HETEROJUNCTION AND HETEROFACE STRUCTURE CELLS

CHAPTER 8

HETEROJUNCTION AND HETEROFACE STRUCTURE CELLS

In this c h a p t e r w e discuss the first-order criteria for the selection of heterojunction (HJ) solar cell materials. W e develop a figure of merit and c o m p a r e s o m e of the many possible candidates for H J c o u p l e s . By using a low lattice-mismatch, isotype Η J to provide a w i n d o w for a homojunction (i.e., a heteroface structure), a considerable reduction of surface recom­ bination loss for direct band-gap materials can be obtained. T h e A l G a A s / G a A s cell is discussed as an example of such a structure and as an e x a m p l e of the design of solar cells for c o n c e n t r a t o r s y s t e m s . Finally, the C d S / I n P cell is considered as an example of an anisotype Η J in which the properties of low lattice m i s m a t c h a p p e a r to be directly a d v a n t a g e o u s to j u n c t i o n transport.

8.1

CHOICE OF HETEROJUNCTION SOLAR CELL COMPONENTS

F o r the selection of a H J pair the following first-order criteria can be considered. (1) T h e a b s e n c e of conduction (or valence) band spikes that could impede p h o t o g e n e r a t e d carrier t r a n s p o r t . This condition is suggested if 299

Heterojunction

8

300

and Heteroface Structure

Cells

AE = Χι - χ < 0 and Δ £ = χ - Χι + E - E > 0 (see Sections 5.3.1 and 5.3.2). (2) ΔΕ (or Δ £ for η-type absorbers) as close to zero as possible to maximize t h e diffusion voltage V and V for the cell. (3) E close to 1.4-1.6 e V to take advantage of t h e m a x i m u m in solar efficiency of the absorber. (4) E as large as possible to p a s s as m u c h of t h e solar s p e c t r u m as possible, while still allowing a low-resistance material. (5) Small lattice mismatch. (6) Small thermal expansion mismatch. Since most j u n c t i o n s are pre­ pared at elevated t e m p e r a t u r e s , the thermal expansion coefficients must be m a t c h e d to some degree. C

2

ν

0

2

G2

GL

ν

d

oc

GL

G2

T h e subject of lattice mismatch is c o m p l e x . Although small lattice mis­ match does not assure large T J and large mismatch does not p r e v e n t large T J , the evidence does point to low mismatch as being beneficial. A n ex­ ample of t h e former situation is the Z n S e / G a A s j u n c t i o n , which h a s a low lattice mismatch a n d , according to the preceding criteria, should have an extremely high efficiency. T h u s far t h e Z n S e / G a A s cell has not realized this promise because of insufficient photogenerated current collection.t The effects of lattice mismatch and interfacial defects are discussed in Chapter 5 and by F a h r e n b r u c h and Aranovich (1979). A m i s m a t c h strain e < 1% appears to be generally beneficial, but for € > 1% t h e r e ap­ pears to be n o strong d e p e n d e n c e of t h e j u n c t i o n properties on t h e magni­ tude of e . In many cases large lattice mismatch does not d e c r e a s e the q u a n t u m efficiency appreciably, b u t leads to current p a t h s that decrease Voc and ff. Ternary c o m p o u n d s such as A L p G a ^ A s and Z n ^ C d ^ ^ S allow an ad­ ditional degree of freedom in the selection of either t h e band-gap or the lattice constant. Quaternary c o m p o u n d s such as A l ^ G a i - ^ A s ^ - j , and Cuxln^xSeyT^-y yield t w o degrees of freedom, and ideally both the b a n d gap and lattice constant can be adjusted to give optimal junction properties. Of course the price of this freedom is t h e additional com­ plexity of t h e system. Several such s y s t e m s are described in C h a p t e r 11. Since the fill factor is relatively constant for efficient cells (ff — 0 . 7 5 - 0 . 9 0 ) , a simple figure of merit for H J couples is the p r o d u c t of J and V . By assuming ηο(λ) = 1, / c a n b e determined approxi­ mately from (E - E ) using the integrated p h o t o n flux shown in Fig. 6.3. T h e open-circuit voltage is a s s u m e d to b e proportional t o t h e dif­ fusion voltage V = E - δ - δ + ΔΕ , w h e r e A £ ^ 0, δ = E - E , s

s

mf

m f

m f

s c

o c

s c

G2

GL

d

GL

η

ρ

0

c

η

C

F

t Gaugash et al. (1976) report 8 to 9% efficiency. See also Balch and Anderson (1972).

8.1

Choice of Heterojunction

Solar Cell

Components

301

8 = E - E and j u n c t i o n s with appreciable AE > 0 are not included. A brief list of H J candidates with p a r a m e t e r s is s h o w n in Table 8.1 with the figure of merit e x p r e s s e d as a " m e r i t efficiency" 7j it by assuming the c o m m o n values ff = 0.75 and Voc/Vd = 0.65.t O t h e r such lists have b e e n compiled by Milnes and F e u c h t (1972), and F a h r e n b r u c h (1977). B u c h e r (1978) gives a c o m p r e h e n s i v e listing of both homojunctions and H J s that have b e e n fabricated and their photovoltaic parameters. T h e r e a r e , of c o u r s e , other considerations for the formation of H J s such as interdiffusion, c o m p o u n d formation at the interface, resistivity, interfacial band pinning, and oxide layers formed prior to or during growth. At the t e m p e r a t u r e of fabrication, interdiffusion of the c o m p o n e n t species can o c c u r at either a small or large scale. In the small-scale case w e are considering transport of only sufficient material to cause changes in doping. T h e diffusion of Cu into C d S during fabrication of the C u ^ S / C d S H J is a prime example h e r e and the extent of Cu doping domi­ n a t e s the properties of this j u n c t i o n . Interdiffusion on a larger scale can lead to the formation of inter­ vening, third-compound layers at the metallurgical j u n c t i o n . A n example of this is provided by the spectral r e s p o n s e c u r v e s s h o w n in Fig. 8.1 for t w o /?-CdTe/rt-ZnSe cells p r o d u c e d at r a t h e r high t e m p e r a t u r e s (~600°C) by C S V T using t w o slightly different m e t h o d s (Buch et al., 1977). Cell M - l shows an interlayer of n-CdSe (E — 1.74 eV) that a b s o r b s p h o t o n s a b o v e its b a n d g a p , but not usefully. T h e spectral r e s p o n s e for cell M-21 indicates an interlayer of p - Z n T e (E — 2.3 eV) that h a s a relatively good η itself but prohibits all but hot p h o t o g e n e r a t e d carriers from t h e CdTe from being collected. An additional consideration is optical mismatch b e t w e e n the c o m p o ­ nents of a H J that can add to the reflection loss. T h e effect is small for most c o m p o u n d semiconductors ( < 1%) but can a p p r o a c h 5% for H J s that include metal oxide films ( Z n O , S n O ) . Of course this reflection loss might be eliminated by making the w i n d o w layer part of an antireflection sandwich or by making textured interfaces. In large-band-gap materials, particularly the I I - V I c o m p o u n d s , the tendency t o w a r d self-compensation is large, and production of suffi­ ciently low-resistivity material is difficult. This is particularly the case for polycrystalline thin films since the self-compensation is m o s t severe in regions of disorder such as grain b o u n d a r i e s . F o r e x a m p l e , n - Z n ^ C d ^ S P

Y

V9

C

mer

g

g

0

x

t Measured Voc/V values at AMI for common cells are Si, 0.59; AlGaAs/GaAs, 0.71; Cu S/CdS, 0.58; CdS/InP, 0.71; and CdS/CdTe, 0.50. d

x

6

d

c

6

a

1.43

1.50

4.5

2

2

c

= [<7

Y

-0.07

m

s

0

s

d

33 8.5

36 0.51

-0.02 0.05 0.76

-0.22 0.05 0

1

25

1.26

1.13 1.33

24 8.8

49

1.43

24 11.0

17.5

17.6

24 24

28

L

21.3 16.4 14.8

17.5

12.8 18.9

6

erit

^

29 18.7

15.0

2

J (mA cm" )

19 24

1.48 1.18

-0.3 1.13

d

V (V)

-0.12 0.05 1.08 0 0.55 27

(dT/dE) dE^ (0.65V ) ff/P , withff= 0.75 and P = 82.3 mW cm" . d

4.28 10.2 4.28 34.0

4.38 0.31 -0.75

0.83 0.04

-0.3 0

-0.12 0

η2

δ (eV)

-0.08 0.05 0.89 0.65 22

-0.3 +0.04

«25 -0.22 «25 -0.22

«15% «15 -0.12

c

A£ (eV)

ITO band gap changes for degenerate material because of Burstein shift. δ„ — E — E . Zn3P is pseudocubic with a = 2.86 A; 2a is used in these lattice match calculations.

rhnerit

a

4.09 4.07 0.23 4.09 3.6 0.93 -0.49 0.05

4.35

1.35 4.5 4.35 3.6 22*

a for hexagonal materials is given as 2 a .

