer, for example, forms a back surface ield junction with the p layer to 20 provide high efficiency current collection. The high doping level in the p~ layer also simplifies the application of an ohmic contact thereto and this high doping level also allows the shallow-homojunction cell to be formed on a different sub-25 strate material which reduces the cost or provides other advantages. In ~his regardl the heavily doped p layer allows tunneling between any heterojunction formed between dissimilar materials thereby making ohmic contact feasible.
The n+ top layer reduces the series resistance of solar cells having this structure. It also sim-plifies the formation of an ohmic contact to the cell because of its high doping level.
The p layer also provides a major advantage in ~3~

the n /p/p structure. Greater cell efficiencies are possible with this structure compared to other shallow homo~unction structures, such as p /n, because of the greatly increased diffusion length of minority carriers, i.e., electxons, in a p layer compared to the diffusion length of minority carriers in an n layer, i.e., holes, The overall cell structure of n /p/p thus provides a shallow-homojunction device which can 10 be manufactureed inexpensively, is capable of pro-viding high efficiencies, can be deposited on sub-strates formed from different materials, and has outstanding resistance to degradation by electron bombardment which is a severe problem encountered 15 in space applications.
Top layer 28 is an antireflection coating which reduces the reflection of GaAs and thus increases absorption of solar energy. The antireflection coating might be, for example, successive layers 20 o~ transparent materials having relativ~ly high and relatively low indices of refract:ion, respectively.
For example, an antireflection coating might be prepared by electron-beam evaporation of titanium dioxide and magnesium fluoride.
It is particularly preferred to apply the anti-reflection coating by anodization. Application of anodic coatings can be done without any vacuum processing and is an inexpensive way of producing excellent antireflection coatings. In addition, the application of an anodic layer necessarily reduces the thickness of the top GaAs layer. For example~ it has been found that application of an anodic layer typically reduces the thickness of the GaAs layer at the rate of about 2/3 part of the 35 volume of GaAs to one part by volume of anodic layer.

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Typically, the thickness of the antireflection layer would be based upon quarterwave theory.
Anodic coatings can be formed by employing the device as the anode in an electrolytic cell. By proper selection and control over the cell parameters, including the electrolyte and voltage applied, thin uniform anodic coatings can be formed Suitable electrolytes are well known and a specific one found to be suitable is a solution formed by mixing 3 grams of tartaric acid into 100 ml of water, adding suffi-cient N~40~ to adjust the pH to about 6 2, and then adding 250 ml of propylene glycol. The voltage applied should be sufficient to produce the anodic coating in the thickness desired.
Although the description above has been limited to GaAs cells, other direct bandgap semi-conductors such as InP and CdTe are also suitable.
Additionally, the direct bandgap semiconductor layers can be deposited on substrates formed from different materials.
FIG. 3 illustrates a shallow-homojunction device 30 ~o~med on p+ germanium substrate 32. The thickness of substrate 32 might be from 0.1 ~m to 500 ~m and it might be ~ormed from single crystal Ge doped with p-dopants to a carrier level of 1013 carriers~cm or greater. GaAs layers are then applied to Ge substrate 32, by chemical vapor deposition or other techniques, to form the desired shallow-homo-junction device from p GaAs layer 22, p GaAs layer 24 and thin n~ GaAs layer 26, which are similar to layers 22, 24 and 26, respectively, in FIG. 2, Antireflection coating 28 is subsequently applied over thin n+ GaAs layer 26 to complete this embodiment.