0

2.67 2.67 1.4

2

/2-ZnSe//?-GaAs H-ZnSe//?-Zn P

3

2.42 3.3 1.5

2

2

A2-CdS//?-CdTe Λ-ΖηΟ/ρ-CdTe

3

3

2.42 3.3 1.4

e

4.28 4.28

0

4.01 «7 -0.49 4.13 0.15

4.5 4.5

4.38 4.38

0

Δα /α (%)

H-CdS/p-InP κ-ΖηΟ/ρ-Ζη Ρ

4.5 4.09

(eV)

4.5 4.5

Xl

1.12 4.09 4.01 4.3 2.42 1.4 4.5 3.6 2.26 -0.70 0.05

1.12 0.66

1.50 1.50

1.35 1.35

2

χ (eV)

n-ZnSe/p-Si 2.67 n-CdS/p-Zn P

Deg. η-ΙΎΟ/ρ-Si 3.35 rt-ZnSe//?-Ge 2.67

3.35 3.05

Deg. n-ITO/p-CdTe H-ITO/p-CdTe

c

3.35 3.05

c

el

E (eV)

Deg. Λ-ΙΤΟ/ρ-ΙηΡ «-ITO/p-InP

g2

Heterojunction E (eV)

Selected Heterojunction Solar Cells —Figure of Merit

Table 8.1

24.0

(%)

8.1

Choice of Heterojunction

Solar Cell

Components

303

ZnTe

1.3

1.5

1.7

1.9 PHOTON

2.1

2.3

2.5

2.7

2.9

ENERGY ( e V )

Fig. 8.1. Spectral response of two p-CdTe/VZnSe heterojunctions prepared by CSVT. Proposed band diagrams of each cell are also shown. [Redrawn from F. Buch, Ph.D. Thesis, Dep. Mat. Sci. and Eng., Stanford Univ., Stanford, California, 1976.]

films with compositions of χ > 20%XE > 2.65 eV) c a n n o t easily be m a d e with ρ < 1 Ω c m (Feigelsen et al., 1977; C h y n o w e t h and B u b e , 1980). A s n o t e d in C h a p t e r 5, the surfaces of m o s t s e m i c o n d u c t o r s t a k e o n a different potential than the bulk. Surface states dominate the electronic properties of the surfaces of covalent s e m i c o n d u c t o r s and pin E at the surface (i.e., the " § r u l e " ) . F o r s o m e materials the surface state effects a p p e a r to b e strong enough to invert the surface. F o r e x a m p l e , an atomically clean surface of /?-InP readily inverts to w-type o n e x p o s u r e to O at even m o d e r a t e t e m p e r a t u r e s (Weider, 1979) or with sputter cleaning in A r (Tsai et al., 1980). E x p o s u r e of the " p u r e " surface (e.g., cleaved) of a s e m i c o n d u c t o r to air very rapidly p r o d u c e s a thin oxide film (—15 A for G a A s or I n P within s e c o n d s at r o o m t e m p e r a t u r e ) that also affects j u n c ­ tion t r a n s p o r t , sometimes beneficially. T h e s e considerations are of criti­ cal i m p o r t a n c e in the fabrication of H J s a n d p r o m p t such techniques as the in situ etching required for stable results with the C d S / I n P Η J cell, g

F

z

304

8

Heterojunction

and Heteroface Structure

Cells

and the melt-back techniques sometimes used for preparation of A l G a A s / G a A s heteroface cells. As was pointed out in Chapter 5 and by F a h r e n b r u c h and Aranovich (1979), t h e b a n d configuration at t h e interface of a ΗJ and the electrical transport there is considerably m o r e complex than w e have suggested by considering j u s t a simple difference in electron affinities across the j u n c ­ tion. Despite the basic nature of this difference, it may play a minor role in junction transport. Indeed, other effects may dominate the situation: (1) microscopic pinning of the Fermi level at the interface (as influenced by chemical reactivity of the H J c o m p o n e n t s and the resulting charged inter­ face states and dipoles), (2) the defect structure in the vicinity of the inter­ face and the preferential segregation of d o p a n t s and impurities t h e r e , and (3) the p r e s e n c e of intervening insulating layers, formed during junction fabrication, which may contain t r a p p e d charge and can act as buffering layers to a c c o m m o d a t e lattice mismatch.

8.2

THE AlGaAs GaAs HETEROFACE SOLAR CELL

T h e band g a p of G a A s (1.43 eV) is near the o p t i m u m for solar conver­ sion, indicating a theoretical efficiency of 2 6 - 2 9 % at A M I . B e c a u s e the high mobility of G a A s allows the fabrication of very-high-frequency de­ vices, b e c a u s e its b a n d gap usefully c o m p l e m e n t s that of Si, and b e c a u s e of the ease of forming lattice-matched ternary c o m p o u n d s , G a A s rivals Si as the most investigated semiconductor material. G a A s has a direct band gap and a large optical absorption coefficient, absorbing 9 7 % of the A M I p h o t o n s within about 2 μπι. The n e a r perfect lattice m a t c h and a b s e n c e of interface state recombination of the A l G a A s / G a A s isotype j u n c t i o n has been used to advantage to r e m o v e the front surface recombination loss from the heteroface structure cell and to yield the highest efficiencies of any solar cell t y p e in 1983. B e c a u s e of t h e high c o s t of t h e material and fabrication, the present goals of the GaAs-based solar cell technology have been strongly directed toward c o n c e n t r a t o r systems and space u s e .

8.2.1

The Materials GaAs and AlGaAs

Although relatively a b u n d a n t in the e a r t h ' s crust (18 p p m as c o m p a r e d to 64 p p m for Cu), the supply of G a is not large in t e r m s of present-day re­ fining practice. In 1980 the cost for highly purified G a w a s s o m e $3000 k g " in small lots. H o w e v e r , the p r e s e n c e of G a in fly ash from the burning of coal promises a large future source should the need w a r r a n t setting up recovery m e t h o d s . Although arsenic is also quite a b u n d a n t and easily 1

8.2

The Al GaAs/GaAs

Heteroface

Solar Cell

305

Table 8.11 Properties of GaAs and AlAs {all at 300°K) AlAs

GaAs Physical properties Structure Melting point Density Thermal expansion Lattice constant

6

Electronic properties n, (297°K) N N X E , direct g

17

3

17

3

μη (N = 10 cm" ) μρ (N = 10 cm" ) r„, typical maximum T , typical maximum L , typical maximum L , typical maximum D

n

p

3a

b

E , indirect

p

3 a

18

g

-1

3

1.8 x 1 0 c m 4.27 x 10 c n r 8.19 x 1 0 c m " 4.07 eV 1.424 eV ί £J = 1.708 eV [Ε* = 1.900 eV 4000 cm V" sec" 250 cm V" sec" 1 0 sec 6 x 10" sec 3 x 10" sec 8 x 10" sec 4 - 8 μ,πι 23 μτη 4 μπι 5 - 6 μτη 17

c

- 3

6

l

e

v

A

Zincblende cubic 1740°C 3.73 g c m 5.2 x 10" ° C 5.661 A

Zincblende cubic 1238° 5.316 5.9 x 10" °C 5.654 A

«3.5 eV 3.018 eV = 2.3 eV E g = 2.168 eV 280 cm V" sec" x

2

1

1

2

1

1

2

1

1

-9

8

9

9

a

From H. C. Casey, Jr., and Μ. B. Panish, "Heterostructure Lasers, Part A," p. 207. Academic Press, New York, 1978. Eg(T) = 1.519 - 5.405 x 10" Γ /(204 + Τ). [From C. D. Thurmond, J. Electrochem. Soc. Ill, 1133 (1975).] 6

4

2

w o n , its chemistry and t h e inefficiency of refining the small lots u s e d at present m a k e s s e m i c o n d u c t o r grade As expensive ($500 k g ) as well. G a A s a b s o r b s strongly in the visible and h a s a zincblende cubic struc­ t u r e . A brief list of its physical and electronic properties is given in Table 8 . I I . t Although minority carrier lifetimes in G a A s are quite short ( ~ 1 0 sec) b e c a u s e of the direct band-gap n a t u r e of t h e material, the large mobility gives diffusion lengths quite sufficient for high q u a n t u m ef­ ficiencies ( 6 - 8 jLtm in L P E G a A s r G e ) . Optical absorption d a t a are s h o w n - 1

- 9

t An excellent source for the fundamental electronic properties of GaAs and AlGaAs alloys and junctions is Casey and Panish (1978a, Chapter 4).

306

8

Heterojunction

and Heteroface Structure

Cells

,ο»,

Ι Π

I

U

Ι6|

0

L_l

1

I

0.4

0.2

ALUMINUM

0.6

I

I

1

0.8

COMPOSITION,

1.0

X

Fig. 8.2. Carrier concentration in Al Ga _ As layers versus Al composition for four do­ pants. The mole percent of dopant in the melt is fixed at 0.5%. Layers are grown on (lOO)-oriented GaAs substrates at temperatures ranging from 810 to 840°C. [From D. Cheung, Ph.D. Thesis, Dep. Electr. Eng., Stanford Univ., Stanford, California, 1975.] x

x

x

in Fig. 3.8. Cells m a d e from A l G a A s must be encapsulated since AlGaAs is hygroscopic at high Al concentrations. Crystal growth of the material is usually by the Bridgman t e c h n i q u e , although t h e Czochralski m e t h o d can also b e u s e d . Synthesis from the elements must be d o n e carefully to avoid explosion. B e c a u s e of the large difference in the v a p o r p r e s s u r e (P = 3 x 10" a t m at 1240°C and P = 1 a t m at 615°C over the elements), an arsenic o v e r p r e s s u r e must b e used during growth to p r e s e r v e stoichiometry. Crystal growth can also be accomplished by chemical v a p o r deposi4

Ga

A s

8.2

The AlGaAs/GaAs

Heteroface

Solar Cell

307

tion and by g r o w t h from solution; t h e s e are the m o s t c o m m o n m e a n s of fabrication of solar cells from t h e s e materials. D o p a n t s include S, Se, T e , Sn, Si, C, and Ge as shallow d o n o r s and Z n , B e , M g , C d , Si, G e , and C as shallow a c c e p t o r s . T h e column IV e l e m e n t s — C , Si, G e , and S n — a r e a m p h o t e r i c in G a A s , and their elec­ trical activity in t h e crystal d e p e n d s on g r o w t h conditions. F o r e x a m p l e , G e is substitutional on an As site, acting as an a c c e p t o r for growth near 900°C, w h e r e a s for growth near t h e melting point Ge substitutes on a G a site as a d o n o r . Z n is volatile a n d fast diffusing and t h u s e a s y t o introduce in diffused j u n c t i o n s , but Ge and Be a p p e a r to give the highest lifetimes a m o n g the c o m m o n a c c e p t o r d o p a n t s . T e a p p e a r s t o be the d o p a n t of choice as a d o n o r impurity. T h e low atomic diffusivity of b o t h G e and T e m a k e it n e c e s s a r y to introduce t h e m during the growth of layers r a t h e r than by p o s t g r o w t h diffusion. D o p a n t s generally b e c o m e less electrically active with increasing Al c o n t e n t in A l G a A s , as s h o w n in Fig. 8.2. R e c o m b i n a t i o n c e n t e r s active in G a A s are similar to t h o s e in Si (Cr, F e , N i , C u , and Ag) but also include o x y g e n . Alj.Gai_j.As fulfills the n e e d for an adjustable band-gap c o u n t e r p a r t for G a A s with an exceptionally good lattice m a t c h (only 0.16% m i s m a t c h b e t w e e n AlAs and G a A s ) , and g r o w t h m e t h o d s involving A l G a A s h a v e the ability to form clean interfaces. Figure 8.3 s h o w s the energy b a n d gap

2.8,

Fig. 8.3. Energy gap versus lattice constant of various III-V binary and ternary com­ pound systems. Direct band gaps are shown by the solid line and indirect band gaps by the dashed line. [From D. Cheung, Ph.D. Thesis, Dep. Electr. Eng., Stanford Univ., Stanford, California, 1975.]

8

308

Heterojunction

and Heteroface Structure

Cells

versus lattice constant for Al, G a , I n , P, A s , Sb c o m p o u n d s . At a c o m p o ­ sition of χ — 0.44, the indirect and direct b a n d gaps of A l ^ G a ^ A s coin­ cide; for higher JC, the indirect b a n d gap of the ternary is lower, as s h o w n in Fig. 3.9.

8.2.2

Fabrication by LPE and CVD

T h e fabrication of layered structures by liquid p h a s e epitaxy L P E de­ p e n d s on the solubility of As in liquid Ga and AlGa alloys and the subse­ quent precipitation of A ^ G a ^ A s layers onto crystalline s u b s t r a t e s , which determine the crystallographic orientation of the layers.t T h e growth apparatus s h o w n in Fig. 8.4 is fabricated of high-purity carbon and q u a r t z , and growth occurs in an ambient of high-purity H . T h e top portion, containing the G a melt, slides o v e r t h e b o t t o m , which contains both the substrate and a GaAs s o u r c e . After the melt has b e e n saturated at Τ — 900°C o v e r the G a A s s o u r c e , the slider carries the G a melt over the substrate and the t e m p e r a t u r e is lowered at a rate of 0 . 1 - 0 . 5 ° C m i n ' . The solubility-versus-temperature data are shown in Fig. 8.5. G r o w t h rates of 0 . 1 - 0 . 5 μπι m i n are obtained. During deposition the relatively large vol­ u m e of G a acts as a sink for segregated impurities. Since the growth pro­ ceeds very near thermal equilibrium, the resulting epitaxial layers are of very high quality. T o stabilize the growth, t e m p e r a t u r e gradients are sometimes established across the freezing front by cooling d e v i c e s , pro­ ducing layers of very uniform thickness and preventing the formation of hillocks. 2

1

- 1

Since the growth takes place in a system of C, S i 0 , and H , the resis­ tivity of the resulting layers d e p e n d s on the growth t e m p e r a t u r e and the resulting activity of the C, which acts as an amphoteric dopant in G a A s . The oxygen partial pressure must be kept as low as possible since Ο is an active recombination center in G a A s . During the initial m o m e n t s of deposition there is, in some c a s e s , a small dissolution of the G a A s substrate, which ensures an atomically clean interface and accounts for the low recombination velocities ob­ served at the metallurgical interface. By adding a small quantity of Zn to the Al, G a melt, a shallow p - t y p e layer can be formed in the G a A s substrate simultaneously with the forma­ tion of the AlGaAs layer. Alternatively, a separate /?-GaAs layer can be grown from a Ge-doped G a melt prior to the growth of the A l G a A s layer. The diffused layer and the grown layer are t w o major alternatives for this sort of device. Naively, one would expect greater crystalline perfection and longer lifetime in the grown layers. 2

t A review of LPE is given by Dawson (1972).

2

8.2

The AlGaAs/GaAs

Heteroface Solar Cell GaAs MELT.

SOURCE 2

309

A £ GaAs MELT

SUBSTRATE

SOURCE I ® NEUTRAL

Π

™ * *

# 3

# 2

# 1

τ'τ Τ V f 'ι e

d

c

b

a

© GROWTH OF GaAs PULL

¥1 © GROWTH OF AlGaAs ©

COOLING RATE = 0.2°C/MIN

© TEMPERATURE CYCLE OF A TYPICAL GROWTH

TIME Fig. 8.4. Cross section of LPE growth system and temperature-time profile during growth of GaAs and AlGaAs layers. The growth system is situated within a quartz tube car­ rying a flow of high purity H at atmospheric pressure. [From D. Cheung, Ph.D. Thesis, Dep. Electr. Eng., Stanford Univ. Stanford, California, 1975.] 2

Several variations on L P E g ro w th are u s e d t o e n h a n c e the properties of the d e v i c e s . Deposition can t a k e place using u n d e r s a t u r a t e d , saturated, or s u p e r s a t u r a t e d m e l t s , t h e latter typically giving smaller minority carrier diffusion length L in the g r o w n layer (Woodall and H o v e l , 1977). " L e a c h i n g " or gettering of impurities from t h e underlying ρ or η substrate layers to e n h a n c e L can be accomplished by annealing the substrate in the G a , Al melt before growth (Woodall and H o v e l , 1975). An " e t c h b a c k r e g r o w t h " m e t h o d , also used by Woodall and H o v e l (1977), accomplishes

Heterojunction

8

310

, -3| n

I U

|

I

and Heteroface

Structure

I

Cells

1

700

800 900 1000 TEMPERATURE, Τ (°C) Fig. 8.5. Solubility of As in liquid Ga versus temperature. [From L. C.-C. Shen, Ph.D. Thesis, Dep. Electr. Eng., Stanford Univ., Stanford, California, 1975.]

both gettering and growth of a compositionally graded A ^ G a ^ layer in a single step. T h e advantages of such compositionally graded layers are discussed in Section 8.2.6. T h e other major growth method for films of A ^ G a ^ A s is chemical vapor deposition C V D [or, m o r e restrictively, v a p o r p h a s e epitaxy (VPE)] by synthesis from gaseous G a and As c o m p o u n d s . By reaction of G a C l and arsine ( A s H ) on a substrate at 600-800°C, growth rates for G a A s of > 1 0 μ,πι h r can be achieved (Johnston and Callahan, 1976). Deposition can also be accomplished by pyrolysis reaction of organometallic G a c o m p o u n d s (e.g., trimethylgallium) and A s H at an rf-heated substrate (MO-CVD)(Dapkus
3

- 1

3

2

8.2

The AlGaAs/GaAs

Heteroface

Solar Cell

311

B e c a u s e of t h e e x t r e m e difference in v a p o r p r e s s u r e s of the elements (a ratio of 1 0 at 400°C) stoichiometry is impossible to maintain in stand­ ard v a c u u m evaporation of G a A s from the c o m p o u n d . High quality layers can be g r o w n by molecular b e a m epitaxy ( M B E ) , h o w e v e r . A review is given by C a s e y and Panish (1978b, p . 132). 11