There is a major cost advantage possible in the manufacture of GaAs shallow homojunction solar cells when the actual gallium arsenide employed can be minimi2ed by depositing the cell on a substrate S of less costly material, such as germanium or silicon.
Gallium arsenide solar cells theoretically have higher conversion eficiencies than cells formed from indirect bandgap materials, such as silicon and germanium. In addition, gallium arsenide cells poten-tially should bP more radiation resistant in space en~vironments. By growing gallium arsenide cells on materials such as silicon or germanium, which have lower costs, the advantagesof both types of materials can be achieved.
Substrates formed from materiàls differen~ from the host semiconductor can offer other advantages in addition to cost advantages. Germanium~ for ex-ample, has higher thermal conductivity than GaAs which is an advantage in heat dissipation. Germanium 20 also has a lower melting point and lower vapor pressure than Ga~s, which might allow easier laser crystalliæation on a substrate such as graphite.
Laser crystallized germanium having large grains would pro~ide a good substrate for chemical ~apor 25 deposition of GaAs.
The use of substrates which are diffèrent lrom the host semiconductor is possible because of th~
heavy doping of the substrate and the p layer o the device. This permits tunneling to occur around 30 the heterojunction between the substrate and p layer so that it does not act as a barrier. Thus, a good ohmic contact oan be formed.

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It should be understood that the embodimenk shown in FIG. 3 is still considered to be a homo-jun~tion cell, even though it technically contains a boundary betwPen dissimilar materials, namely the boundary between p 5e substrate 32 and p GaAs layer 22. Although this boundary might techni-cally be referred to as a heterojunction, the heavy doping allows ohmic contact. This should be con-trasted with the heteroface between p GaAs layer 14 and thin p Gal xAlxAs layer 16 in FI~. 1, which serves the function of reducing surface recombina-tion velocity of carriers generated in layer 14 upon solar irradiation. It should also be con-trasted with a typical heterojunction between dis-similar materials which is used to create a barrierto current flow in heterojunction devices. Because of these differences in purpose, solar cell 30 il-lustrated in FXG. 3, and other similar cells, will be referred to herein as shallow-homojunction solar cells formed from direct gap semiconductors deposited on different substrate materials.
Although substrate 32 has been illustrated to be single crystal Ge, other substrates could be employed. Single crystal silicon, for example, could also be employ~d. In fact, substrate 32 might also be formed from polycrystalline or amorphous materials, including silicon and germanium.
Electrical contact to the thin n+ layer can be easily made because of the high doping level therein.
30 Contacts can be formed by electroplating metals such as gold, tin, etc. Specific procedures for the electroplating of these metals differ, however, and are respectively illustrated in FIGS. 4 and 5. In ~376~

both of these figures, the shallow-homojunction solar cell has the structure illustrated in FIG. 3.
In FIGS. 4(a) and 4(b), a typical application of gold contacts is illustrated. In this technique, 5 the thin n+ layer 26 is first anodized to form anodic coating 28 while simultaneously thinning n+
layer 26.
The anodization potential can be set to achieve the appropriate thickness for the antireflection co~ting -and n layer. A photoresist mask 30 is then placedover anodic coating 28 and finger openings 32 (FIG.
4[a]) are etched through anodic coating 28 employ-ing an etch such as dilute hydrochloric acid. Gold contacts 34 are then electroplated onto n layer 26 through photoresist mask 30. Photoresist mask 30 is then removed by dissolving it in acetone to produce the device of FIG. 4(b~.
The application of tin contacts is illustrated in FIGS. 5(a) and 5(b). Photoresist mask 30 is directly applied to thin n~ layer 25 and tin con-tacts 36 are electroplated onto layer 26 through mask 30 (FIG. 5[a]). Photoresist mask 30 is then removed and the thin n layer 26 is anodized (FIG. 5 ~b]). When this procedure is employed, the thin n layer 26 remains thicker under tin contacts 36 than under the remainder of anodic coating 28. Thus, layer

2~ has raised shoulders 38 directly beneath tin contacts 36 as can be seen in FIG~ 5tb). Thus, there is a larger separation between the metal contacts and the p-n junction for tin contacts than for gold contacts. Because the n+ layer is extremely thin after anodization, this in-creased separation should imp~ove device yield and reliability.

,, .. .