8.2.3

The Ga As-Based Solar Cell—Basic Structure and Preliminary Optimization

E x c e p t for an isolated report of η = 4 % for a p/n homojunction by J e n n y et al. (1956), the history of the G a A s cell is m u c h m o r e r e c e n t than that of the Si cell; o n e of the early reports of a G a A s p/n j u n c t i o n cell w a s by G o b a t et al. (1962), w h o q u o t e d T J = 1 1 % and J = 17 m A c m " at A M I , with a large V . In 1970 Alferov et al. (1971) r e p o r t e d t h e firstpAlGaAs/rc-GaAs heterojunction structure with an AMO efficiency of 1 0 - 1 1 % and a strongly increased short-wavelength r e s p o n s e with respect to t h e p/n homojunction. A large j u m p in efficiency o c c u r r e d in 1972 with the heteroface s t r u c t u r e , apparently first r e p o r t e d by Woodall and H o v e l (1972), w h o obtained η = 15.3% at A M I and 1 9 . 1 % at A M 2 with a p Al , Gao. As//?-GaAs/At-GaAs device fabricated by L P E . Diffusion of Zn during the A l G a A s deposition formed the intermediate /?-type layer. Be­ c a u s e of its high efficiency, t h e G a A s - b a s e d cell w a s soon associated with solar concentration w h e r e t h e high cell cost could be a b s o r b e d in t h e cost of t h e c o n c e n t r a t o r s y s t e m . 8

2

s

L

o c

8

0

7

3

+

In 1978 F a n and Bozler (1978) r e p o r t e d a n+/p/p homojunction G a A s cell with a structure m u c h like that of the standard B S F Si cell. T h e ρ and n layers of t h e cell w e r e fabricated by C V D and the thin n layer w a s re­ d u c e d to its final thickness (0.045 μπι) by anodization. T h e anodic oxide layer w a s left on to function as an antireflection coating for the cell. This cell had an A M I efficiency of 2 0 % with no c o n c e n t r a t i o n . Heteroface structures still h a v e a s o m e w h a t higher η with r e p o r t s of 24.7% at A M I (C = 180) by Sahai et al. (1978) for a heteroface structure with a graded A l G a A s layer, T J = 2 1 % at A M I . 4 (no concentration) by J a m e s and M o o n (1975), and of 21.9% at A M I (no concentration) by Woodall and H o v e l (1977). Woodall and H o v e l ' s cell configuration, similar t o t h a t s h o w n in Fig. 8.6, has b e e n changed little for most of t h e p r e s e n t designs e x c e p t for vari­ ations in t h i c k nes s and composition of t h e l a y e r s . Typical values of the p a r a m e t e r s of an experimental cell (without concentration) are listed in Table 8.III. At t h e light-incident surface x + a. considerable portion of the p h o ­ t o g e n e r a t e d carriers is lost to surface recombination ( 5 = 10 c m s e c " ) . H o w e v e r , b e c a u s e of t h e large indirect b a n d gap of A l G a A s , only a small +

+

8

s

p

6

1

372

Heterojunction

8

and Heteroface

Structure

hv

Fig. 8.6. Typical heteroface AlGaAs/GaAs solar cell structure. Table 8.III AlGaAs /GaAs Heteroface Cell / 7 - A l G A I _ A s window layer Dopant

0

Parameters

+

X

-

WP

J

+

*'p \

Ge 7 ΜΓΗ 0.7 10 cm" 2.08 eV 2.41 eV 18

Composition, χ ΝΑ

Ε , indirect E , direct p-GaAs layer Dopant \x ~ x \ N

3

8

g

p

Ge 1.2 ΜΠΙ 5 - 8 μτη 9 x 10 cm"

P

17

A

n-GaAs substrate Dopant Κ

-

*n\

3

Te - 2 0 0 ΜΠΙ 1.6 x ΙΟ cm" 18

Cell \x

n

Jo

A factor Voe ( A M I ) Jsc ( A M I )

ff

α

Xp

0.04 ΜΠΙ 10~ A cm" -2 0.88 V 0.02 A cm~ 0.75 14.8% 10

2

2

L. C.-C. Shen, Ph.D. Thesis, Dep. Electr. Eng. Stanford Univ. Stanford, California, 1976.

3

Cells

8.2

The AIGaAs/GaAs

Heteroface

313

Solar Cell

p-AlGaAs

-4 -2

0

DISTANCE

2

4

V

-4

-2

(Μ™)

0

2

DISTANCE

(>im)

Fig. 8.7. Electron-beam-induced current versus distance for (left) a structure with a pGaAs/Zn layer formed by diffusion during AlGaAs layer growth and (right) a structure with a p-GaAs/Ge layer grown by LPE with subsequent growth of the AlGaAs layer. [Redrawn from L. C.-C. Shen, Ph.D. Thesis, Dep. Electr. Eng., Stanford Univ., Stanford, California, 1976.]

fraction of the total J is generated in this w i n d o w layer, and so the rela­ tive loss is small. B e c a u s e of good lattice matching and b e c a u s e the fabri­ cation p r o c e s s e s can p r o d u c e clean interfaces, the recombination loss at the A l G a A s / G a A s interface is small. M e a s u r e m e n t s by E t t e n b e r g and K r e s s e l (1976) indicate values of Si < 10 c m s e c . . B e c a u s e the /? -layer also serves for lateral current collection in most cases (see Section 8.2.4 for an exception), the resistivity of the thin /?layer, w h e r e m o s t of the useful absorption o c c u r s , c a n be optimized with regard for L r a t h e r than for current collection considerations. Shen (1976) c o m p a r e d Zn-diffused j u n c t i o n s with Ge-grown j u n c t i o n s , finding greater L and effective collection for the latter c a s e , as illustrated by the E B I C plots of Fig. 8.7. O t h e r E B I C m e a s u r e m e n t s indicated L = 6.7 μ,πι for Ge doping at a doping level of 2 x 1 0 c m . T h e diode c u r r e n t - v o l t a g e characteristics of G a A s - b a s e d cells are r e p r e s e n t e d remarkably well by the Shockley injection and recombina­ t i o n / g e n e r a t i o n m o d e l s . F o r current levels corresponding t o u n c o n c e n trated A M I illumination either injection or r e c o m b i n a t i o n / g e n e r a t i o n L

4

- 1

+

n

n

n

17

- 3

314

8

Heterojunction

and Heteroface Structure

Cells

Table 8.IV Diode Parameters of Selected GaAs-Based Solar Cells Reference a

Alferov et al. Van der Plas et al.

b

c

Ewan et al.

Type

Jo

A

Homojunction Heteroface

0.25-1.7 x IO"

Heteroface

2.08 x IO"

19

12

Details

-2 1 2 -1

Measured in dark At high concentration At AMI Curve fitting of illumi­ nated data for 200 < C < 10 log 7 versus VV 3

d

Fan et al.

+

e

Loo et al. Shen' Calculated

9

n /p Homojunction Heteroface Heteroface Heteroface

1.4 x IO"

17

9

1.6 x IO" 1.1 x 10" 2 x 10~ 18

10

1.1 2.28 -2 1

SC

Dark; C = 1 type cell Dark

a

Zh. K. Alferov, U. M. Andreev, Μ. B. Kagan, I. I. Protasov, and V. G. Trofim, Sov. Phys.—Semicond. 4 , 2047 (1971). H. Van der Plas, personal communication, 1979. J. Ewan, R. C. Knechtli, R. Loo, and G. S. Kamath, Proc. 13th IEEE Photovoltaic Specialists Conf (1978) p. 941. J. C.-C. Fan and C. O. Bozler, Proc. 13th IEEE Photovoltaic Specialists Conf (1978) p. 953. R. Loo, L. Goldhammer, B. Anspaugh, R. C. Knechtli, and G. S. Kamath, Proc. 13th IEEE Photovoltaic Specialists Conf (1978) p. 562. L. C.-C. Shen, Ph.D. Thesis, Dep. Electr. Eng., Stanford Univ., Stanford, California, 1976. Ideal, A = 1 homojunction current calculated using the material properties and dimen­ sions of Table 8.III. b

c

d

€

F

9

modes may d o m i n a t e , but u n d e r high concentration most cells show the A = 1 injection behavior with corresponding low J . Table 8.IV lists the diode A factors and J values of several cells, along with a J calculated from the materials p a r a m e t e r s of S h e n ' s cell for comparison. H o v e l (1973) c o m m e n t s on the junction transport m e c h a n i s m s , using the S a h N o y c e - S h o c k l e y and C h o o models to calculate ultimate efficiencies for GaAs-based cells. T h e heteroface cell may be optimized in several w a y s : 0

0

0

+

(1) Decreasing the window (p )-Iayer thickness. Figure 8.8 s h o w s the calculated effect of decreasing the w i n d o w thickness \x' — x' \ for Α Μ 0 conditions. T h e results suggest making the window as thin as pos­ sible consistent with its spreading resistance. T h e s e results a r e , of c o u r s e , strongly d e p e n d e n t on the A l / G a composition chosen and the air m a s s n u m b e r . T h e effect on η is shown in Fig. 8.9. p+

8

p

8.2

The AlGaAs/GaAs § ο

Heteroface

315

Solar Cell

I.Oi

-

£ 0.8 -

\ρ=0.2μπα

CO

δ

X

V

-

&0.4I

-

«0.2

V

Ι­ Ο UJ QCO

I I ..5

I.O

I

I

2.0

2.5

\

3.0

3.5

4.0

PHOTON ENERGY, hv (eV)

Fig. 8.8. Calculated spectral response of an Alj.Gaj_a.As/GaAs cell with the A l - G a ^ A s thickness D as the independent variable. The parameters used here are \x - - 4 | = W = S(x' -d = S(x' ) = p

d

p

p

1 μπί, 0.01 μτη, 10 cm s e c 10 cm s e c

L - 0.5 μτη for AlGaAs, L = 5.0 μτη for p-GaAs, L = 1.0 μτη for /z-GaAs, JC = 0.86. n

n

6

-

4

-

p

[From L. C.-C. Shen, Ph.D. Thesis, Dep. Electr. Eng., Stanford Univ., Stanford, Cali­ fornia, 1976.]