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The use of tin contacts is also advantageous for optimizing the n~~ layer thickness since the anodic oxide formed on gallium arsenide can be stripped with dilue HCl and the cell reanodized 5 without removing the contacts. Because the thickness of the oxide layer is very uniform and easily controlled by adjusting the anodizing voltage, a series of alternating anodization and stripping steps can therefore be used for con-10 trolled reduction of n+ layer thickness. Thethickness of the gallium arsenide removed during each anodization can be accurately determined by using elliposometry to measure the anodic oxide thickness and multiplying this value by an appro-15 priate factor Some anodic antireflection coatings may besomewhat unstable in harsh environments. If this is a problem, it can be overcome by application of a thin (e.g., 100 A), trans~arent, protective 20 coating of a material such as SiO2 or phosphosilicate glass. Such protective coati~c~s can be applied by pyrolytic deposition techniques.
FIG. 6 is a cross-sectional view illustrating one finger of a solar cell having such a protective ~5 SiO2 coating 40. ~evice fabrication is similar to that described above for FIGS. 4(a) and 4(b), except that a hydrofluoric acid etch is employed prior to the hydrochloric acid etch. Contact finger 34 can be formed from gold plated to a thic~ness of about 30 4 ~m. The back contact 42 can also be formed from plated gold~ Although the SiO~ protective layer was described as being applied prior to contact formation, it could also be applied after the con-tacts have been formed.

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As those skilled in the art will recognize, other metals could be used in place of gold and tin for purposes of establishing electrical contact with the photovoltaic device which is described herein, In devices fabricated as illustrated in FIG. 4, any metal could be employed including gold, silver, plati-num, tin, aluminum, copper, etc. In devices fabricated as illustrated in FIG. 5, those metals can be used which form a sufficiently thick oxide layer during anodization such that current leakage through metal contacts is reduced enough to allow the semiconductor surface to be anodized. Tin, aluminum and copper are examples.
Devices of this invention have at least one n-p homojunction and at least one other junction suffi-cient to increase current collection. This other junction requires an impurity profile wherein the majority carriers all have the same charge and wherein an electrical field is created by the impurity profile which aids in collecting minority carriers. The p layer also allows the use of substrate materials other than the host semiconductor material. Specific examples of suitable junctions include high/low homojuncti~ns and graded pro~ile junctions where the impurity doping level increases with distance from the n-p junction.
This invention canbe further specifically illustrated by the following Examp~es.

GALLIUM ARSENIDE SHALLOW-HOMOJUNCTION PHOTOVOLTAIC DEVICE
GaAs layers were grown in an AsC13-Ga-H2 system.
The reactor tube had an inner diameter of 55 mm, and the H2 flow through the AsC13 evaporator and .~

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over the Ga boat was in the range 300-500 cm3/min.
The p and n dopants were introduced in the vapor phase by using ~C2~s)2Zn and H~S, respectively. The reactor tube was vertical, allowing rotation of the substrate, which resulted in greater doping uniformity in the layersO Use of high purge flows allowed the reactor tube to be opened at the bottom to load and unload substrates without losing the ~2 atmosphere inside the tube. Thus, the furnace could remain at growth temperature during the loading procedure, decreasing the cycle time between runs. Once inside the reactor tube, the substrate could be preheated in pure H2 just before being introduced into the reactant gas flow at the growth posit.on. For a more detailed description, see Bozler, C. O., Solid State Research Report, 2, 52, Lincoln Laboratories, M.I.T. (1975).
A p layer, 1.7 ~m thick, was first grown on â
p , Zn-doped (100)-oriented GaAs substrate with a carrier concentration of 1018 carriers/cm3~followed by a thin n layerO The p layer (p~ l~cm 3) and n+
layer (n-~ x1018cm 3) were doped with Zn and S, respectively, by using (C2Hs)2Zn and ~2S sources.
The sheet resistance of the n+ layer w~s 7Q~/c.
To determine the thickness of this layer, the I-V
characteristic between two ohmic contacts to the layer was measured while a channel was being etched between the contacts. When the I-V char-acteristic for back-to-back diodes was observed, etching was immediatedly stopped, and the channel depth was measured with a profilometer. The n~
layer thickness measured by this technique was 1300A.
The initial fabrication step following layer growth was the pyrolytic deposition of SiO2 glass ~L9.37~

(1000 ~) on the GaAs wafer at 400C, in order to protect the n+ layer during the succeeding steps. Openings for ohmic contact fingers were etched in the ylass coating using photclitho-graphic techniques. There were 10 openings, 0.5cm long and 12 ym wide, spaced 1 mm apart~ The wafer was sputter-etched to remove GaAs to a depth of 40 ~ in the finger openings, then sputter-coated with successive layers of Au-12~ Ge (300 ~) and Au (2000 A). The Au/AuGe filmwas defined photolithographically into 25-lum-wide fingers, interconnected at one end, that overlaid the openings in the glass. All of the photolitho-graphic steps were carried out with standard equip-ment used for silicon wafer processing~ The waferwas then annealed under flowing N2 for one second at 300C on a graphite heater strip to establish ohmic contact between the AuGe fingers and the n+
layer, as verified by measurements of test con-tacts on the wafer. The conventional techniqueof alloying at ~50C was not used because it was found to cause penetration of t,he n layer, a~d su~sequent destruction of the homojunction.
The contact ingers to the n~~ layer were electroplated with Au to a thickness oS 4 ~n.
The back contact to the p+ substrate was made with sputtered Au. The active area of the cell was de-fined by etching a 1 cm x 0.5 cm rectangular mesa in the Ga~s, and the glass layer was removed with buffered HF. The fingers of the cell at this stage had a cross sectional configuration similar to that illustrated in FIG. 6, except that there was no anodic layer 28. Finally, the cell was antireflec-tion-coated with successive layers of Si0 (700 A) ~3~

and MgF2 (1200 A) ormed by electron beam evapora-tion. For GaAs with this two-layer coating, the average reflectivity measured over the 0.5-0.9 ~m wavelength band was less than 5%.
An efficiency measurement of the cell was made by using a high-pressure Xe lamp with a water filter as a simulated AM 1 solar source. The incident intensity was adjusted to 100mW/cm2, using a NASA
standard Si solar cell, c~librated for AM 1, as a reference. The open-circuit voltage was 0.91 V, the short-circuit current 10.3 mA, and the fill factor 0.82, giving a measured conversion effi-ciency of 15.3%. When the contact area is subtracted, the corrected efficiency is 17~. The 15 n factor at 100 mA/cm2 is 1.25, as obtained from the dark I-V characteristic, indicating good material quality with long carrier diffusion lengths. The series resistance is 0.5~.
The quantum efficienc~r of this cell as a func-20 tion of wavelength is shown in FXG. 7, and, as can be seen, quantum efficiency is hlghest at the longer wavelengths, with a gradual decrease at shorter wavelengths.
This cell was fabricated by sputtering and 25 alloying techniques, which although possible because of the relatively thick n layer, are not preferred.

GALLIUM ARSENIDE SHALLOW-HOMOJUNCTION PHOTOVOLTAIC
DEVICE HAVING ANODIC ANTIREFLECTION COATING
A photovoltaic device was prepared as in Example 1 except that the antireflection coating was an anodic coating, and all ohmic contacts were electro-plated.