(2)

Optimization

of the p-layer

the / 7 - l a y e r t h i c k n e s s d =

x\ p

thickness.

T h e effect of c h a n g i n g

to m a x i m i z e c a r r i e r g e n e r a t i o n a n d

collection is s h o w n in Fig. 8.10, w h i c h indicates a b r o a d m a x i m u m for d — 1 μ π ι for t h e larger L

v a l u e s using t h e s a m e c o n d i t i o n s as t h e p r e -

n

IOL

Ι

Ι

1

2

Al Ga x

h x

1

1

3

4

J - — — I

5

6

A s THICKNESS, D (j_m)

Fig. 8.9. Calculated AMO efficiency as a function of the AlGaAs thickness D. Parame­ ters used are the same as for Fig. 8.8. V = 1.0 V and ff = 0.8 are assumed. [From L. C.-C. Shen, Ph.D. Thesis, Dep. Electr. Eng., Stanford Univ., Stanford, California, 1976.] oc

316

Heterojunction

8

and Heteroface Structure

Cells

§40,

<

L =l0/im

Ε

2

I

I.5

2.0

JUNCTION DEPTH, d

2.5 2.

3.0

{μη\)

Fig. 8.10. Calculated J at AMO as a function of the p-GaAs layer thickness for several values of L . Parameters used are the same as those of Fig. 8.8. [L. C.-C. Shen, Ph.D. Thesis, Dep. Electr. Eng., Stanford Univ., Stanford, California, 1976.] sc

n

ceding.t Van der Plas et al. (1978), optimized the total efficiency versus /?-layer thickness in concentrator cells and found similar b r o a d m a x i m a for d LJ4. (3) Optimization of doping level. T h e doping of the η and ρ regions can be varied to maximize V while retaining a long L in the p-layer. In junctions w h e r e transport is controlled by r e c o m b i n a t i o n / g e n e r a t i o n this optimization involves the adjustment of W with consideration for the ef­ fect of doping level in the quasi-neutral regions on the lifetime in the deple­ tion layer. T h e d e p e n d e n c e of T J on the Μ - t y p e quasi-neutral region doping level for cells controlled by r e c o m b i n a t i o n / g e n e r a t i o n is discussed by Sekela et al. (1977), w h o a s s u m e d that the lifetime in the depletion layer is controlled by τ in the η-type quasi-neutral region. Their calculations indi­ cate that an optimum exists in the range Ν = 10 to 1 0 c m but with no strong d e p e n d e n c e on N . T h e y estimate that cells with r) < 2 2 % at AMO with no concentration can b e m a d e with presently available mate­ rials (assuming no grid coverage or reflection losses). F o r cells in which injection and diffusion dominate, the optimization variables are more directly accessible and higher doping densities might be favored. oc

n

a

s

ρ

16

17

- 3

Ό

D

s

One of the principal advantages of the GaAs-based cell is the small de­ crease of efficiency with t e m p e r a t u r e . M o s t m e a s u r e m e n t s show a m o n o tonic d e c r e a s e of p e r c e n t efficiency of 0.033 percentage points p e r °C (e.g., from 20.000% to 19.967% for 1°C t e m p e r a t u r e increase) such as that shown in Fig. 6.23. t For these conditions the J increase is approximately linear with p-layer thickness (if transport is dominated by J^) as indicated by Fig. 5.6 and so the change in with thickness is rather small. 0

8.2

The AlGaAs/GaAs

Heteroface Solar Cell

317

30.00P

25.00h

20.00h

0.00 1.00

100.00 CONCENTRATION

10.00

1000.00

1.00E4

Fig. 8.11. Calculated solar efficiency as a function of concentration ratio for an AlGaAs/GaAs LPE cell for various values of lumped series resistance. [Courtesy of R. Sahai, D. D. Edwall, and J. S. Harris, Jr., Proc. 13th IEEE Photovoltaic Specialists Conf. (1978), p. 946. © 1978 IEEE.]

8.2.4

AlGaAs/GaAs Concentrator Cells

At t h e high concentration ratios favored for terrestrial concentration systems using GaAs-based cells, o t h e r optimization p r o c e d u r e s are re­ quired. Since efficiency generally increases with intensity as s h o w n in Fig. 8.11, concentration ratios of 10 and a b o v e are favored. T h e major limitation at these high ratios arises from current collection, requiring total R values of less than ΙΟ" Ω c m . t T h e w i n d o w layer plays a major role in current collection, so some sort of c o m p r o m i s e b e t w e e n A l G a A s sheet resistance (thickness and resistivity), grid spacing, a n d t h e a m o u n t of light transmitted by the A l G a A s layer must be m a d e . T h e design of the grid itself is critical, a n d every a s p e c t — c o n t a c t resistivity; finger width, thickness, and spacing; a n d b u s r e s i s t a n c e — m u s t be considered in pro­ p e r optimization. 3

3

s

t Thermal considerations at high concentration ratios are discussed in Section 12.2.2.

8

318

Heterojunction

and Heteroface Structure

Cells

F o r GaAs-based cells the D e m b e r voltage has only a small effect ( < 1 m V ) , since the photogenerated carrier density ( « 2 x 10 c m at C = 10 ) is small c o m p a r e d to the majority carrier density. N e v e r t h e l e s s , the high-intensity designs have light incident on the /?-side so that the D e m b e r voltage adds to V . T h r e e c o n t e m p o r a r y designs, all using L P E growth and all with 7j > 20% under high concentration, are considered h e r e : a design by L. J a m e s reported by V a n d e r Plas et al. (1978) and designs reported by E w a n et al. (1978) and Sahai et al. (1978). P e r h a p s the major variation among t h e m is in the thickness of the A l G a A s window: 1.2 μπι (Sahai et al., der P l a s m a / . , 1978), 10 μπι (Ewtmetal., 1978), a n d 0 . 0 5 μπι ( S a h a i e t a i , 1978). Compositions of χ > 0.9 are used in all. T h e p a r a m e t e r s of these cells are summarized in Table 8.V. In the design by Sahai the 0.05 μπι A l G a A s layer is used only to re­ duce surface recombination rather than for current collection. T h e s e cells show a considerably larger J (—31 m A c m " w h e n normalized to C = 1, 100 m W c m ) than do the cells designed by E w a n ( ~ 2 3 . 4 m A c m at C = 1) or J a m e s (—25.7 m A c m " at C = 1), p r e s u m a b l y b e c a u s e of the difference in A l G a A s t h i c k n e s s . In the design reported by Sahai et al. the grid c o n t r a c t s are m a d e through the A l G a A s layer, directly to the /?-GaAs in o r d e r to reduce con­ tact resistance. H o w e v e r , surface recombination is generally m u c h larger at metal//7-GaAs interfaces, possibly resulting in higher J despite the small contact area. This may be responsible for the s o m e w h a t lower V seen in these cells as c o m p a r e d with those in which c o n t a c t is m a d e to the AlGaAs layer. T h e p-layers of these cells are more uniform in thickness. Doping is by B e , Mg, or Zn and L = 4 - 5 μπι. T h e lowest effective series resistance is for the cell designed by J a m e s , which utilizes a / ? - G a A s layer b e t w e e n the contact metallization and the AlGaAs w i n d o w to obtain contact resistivities in the 1 0 " - 1 0 " Ω c m range and an overall R of 4 x IO" to 4 χ 10~ Ω c m . All of these cells, particularly the o n e by J a m e s , s h o w e d almost exact agreement with the elementary theory utilized in the c o m p u t e r simulation used for their design. 14

3

3

oc

s

2

s c

- 2

- 2

2

0

oc

n

+

5

5

4

4

2

2

s

8.2.5

GaAs-Based Cells for Space Power Systems

A promising use for G a A s solar cells is in s p a c e , w h e r e the powerto-weight ratio and radiation resistance are of primary i m p o r t a n c e . By 1980 several experimental cell test groups had undergone testing in simu­ lated space radiation environments and other groups have b e e n tested in space. G a A s cells appear to have significantly better radiation resistance

8.2

The AlGaAs/GaAs

Heteroface

Solar Cell

319

Table 8.V Parameters for Selected AlGaAs /GaAs Concentrator Cells Van der Plas° Alj.Gai_j.As Layer χ

0.93 1.2 5 x 10 Mg

D (/LTM) 3

NA (cm" ) Dopant p-GaAs Layer

17

Ewan

>0.90 10 1.5 x 10 Be

6

Sahai

0.05 3 x 10 Be

18

c

18

Diffused 1.2 7 x 10 Mg 4

Diffused

n-GaAs Base Ν (cm" ) Dopant A> (i-m)

7 x 10" Sn 4

2 x 10 Sn 2 (calculated)

Device R (Ω) Grid spacing (μπι)

62

0.01 250

0.02 90-100

1006 (AMI) 23.7 1.19 20 23.6

178 (AM2) 4.35 1.07 24.7 50 -31

22.3 (at C = 197)

20.3 (at C = 440)

d (/LTM) 3

NA (cm" ) Dopant L (μπι)

LPE grown 2 2 x 10 Zn 5

17

n

3

Ό

s

Photovoltaic parameters Concentration ratio J (A c m ) - 2

sc

Voc(V)

Temperature (°C) J (mA c m ) , normalized t o C = 1, 100 mW cm" T7s (%), at other concentration ratios -2

sc

945 (AM 1.5) 20.7 1.14 23 50 25.6

18

Be

17

2

a

H. A. Van der Plas, L. W. James, R. L. Moon, and N. J. Nelson, Proc. 13th IEEE Photovoltaic Specialists Conf. (1978), p. 934. J. Ewan, R. C. Knechtli, R. Loo, and G. S. Kamath, Proc. 13th IEEE Photovoltaic Specialists Conf (1978), p. 941. R. Sahai, D. D. Edwall, and J. S. Harris, Jr., Proc. 13th IEEE Photovoltaic Specialists Conf. (1978), p. 946. 6

c

t h a n Si c e l l s , a l t h o u g h this r e s i s t a n c e a p p e a r s t o b e m o r e strongly d e p e n d ­ ent on fabrication v a r i a b l e s t h a n t h a t for Si ( L o o et al.,

1978). T h e d e ­

c r e a s e s in efficiency o n e x p o s u r e t o r a d i a t i o n a p p e a r t o fit t h e m o d e l s set u p t o quantify radiation effects in Si cells [ e . g . , E q . (7.7)]. S e v e r a l m e t h o d s exist for minimizing o r alleviating r a d i a t i o n d a m a g e . P o s t e x p o s u r e annealing (e.g., 30 h at 200°C) h a s b e e n u s e d t o r e s t o r e t h e

320

8

Heterojunction

and Heteroface Structure

Cells

major part of lost efficiency (Walker and C o n w a y , 1978). G r a d e d junction structures (discussed in the next section) utilizing built-in electric fields can reduce the effects of the radiation-lowered L . H u t c h b y ' s (1978) de­ tailed c o m p u t e r analysis for a doubly graded structure indicates substan­ tially increased radiation resistance. This is particularly appealing for the AlGaAs system since grading is rather easy to accomplish during fabrica­ tion by C V D or L P E . n

8.2.6

GaAs-Based Graded Band Gap and Schottky Barrier Cells

T w o alternative GaAs-based structures are discussed briefly here: compositionally graded structures and Schottky barrier cells. Poly crystal­ line, thin-film G a A s cells are discussed in Chapters 9 and 11 and by H o v e l (1975). Introduction of a band-gap gradient by compositional grading has sprung from three s o m e w h a t different but interrelated goals: 1. Grading is a natural extension of the window c o n c e p t for opti­ mizing both the admission of light and the absorption of light in a layer backed by a p/n j u n c t i o n . 2. Grading and c o n s e q u e n t high electric fields n e a r the incident sur­ face act to reflect carriers away from the surface and reduce the surface recombination loss. 3. High electric fields throughout the generation volume can substan­ tially increase the effective L there [L ff — L (%qL /kT) as in E q . (4.28)]. n

e

n0

n0

T h e s e ideas have been considered for a long time, and the A l G a A s - G a A s system appears to be ideal for exploring their effects. F o r e x a m p l e , T a u c (1957) apparently first p r o p o s e d the reduction of the surface recombina­ tion velocity by a gradient-induced electric field in 1957. T a u c also showed that a potential is generated across an illuminated graded bandgap semiconductor because of the gradients t h e r e . Marfaing and Che valuer (1971) e x t e n d e d the theory of T a u c to graded structures in which band gap, mobilities, lifetimes, carrier densities, and effective masses varied with position and c o m p a r e d their calculations with experimental H g ^ C d ^ T e structures, finding good a g r e e m e n t . T h e y found that a small p h o t o voltage of < 1 . 5 m V could be generated by the gradients at very high illumination levels. H o w e v e r , for all practical solar cell p u r p o s e s the voltage generated by the gradients is completely neg­ ligible and the primary effects of junction grading are those listed at the beginning of this section. H u t c h b y and Fudurich (1976) have m a d e a very detailed (and quite conservative) c o m p u t e r simulation of rt-AlGaAs//?-GaAs graded struc-

8.2

The AlGaAs/GaAs

Heteroface Solar Cell

AlGaAs

-IR

321

GaAs

Fig. 8.12. Energy band diagram of Au/n-AlGaAs/w-GaAs Schottky barrier solar cells. The thickness of the AlGaAs layer is greater than the depletion layer width in part (a) and smaller than the depletion layer width in part (b). [From Y.-D. Shen, Ph.D. Thesis, Stan­ ford Univ., Stanford, California, 1978.]

t u r e s , using p a r a m e t e r s for available materials as far as k n o w n and con­ sidering all k n o w n loss m e c h a n i s m s , e v e n to the antireflection coating. Their optimal cell has an A l - G a ^ - A s layer graded o v e r 1.0 μ,πι from x = 0 . 3 5 t o j c = 0, yielding a solar efficiency of 17.7% at AMO. T h e grading provides increased resistance to degradation of L and S in radia­ tion e n v i r o n m e n t s . Woodall and H o v e l (1977) used an ' i s o t h e r m a l e t c h b a c k - r e g r o w t h " n

4

8

322

Heterojunction

and Heteroface Structure

Cells

technique for growth of p - A l - G a ^ - A s layers graded from χ ~ 0.9 to χ = 0 o v e r a distance of 0 . 2 - 0 . 4 μ π ι . The G a - A l - Z n melt w a s slightly u n d e r s a t u r a t e d when positioned o v e r the ft-GaAs substrate. This first dis­ solves a portion of the substrate and then regrows a graded layer of A l G a A s as a result of the concentration gradients set u p in the melt during the dissolution part of the p r o c e s s . Diffusion of Zn during the p r o c e s s re­ sults in a 0 . 8 - 2 μπι /?-GaAs:Zn layer. T h e resulting cells have 2 1 . 8 % effi­ ciency at A M I (T7 = 18.4% at AMO, simulated) and considerably im­ proved short-wavelength r e s p o n s e . K o r d o s et al. (1979) prepared 0.2 μ,πι A l G a A s layers by a similar m e t h o d . Using Auger depth profiling to verify the grading profile, they found good agreement with calculations assuming aluminum diffusion through a concentration gradient in the liquid adjacent to the growth inter­ face. T h e y also found that the graded layer depth (or e t c h b a c k thickness) could be controlled by the degree of undersaturation within a range of 0 . 1 5 - 0 . 2 5 μπι. Solar cells m a d e using this technique by K o r d o s and Pearson (1981) suggested strongly improved high-energy r e s p o n s e could be obtained using these m e t h o d s . Schottky and M I S structures on G a A s are discussed in detail in Sec­ tion 11.1.2, but an interesting Au/rt-AlGaAs/w-GaAs structure r e p o r t e d by Shen and Pearson (1979) deserves mention h e r e . Theoretical analyses predict that this structure, shown in Fig. 8.12, can h a v e J as large as a conventional Au/rt-GaAs cell and that a considerably greater V can be obtained. Experimental results show that V increases with Al mole frac­ tion χ from 0.53 V (x = 0) to 0.70 V (x = 0.5). Since a band-gap difference Δ Ε ( « 0 . 4 e V for χ = 0.3) exists that could impede hole transport to the Au (as indicated by Fig. 8.12a), the AlGaAs layer must be thin enough so that both A l G a A s and G a A s are de­ pleted at the H J and so that the H J interface o c c u r s in a high electric field region. T h e result of variation of t h e A l G a A s thickness o n the m e a s u r e d spectral r e s p o n s e is shown dramatically in Fig. 8.13. A family of c u r v e s of similar a p p e a r a n c e is generated by variation of reverse bias V in a cell with a larger thickness (e.g., d = 2000 A ) as increasing V r e d u c e s the ef­ fect of ΔΕς. Some j u n c t i o n grading at the H J interface may also r e d u c e the effec­ tive Δ/ig, as discussed in Section 5.3.3. Such m e a s u r e m e n t s d o m u c h to restore o n e ' s faith in the c o n c e p t of H J band discontinuities. An unoptimized A u / n - A l G a A s / n - G a A s cell recorded V = 0.88 V and 7j = 10.5% (AMI simulation, with n o antireflection coating, and a AlGaAs layer thickness of 300 A ) (Yang et al., 1980). A m o r e general t r e a t m e n t of t h e s e double layer Schottky barrier devices is given by L e e and Pearson (1980). s

s c

oc

oc

8

R

R

oc

s

8.3

InP-Based

Cells

323 WAVE L E N G T H , λ (/_m)

0.8856

1.2398

0.6888

0.5635

0.4768

0.4133

- -^tr

ο

d= 8 0 0 A 5000 A o

1030 A

0

ZL

1.2

I

1.4

I

1.6

I

I

1.8

4 _ «

I

I

2

I

I

I

2.2

I

2.4

I

2.6

I

L_

2.8

3

PHOTON ENERGY, hi/ (eV)

Fig. 8.13. Normalized collection efficiency versus photon energy of A u / «-Alo.43Gao.57As/rc-GaAs Schottky barrier solar cell. The independent variable is the AlGaAs layer thickness d. The characteristic of a Au/GaAs cell is shown for comparison purposes. [From Y.-D. Shen, Ph.D. Thesis, Stanford Univ. Stanford, California, 1978.]