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The GaAs layer used was grown in an AsC13-Ga-H2 CVD system on p+ Zn-doped (100)-oriented substrate with a carrier concentration of 1018cm 3. A p layer about 2 ~m thick was first grown on the substrate 5 followed by an n+ layer (n~5 ~ 1018cm~3) were doped with Zn and S, respectively~ by using (C2H5)2 Zn and H2S sources. Following GaAs growth, the n+ layer was anodically oxidized as follows.
The electrolyte solution used for anodization was prepared by mixing 3 g of tartaric acid with 100 ml of H20, adding sufficient NH40H to adjust the pH to about 6.2, and then adding 250 ml of propylene glycol. The final pH was 4.6-5.8. Anodization of 15 the GaAs was performed at room temperature, using a platinum wire as cathode. A smooth anodic layer of uniform thickness was obtained by using a constant current source with a voltage limiter. The source was set at a current corresponding to a 20 current density of about 750 ~A/cm2 ~or the GaAs anode, and the maximum output voltage was set at about 43 V. The current initially remained con-stant until the voltage increased to its limiting value, after which the voltage remained constant ~5 and the current decreased. Anodization was termin-ated when the current fell to one-tenth of its ini-tial value. The thickness (measured by ellipsometry using a He-Ne laser) of the anodic layer was abo~t 20 A/V and did not depend strongly on current density.
30 The layer produced took less than 5 minutes, was about 850 A thick, and consumed about 550 A of the GaAs layer. The anodic layer was stable up to at least 250C in air. The optical constants were mea-~7~

sured by ellipsomtery at 4358 and 5~61 A using a Hg lamp and at 6328 A using a He-Ne laser. The values of the refractive index n at these wavelengths are 1.91, 1.85 and 1.33 respectively, as shown in the inset of FIG. 8. The values of extinction coefficient k axe very low, and for the thickness of anodic layers used, absorption of the optical constants (and the effectiveness of the antireflection coating) is illustrated in FIG. 8 by the close agreement between the measured reflectivity spectrum of an 800 A thick anodic layer on GaAs and -the value for this struc-ture calculated using values of n obtained from the curve shown in the inset of FIG. 8, k = 0 for the anodic layer, and bulk optical constants 15 for GaAs.
A layer of Au about 3 ,um thick was then electro-plated on the p+ substrate as the back contact.
Photoresist AZ 1350J was spun on the anodic layer, and photolithographic techniques were used to etch 20 openings for ohmic contact fingers in the anodic layer. (The anodic layer dissolves readily in AZ
photoresist developer, as well as in HCl.) There were 10 openings, 0.5 cm long and 12 ~un~ wide, spaced 1 mm apart and interconnected with a bar at 25 one end. A layer of Au about 3 ~m thick was then electroplated into the openings, and the wafer was annealed in N2 for 1 sec at 300C on a graphite heater strip to produce ohmic contact between the Au fingers and the n+ layers. Finally, the active 30 area of the cell was defined by etching the GaAs to form a l-cm x 0.5 cm rectangular mesa.

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Efficiency measurements, using a high-pressure Xe lamp with a water filter as a simulated AM 1 source, werc made. The incident intensity was ad-justed to 100 mW/cm2, using a NASA-measured GaAs solar cell as a reference. The cell was also measured on the roof of the laboratory at an ambient temperature of about 20C. The solar flux density measured with a pyranometer was 98 mW/cm2, close to AM 1 conditions. The open-circuit voltage was found to be 0.9~ V, the short-circuit current 25.6 mA/cm2, and the fill factor 0.81, giving a measured conversion efficiency of 20.~ per cent, without correcting for the area of the contact fingers. The quantum efficiency of this cell as a function of wavelength is shown in FIG.
9. The quantum efficiency exceeds 90 percent at the maximum, but it decreases quite strongly at shorter wavelengths.
EXAMPLE 3 0 GALLIUM ARS~NIDE SHALLOW-HOMOJUNCTION PHOTOVOLTAIC
DEVICES ON GERMANIUM SUBSTRATES
.
The growth procedures and apparatus of Example 1 were used, except as noted. The Ge substrates were oriented (100) 2 off toward (110) and were prepared by coating them with Si02 on the backside to reduce Ge autodoping of the GaAs 1ayexs during growth. The electron concentration in nominally undoped layers deposited on these coated substrates was 5 x 1015cm 3. The lattice constants and expansion coefficients of Ge and GaAs are well matched, a favorable condition for obtaining good ~uality epitaxial layers~

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The doping profile used for the solar cells is shown in FIG. 10. The p+ Ge substrate was highly doped with Ga (8 x 1018cm 3) in order to overdope any As that might diffuse into the Ge during the 5 deposition of the GaAs and to assure tunneling through any thin barriers which could arise at the heterojunction interface. The p~ GaAs buffer layer was highly doped with ~n, again to assure tunneling and also to overdope Ge diffusing into 10 the GaAs during growth. The change in hole con-centration from 5 x 1018cm~3 in the buffer layer to 1 x 1017cm 3 in the active layer provided a backsurface field to increase the collection effi-ciency. The n+ layers were doped to a carrier con-15 centration of 5 x 1018cm~3 with sulfur, had asheet resistivity in the range 45-100~, and their electron mobility was ~1000 cm2/V-sec.
An AR coating was produced on the n+ layer by anodic oxidation, which consumed a thickness of 20 GaAs equal to 0.66 times the thickness of oxide produced. The anodizing solution was prepared by adding 3 g tartaric acid to 100 ml ~2~ adjust-ing the pH to 6.2 with NH40H, and adding 250 ml propylene glycol. The thickness of the oxide 25 layer was proportional to the limiting voltage used for anodi~ation. The thickness required for an optimum AR coating was 850 A, which waC
obtained for a limiting voltage of 43V~
Contact to the very thin n+ layer was made 30 easily because of its high doping level. Elec-troplated Au formed ohmic contacts with a spec-ific resistance of 8 x 10-5J~-cm2. Electroplated Sn also formed ohmic contacts, although their resistance was not measured. Sn had the advan-~3~

tage t~l~.t in the solution used for GaAs anodi-zation, Sn was also anodized, forming an oxide resistive enough to allow the GaAs to be anodized in the presence of Sn contacts.
Two different fabrication procedures were used for cells with Au and Sn contact fingers, as illustrated in FIGS. 4 and 5. For devices with Au contacts, the n+ layer was anodized first, finger openings were etched through the oxide 10 using a photoresist mask, and Au was plated using the same mas~. For cells with Sn contacts the Sn fingers were plated first, using a photo-resist mask, the photoresist was then removed, and the n+ layer was anodized. With this pro-15 cedure the n+ layer was thicker under the Sncontacts than ~Inder the anodic oxide, so that there was a larger separation of the metal from the p-n junction than with Au contacts. Because the n+
layer is so thin, the increased separation was 20 believed to be better for device yield and reli-ability.
A mesa etch of the GaAs was used to define the active area of the cells, and the back contact to the Ge substrate was made by Au plating. No alloy-25 ing or ~acuum processing was used in cell fahrica-tion.
Measurements of spectral response as a function of n+ layer thickness were made on small cells, 0.05 cm2 in area, having two Sn contact fingers 0.5 mm 30 apart connected to a Sn bar at one end. The n~
layer, which was initially 2000A thick with a sheet resistance of 45 Q~, was thinned by alter-nate anodization and stripping. The external quantum efficiency, which is the ratio of the 35number of carriers collected (Isc~q) to the number l3~6C~

of incident photons, was measured after each of three anodizations at ~3 ~, so that the cells were antireflection-coated during each measure-ment. The values of IsC and incident photon flux 5 were measured as a function of wavelength in a spectrometer which was arranged so that all th`e light fell between the two contact fingers. The results for cell 1 are given in FIG. 11, which shows that thinning the n+ layer results in a 10 marked improvement in quantum efficiency, espec-ially at shorter wavelengths This is expected because of the high absorption coefficients for GaAs (104-105cm 1), which increase with decreas-ing wavelength, and the high surface recombina-15 tion velocity, which is believed to be around107cm/sec. The power conversion efficiency for each n+ layer thickness is also given in FIG. 11.
These values were measured with the cell fully illuminated by a simulated AMl source, with no 20 correction made for the finger area.
FIG. 12 shows the final spectral response of cell 2, which was fabricated next to cell 1.
The n~ layer was sligh-ly thinner than that of cell 1, and the response was therefore slightly 25 improved at the short wavelengths. The curve ~or internal quantum efficiency, which is the ratio ph"t,or,S
~;~ of Isc/q to the rate at which ph~t~ enter the semiconductor, was o~tained from the measured external efficiency ~y correcting for the spectral 30 reflectivity of the AR-coated cell. This curve indicates that the cell design is very near the optimum.
The AMl power efficiencies of cells 1 and 2 ~' .