8.3

InP-BASED CELLS

T h e C d S / I n P system is discussed as an e x a m p l e of a true H J in which excellent lattice matching and i n t e r f a c i a l ' c l e a n l i n e s s " a p p e a r to play an important role in the junction current t r a n s p o r t . Highly efficient cells have b e e n fabricated by v a c u u m evaporation, chemical v a p o r deposition, and close-spaced v a p o r transport (CSVT) m e t h o d s . I n P is a direct-band-gap material with a b a n d gap of 1.34 e V , close to the o p t i m u m for solar energy conversion. Like o t h e r homojunctions of direct-band-gap materials, the efficiency I n P homojunction a p p e a r s to be limited by surface recombination at the illuminated surface (Galavnov et al., 1967). H o w e v e r , in a heterojunction configuration the potential ad­ vantages of I n P can be realized. Indium is a rather expensive metal ($2000 k g in small lots in 1980) with a natural a b u n d a n c e of 0.14 p p m (as c o m p a r e d to 64 p p m for Cu) in the e a r t h ' s crust. P h o s p h o r u s , although a b u n d a n t and inexpensive in its impure forms, is expensive to purify ($4000 k g " in small lots of 6n purity). I n P crystals are grown by the Czochralski m e t h o d at high p r e s s u r e s or by using a liquid encapsulation technique to p r e s e r v e stoichiometry. R e s e a r c h o n I n P heterojunctions did not start until about 1974, proba­ bly b e c a u s e of unavailability of high-quality single crystals of p - t y p e I n P . 4

- 1

1

324

Heterojunction

8

and Heteroface Structure

Cells

The first reported results on n-CdS/p-InP heterojunctions, by W a g n e r et al. (1975), described a 12.5% efficient cell. T h e choice of C d S a p p e a r s to be particularly well suited for such a heterojunction. B e c a u s e the lattice constant of zincblende I n P is 5.869 A and the corresponding p a r a m e t e r (2 a) of wurtzite C d S is 5.850 A , there is only 0.32% lattice mismatch b e t w e e n the (111) plane of I n P and the basal plane of hexagonal C d S . In addition, the tetrahedral atomic distance is 2.533 A in I n P and 2.532 A in C d S , indicating that the orientation of the I n P is not critical, as has b e e n experimentaly b o r n e out. W a g n e r ' s first cells were p r e p a r e d by v a c u u m deposition of C d S on 0.4 Ω c m I n P single crystals doped with Cd. T h e y showed a q u a n t u m efficiency of 0.70 b e t w e e n 0.55 and 0.91 μ,πι, and re­ combination/generation type transport with a low J = 5 x 10~ A c m , suggestive of a good lattice m a t c h . T h e solar efficiency η = 12.5% w a s obtained at a relatively low solar intensity of 53 m W c m (a cloudy day in N e w J e r s e y ) , which gave V = 0.63 V , / = 15 m A c m , and a fill factor ff = 0.71 (with an antireflection coating). It w a s discovered that a heat t r e a t m e n t at Τ — 600°C in a nonoxidizing a t m o s p h e r e improves this type of C d S / I n P cell, yielding V = 0.72 V and a solar efficiency of 14% (Shay et al., 1976). N o change i n j u n c t i o n properties w a s o b s e r v e d for a subsequent 15-min anneal at t e m p e r a t u r e s less than 550°C, indicating good operating stability of such a cell. By assuming that o p t i m u m j u n c t i o n quality c o r r e s p o n d s to that of an I n P ho­ mojunction, extrapolated m a x i m u m theoretical values for the C d S / I n P junction of V = 0.85 V, / = 20 m A c m " , ff = 0.75, and T J = 17.2% were estimated. F u r t h e r i m p r o v e m e n t of actual cell performance w a s achieved by going to a chemical vapor deposition of C d S on I n P using an open-tube H S / H flow s y s t e m (Shay et al., 1977; Bettini et al., 1977, 1978). At an appropriate concentration of H S (about 2 mol%) in the gas flow, the sur­ face of the I n P is continuously etched by the formation and sublimation of indium sulfides; C d S nucleates on this clean surface and prevents further attack of the I n P by the H S . With single-crystal I n P , values were ob­ tained as follows: V = 0.79 V , J = 18.7 m A c m " , ff = 0.735, and η = 15.0% at A M 2 . Capacitance data on t h e s e cells again indicated an abrupt junction but with a diffusion voltage of 1.00 ± 0.15 V, close to the value of 1.25 V for the diffusion voltage of an I n P homojunction. C d S / I n P j u n c t i o n cells have also been prepared by the v a c u u m evapo­ ration of C d S o n t o single-crystal p - t y p e I n P homoepitaxial layers, which w e r e , in turn, grown by a chemical vapor deposition method on p single-crystal I n P s u b s t r a t e s . t Cells showing b e t w e e n 8 and 12% effill2

9

- 2

0

8

- 2

- 2

oc

s c

oc

2

oc

2

s c

s

2

2

2

2

oc

s c

8

+

t In these cells, the surface of the CdS layer is slightly textured due to faceting during growth, resulting in some antireflective properties (Manasevit et al., 1978).

8.3

InP-Based

Cells

325

ciency w e r e p r e p a r e d , with the best cell having V = 0.72 V, J = 23.2 m A c m " , ff = 0.63, and η = 11.9% (without an antireflection coating, A M ~ 1 illumination of 94 m W c m , active area basis). E x t r a p o ­ lation of the open-circuit voltage to Τ = 0°K yielded a diffusion voltage of 1.06 e V in this c a s e . T h e highest V o b s e r v e d to date w a s reported in an excellent w o r k by Y o s h i k a w a and Sakai (1977) for a cell prepared by close-spaced-vaportransport. T h e photovoltaic p a r a m e t e r s w e r e V = 0.807 V , J = 18.6 m A c m " , ff = 0.74, and η = 14.4% ( A M 2 , 77 m W c m " ) . In con­ trast with the cells previously mentioned, these w e r e grown on the (110) face rather than the (111) face. T h e growth t e m p e r a t u r e w a s — 710°C. T h e s e r e s e a r c h e r s found that V = 1.14 e V , a value in agreement with the Voc values seen by t h e m and other w o r k e r s as well as being in almost exact agreement with published electron affinity values for C d S and I n P . Capacitance data on these cells indicated an abrupt j u n c t i o n corre­ sponding to the band diagram s h o w n in Fig. 8.14. In view of the preceding c o m m e n t s on the degree of lattice matching b e t w e e n C d S and I n P and the implied relevance of this m a t c h for t h e good cell properties o b s e r v e d , it is something of a surprise to find that essen­ tially equally good cells can be m a d e with I T O / I n P j u n c t i o n s for which the lattice mismatch is large. Ion-beam-deposited I T O layers with resis­ tivity of 10~ Ω c m on Zn-doped p - t y p e single crystals of I n P yield solar cells with V = 0.76 V , J = 21.5 m A c m " , ff = 0.65, and T J = 14.4% oc

s c

2

8

- 2

oc

oc

2

s c

2

8

D

3

2

oc

s c

s

0.0475;um

CdS 19 -3 η = 2.8 χ 1 0 cm , y

J

Fig. 8.14. Energy band diagram for CdS/InP heterojunction prepared by close-spaced vapor transport of CdS film onto single crystal InP. [Redrawn from A. Yoshikawa and Y. S. Sakai, Solid State Electron. 20, 133 (1977). Copyright (1977), Pergamon Press, Ltd.]

Heterojunction

8

326

and Heteroface Structure Cells

(using a M g F antireflection coating) (Sree H a r s h a et al., 1977). Similar re­ sults have been obtained for sputtered I T O layers on homoepitaxial p - I n P layers grown by chemical v a p o r deposition on p - I n P substrates (Manasevit et al., 1978). T h e best cell reported for this process had V = 0.69 V , J = 23.4 m A c m " , ff = 0.65, and η = 12.4% ( ~ A M 1.5 illumi­ nation at 85 m W c m without an antireflection coating). Extrapolation of the open-circuit voltage to Τ = 0°K gives a diffusion voltage of 1.09 V, in good agreement with the values cited previously. In the latter case some heat treatment is required to p r o d u c e the m a x i m u m efficiency. T h e suc­ cess of the I T O / I n P junctions in spite of lattice mismatch can probably be attributable to the existence of a buried homojunction in the InP. Evidence for a buried homojunction has b e e n obtained by m e a s u r e ­ ments of rf sputtering deposited I T O / I n P cells by B a c h m a n n et al. (1979). Ion probe analyses showed the p r e s e n c e of sufficient Sn penetration into the p - I n P substrate to change the conductivity type for nominal substrate t e m p e r a t u r e s of 250°C. W h e n the I T O w a s deposited at a nominal 27°C substrate t e m p e r a t u r e , no evidence of Sn penetration w a s found, although this does not preclude the existence of a very thin homojunction. Tsai et al. (1980) found that buried homojunctions in the ITO//?-InP system were produced by the action of sputter deposition. Schottky diodes were formed by v a c u u m evaporation of thin films of Au on p - I n P substrates. W h e n these substrates had been previously sputter cleaned in Ar u n d e r the same conditions as those for the I T O deposition, spectral response curves with sharply reduced high-energy r e s p o n s e w e r e ob­ tained, typical of homojunction behavior. T h e s e spectral r e s p o n s e s were nearly the same as those for the ITO//?-InP cells. H o w e v e r , w h e n A u / p - I n P Schottky barriers w e r e formed on substrates that w e r e not sputter cleaned after etching, the high-energy spectral r e s p o n s e w a s m u c h larger, typical of that seen for good Schottky barrier devices. T h e s e last conclusions may be further c o m p a r e d with those for C d S / G a A s heterojunctions fabricated by Bettini et al. (1978) and by Yo­ shikawa and Sakai (1975) by C V D (lattice mismatch = 3 . 5 % ) . Surface etching and heat treatment are again required to obtain the best diodes. Although diodes with moderately good diode characteristics w e r e ob­ tained (e.g., J = 10" A c m " with A = 1.80; Bettini et al., 1978), both T J and V for the C d S / G a A s heterojunctions were too low to be competi­ tive. Indium-tin-oxide/GaAs cells m a d e by B a c h m a n n et al. (1979) using rf and magnetron sputtering and ion b e a m deposition show considerably lower J , V , and η ( < 5 % ) than d o similar I T O / I n P cells ( η > 14%). Apparently the details of the junction formation permit good TJQ and V in the p r e s e n c e of large mismatch in the case of the I T O / I n P heterojunction but not in the case of C d S / G a A s or I T O / G a A s . 2

+

oc

2

sc

8

- 2

8

2

Q

0

oc

s c

oc

5

8

oc

8.4

Summary

8.4

SUMMARY

327

+

+

T h e G a A s heteroface structure, the G a A s n /p/p homojunction, and the A u / A l G a A s / G a A s Schottky diode cells are all in remarkable agree­ ment with solar cell t h e o r y . In particular, J c a n be calculated with rea­ sonable a c c u r a c y from m e a s u r e d materials properties. T h e C d S / I n P cell also gives a close to theoretical efficiency, but these cells frequently show tunneling transport rather than r e c o m b i n a t i o n / g e n eration transport, for reasons that are not completely u n d e r s t o o d . In addition to the fortuitous lattice matching of the C d S / I n P j u n c t i o n , the experiments reported here indicate that either heat treatment (at Τ > 500°C) or in situ etching is required to realize the highest and m o s t uniform efficiencies. This suggests one o r m o r e of the following possibili­ ties. 0

(1) Interfacial debris is r e m o v e d or inactivated by these t r e a t m e n t s . (2) S o m e sort of c o m p o u n d formation o c c u r s at the interface that neutralizes the interfacial recombination. (3) A heteroface structure is p r o d u c e d that effectively m o v e s the p/n j u n c t i o n a w a y from the metallurgical interface. T w o of the strongest general guiding principles in the historical devel­ o p m e n t of heterojunction cells h a v e b e e n the use of a heteroface structure to avoid surface recombination loss, and the d e m a n d for excellent lattice m a t c h . T h u s it is rather ironic t o consider the existence of important c o u n t e r examples for both these criteria: the p /n G a A s homojunction with 20% efficiency reported by F a n and Bozler (1978) (Section 8.2.3) and the I T O / I n P heterojunction with 14.4% solar efficiency reported by Sree H a r s h a ^ r al. (1977). T h e first example shows that surface recombination loss can be controlled by suitable fabrication t e c h n i q u e s , in this case by making the n - l a y e r extremely thin (and by the possible a d v a n t a g e o u s ef­ fect of passivation of the « - G a A s surface recombination by anodization).t T h u s other direct band-gap materials m a y p r o v e to be highly effi­ cient in the homojunction configuration, provided suitable fabrication m e t h o d s c a n be found. Although the second c o u n t e r e x a m p l e , ITO//?-InP, is explained by the formation of a buried homojunction, o u r understanding of the details of the heterojunction interface still d o not permit us to predict the formation of the buried homojunction or its effect on carrier t r a n s p o r t . F o r e x a m p l e , w h y d o e s the ITO//?-GaAs analog apparently not p r o d u c e good cells? +

+

+

t On the other hand, these thin AiMayer cells do appear to be much more difficult to fabri­ cate than the heteroface AlGaAs/GaAs cells.

328

8

Heterojunction

and Heteroface Structure

Cells

T h u s still other guiding principles, in particular those involving t h e micro­ scopic chemistry of the interface region, must be added t o the list begin­ ning this chapter. A n u m b e r of other heterojunctions and heteroface cells, including both single crystal and polycrystalline materials are considered in C h a p t e r 11.

REFERENCES Alferov, Ζ. I., Andreev, U. M., Kagan, M. B.,Protasov, 1.1., andTrofim, V. G. (1971). Sov. Phys.—Semicond. 4, 2047. [Fiz. Tekh. Poluprovodn. 4, 2378 (1970).] Bachmann, K. J., Schreiber, H., Jr., Sinclair, W. R., Schmidt, P. H., Thiel, F. Α., Spencer, E. G., Pasteur, G. Feldmann, W. L., and Sree Harsha, K. S. (1979). J. Appl. Phys. 50, 3441. Balch, J. W., and Anderson, W. W. (1972). Phys. Status Solidi A 9, 567. Bettini, M., Bachmann, K. J., Buehler, E., Shay, J. L., and Wagner, S. (1977). J. Appl. Phys. 48, 1603. Bettini, M., Bachmann, K. J., and Shay, J. L. (1978). J. Appl. Phys. 49, 865. Buch, F., Fahrenbruch, A. L., and Bube, R. H. (1977). J. Appl. Phys. 48, 1596. Bucher, E. (1978). Appl. Phys. 17, 1. Casey, H. C , Jr., and Panish, Μ. B. (1978a). "Heterostructure Lasers, Part A." Academic Press, New York. Casey, H. C , Jr., and Panish, Μ. B. (1978b). "Heterostructure Lasers, Part B." Academic Press, New York. Chynoweth, Τ. Α., and Bube, R. H. (1980). J. Appl. Phys. 51, 1844. Dapkus, P. D., Dupuis, R. D., Yingling, R. D., Yang, J. J., Simpson, W. I., Moudy, L. Α., Johnson, R. E., Campbell, A. G., Manasevit, Η. M., and Ruth, R. P. (1978). Proc. 13th IEEE Photovoltaic Specialists Conf., p. 960. Dawson, L. R. (1972). In "Progress in Solid-State Chemistry" (H. Reiss and J. O. McCaldin, eds.), p. 117. Pergamon, Oxford. Dupuis, R. D., Dapkus, P. D., Yingling, R. D., and Moudy, L. A. (1977). Appl. Phys. Lett. 31, 201. Ettenberg, M., and Kressel, H. (1976). J. Appl. Phys. 47, 1538. Ewan, J., Knechtli, R. C , Loo, R., and Kamath, G. S. (1978). Proc. 13th IEEE Photovoltaic Specialists Conf., p. 941. Fahrenbruch, A. L. (1977). J. Cryst. Growth 39, 73. Fahrenbruch, A. L., and Aranovich, J. (1979). Heterojunction phenomena and interfacial defects in photovoltaic converters. In "Solar Energy Conversion—Solid State Physics Aspects" (B. O. Seraphin, ed.), Topics in Applied Physics, Vol. 31, p. 257. Springer-Verlag, Berlin and New York. Fan, J. C.-C, and Bozler, C. O. (1978). Proc. 13th IEEE Photovoltaic Specialists Conf, p. 953. [Also see J. C.-C. Fan, C. O. Bozler, and R. L. Chapman, Appl. Phys. Lett. 32, 390 (1978).] Feigelsen, R. S., N'Diaye, Α., Yin, S.-Y., and Bube, R. H. (1977). J. Appl. Phys. 48, 3162. Galavanov, V. V., Kundukhov, R. M., and Nasledov, D. N. (1967). Sov. Phys.—Solid State 8, 2723. Gaugash, P. V., Kas'yan, V. Α., Kovol'kov, V. I., and Rakhimov, N. R. (1976). Sov. Phys.—Semicond. 9, 1239. Gobat, A. R., Lamorte, M. F., and Mclver, G. W., (1962). IRE Trans. Mil. Electron. 6, 20.

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