~, 17~

- 25 ~
and two other small cells fabricated side by side on the same wafer are plotted in FIG. 13 as a function of n+ layer thickness. In order to ob-tain additional thickness values, cells 2, 3 and 4 were first anodized to 10, 20 and 30 V, respect-ively, after which the oxide was stripp~d. All 4 cells were then anodized together to 43V to pro-vide an AR coating, stripped, anodized to 43 V, stripped again and once more anodized to 43 V.
10 Efficiency measurements were made after each 43 V
anodization. After the third such anodization~ the efficient of cell 3 had dropped to 10% while cell 4 had essentially no sensitivity to light.
~ssuming that the cell output drops to zero 15 when removal of the n+ layer is completed, for cells 1-4 complete removal would occur at a total anodi2ation voltage between 149 and 159 V, the ~inal values for cells 3 and ~, respectively. It was assumed for the purposed of plotting the data 20 that complete removal would occur at 154 V. Using the removal rate of 13.3 A/V, the initial n+ layer thickness was found to be 2050 A. To obtain the thickness of the n~ layer remaining after each successive anodization the total thickness removed 25 was calculated from the sum of the voltages used to that point and subtracted from the initial thickness.
The thickness values given in FIGS. 11 and 12, as well as those of FIG. 13, were obtained in this manner. For all the points in FIG. 13 the fill factor was 0.82 and Vn~ was 0.97 V. As the thick-ness of the n layer is reduced below 200 A thick, the efficiency drops precipitously from the maximum value of 21.2%.

~3~

Seven larger cells, lcm x 0~49 cm, were made from three different wafers. Either Au or Sn was used for the contact finger pattern, which consisted of 20 fingers 0.5 cm apart with a connecting bar at one end. The fingers covered 4% of the total area. One cell with Au contacts was partially shorted and one with Sn contacts was thinned too much. Power efficiency measurements, using a high pressure Xe lamp with a wafer filter as a simulated AMl source, were made on the other five cells. The incident intensity was adjusted to 100mW/cm2 using a NASA-measured GaAs solar cell as a reference.
Table 1 lists the measured values of VOC, ISc, fill factor and efficiency, as well as the initial sheet resistance and the total anodization voltage.
`~ Independent measurements ~ NASA Lewis Research Cen-ter have confirmed these results. The sheet re-sistance value gives some indication of the initial thickness of the n+ layer; a value of 100 ~/O corres-ponds to approximately 1200 A. The total anodization voltage is the sum of the limiting voltages used in a series of anodization~strip steps where the thin-ing ratio is 20 A/V. For each cell the final ano-dization was carried out at 43 V to provide the AR coating. The conve-cion efficiency values, not corrected for contact areas, are all in the 17 20%
range.

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~l37~i~4 ELECTRON BOMBARD~ENT OF GaAs _HALLOW-HOMOJUNCTION SOLAR CELLS
It has been reported that irradiation of Gal xAlxAs/
5 GaAs heteroface solar cells with electrons,causes the conversion efficiencies of these cells to be dramatically decreased. This is believed to be partly because of the ~ ~ n lengths of minority carriers in p and n layers decrease with increasing 10 electron irradiation, and partly because the sur-face recombination velocity at the Gal Al As/GaAs interface increases with increasing electron irrad-iation. It was hypothesized that the n /p/p structure of the cells described herein would 15 dramatically increase the resistance to cell desradation under electron irradiation. If so, the n /p/p GaAs shallow-homojunction solar cells would be outstanding candidates for space appli-cations, involving space vehicles, solar powered 20 satellites, etc.
To test the hypothesis, a series of experi-ments was run in which shallow-homojunction GaAs solar cells, as described herein, were irradiated with 1 MeV electrons from electron fluences rang-~5 ing from 1014 to 1016 electrons/cm2. The 1016electron/cm2 fluence is equivalent to dosages which electronic devices would be subjected to over 50 years in synchronous orbit.
In one experiment, a GaAs shallow-homojunction 30 solar cell prepared according to Example 1 and having a thin n+ layer of about 1000A without an antireflection coating thereon was subjected to electron fluences of 0, 8 x 1014, 3.2 x 1015 and 1.0 x 1016 electron/cm2. The change in the short 35 circuit current, ISc, caused by these electron 6~

fluences was determined by integrating the quantum efficiencies with the solar spectrum at air mass 0 (AM O), which is representative of space conditions.
FIG. 14 is a plot of Isc/(Isc)Owhere (ISc)o is the 5 original short circuit current before electron bom-bardment. As can be seen, at a fluence of 1 x 1016 electron/cm2, the decrease in ISc was only about 7%.
The corresponding Gal xAlxAs/GaAs cell is reported in the literature to undergo a 99% decrease in ISc 10 under a corresponding electron fluence. See l~alker, C.E., Byvik, C.E., Conway, E.J., ~-leinbockel, J.I~., and Doviak, M. J., "Analytical and Experimental Study of 1 MeV Electron Irradiated GaAl~s/GaAs Heteroface Solar Cells," J. ~lectrochem. Soc.: Solid-State 15 Science and Technology, 2034-36 (1978).
FIG. 15 illustrates corresponding data for a GaAs shallow-homojunction solar cell having a thin n~ layer of about 1400 A with an anodic AR coating under electron fluences ranging from 20 5 x 1013 to 7 x 1015 electron/cm~. This cell was also measured under simulate~ AM O conditions at the above range of dosages. As can be seen from FIG. 15 where items having the subscript O in-dicate values prior to electron bombardment, short 25 circuit current (ISc) decreased by about 20% at 7 x 1015 electron/cm2, the open circuit voltage, VOC, also decreased gradually. The fill factor, ff, actually increased slightly before it began to decrease with increasing fluences. The conversion 30 efficiency~~ of the cell, which was about 14% at AM O
before electron irradiation, still had o~er ~0~
of its original efficiency after being bombarded with 7 x 1015 electrons/cm2.

~L~37~

Industrial Applicability This invention has industrial applicability in the fabrication of solar cells, particularly solar cells for use in space applications.

Equivalents Those skilled in the art will recogni~e, or be able to ascertai.n using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.

Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A shallow-homojunction photovoltaic device formed from a direct bandgap semiconductor material doped to provide a n+/p/p+ layered structure, said n+ layer being sufficiently thin to allow significant carrier generation to occur in the p layer upon irradiation of said device with light.

2. A shallow-homojunction photovoltaic device of Claim 1 wherein said n+ layer has a thickness below about 1500 .ANG.,

3. A shallow-homojunction photovoltaic device of Claim 2 wherein said device has an antireflection coating over said n+ layer.

4. A shallow-homojunction photovoltaic device of Claim 3 wherein said direct bandgap semiconductor material comprises GaAs.

5. A shallow-homojunction photovoltaic device of Claim 4 wherein said device has a substrate formed from a different material than GaAs.

6. A shallow-homojunction photovoltaic device of Claim 5 wherein said substrate is formed from silicon or germanium.

7. A shallow-homojunction photovoltaic device of Claims 3, 4 or 5 wherein said antireflection coating comprises an anodic coating.