the preparation and properties of vapor-grown gaas-gap alloys

Vol. 110, No. 9 P R O P E R T I E S O F E P I T A X I A L G E R M A N I U M 991

The h igh va lue s of m o b i l i t y (F ig . 8) e spec ia l Iy a t low t e m p e r a t u r e a r e cons ide r ed to be our mos t s ens i t i ve t e s t of the h igh q u a l i t y of t he p r e s e n t films. In c o n t r a s t a r e t h e low mob i l i t i e s w h i c h h a v e b e e n r e p o r t e d in e v a p o r a t e d (4, 13) a n d s p u t t e r e d (14) g e r m a n i u m s ingle c r y s t a l films. T h e a n n e a l e d f i lms of R u t h et al. also do no t show i d e a l l a t t i c e s c a t t e r e d mob i l i t y .

Conclusion A g e r m a n i u m fi lm g r o w n f r o m the v a p o r b y the

h y d r o g e n r e d u c t i o n of GeC14 has been s h o w n b y e l ec t ron t r a n s m i s s i o n m i c r o s c o p y to be of c r y s t a l q u a l i t y c o m p a r a b l e to m e l t - g r o w n g e r m a n i u m a n d to be n e a r i n t r i n s i c a t r o o m t e m p e r a t u r e a f t e r a n - n e a l i n g at 550~ fo r 250 hr .

C o n d u c t i v i t y and H a l l m e a s u r e m e n t s a r e i n t e r - p r e t e d as showing c o n v e n t i o n a l l a t t i c e s c a t t e r e d m o b i l i t y a n d the p r e s e n c e n e a r r o o m t e m p e r a t u r e of abou t 1014 c m -~ to t a l ion ized impur i t i e s . The r e su l t s a r e cons i s t en t w i t h t he p r e s e n c e of 5.6 x 10 TM cm -3 copper as e x p e c t e d f r o m the a n n e a l i n g and ~ 4 x 10 TM

cm -3 donors . The e x p e r i m e n t i nd i ca t e s t h a t t r a n s p o r t in g e r -

m a n i u m dev ices m a d e f r o m such a n n e a l e d f i lms d o p e d 1015 cm -~ or m o r e wi l l b e s u b s t a n t i a l l y t h e same as in c o n v e n t i o n a l g e r m a n i u m .

Acknowledgment The a u t h o r s w i sh to t h a n k B. G. Cohen and R. L.

J o h n s t o n fo r t h e i r g e n e r o u s coope ra t i on in m a k i n g t h e H a l l m e a s u r e m e n t s . T h e y w o u l d also l i k e to t h a n k R. A. L o g a n for m a k i n g s e v e r a l p r e l i m i n a r y Ha l l m e a s u r e m e n t s , J. T. Nelson, B. S c h w a r t z , J.

Godf rey , and J. M. W h e l a n for h e l p f u l d iscuss ion, a n d O. H. A. H a a s e a n d Mrs. M. H. R e a d for t h e i r a id in o b t a i n i n g the e l e c t r o n m i c r o g r a p h s and d i f - f r a c t i o n p a t t e r n s

Manuscr ip t rece ived Feb. 11, 1963; rev ised m a n u - scr ipt received Apr i l 11, 1963. This paper was de l ivered at the Det ro i t Meeting, Oct. 1-5, 1961.

Any discussion of this paper wi l l appear in a Discus- sion Section to be publ i shed in the June 1964 JOURNAL.

REFERENCES

1. H. C. Theuere r and H. Christensen, This Journal, 107, 268C (1960).

2. H. C. Theuerer , J. J. Kle imack, H. H. Loar, and H. Christensen, Proc. IRE, 48, 1642 (1960).

3. R. P. Riesz and C. G. Bjor l ing, Rev. Sci. Instr., 32, 889 (1961).

4. R. P. Riesz and L. V. Sharp, Trans. IRE, ED-8, 430 (1961).

5. H. A. Woodbury and W. W. Tyler , Phys. Rev., 105, 84 (1957).

6. A. G. Tweet, ibid., 106, 221 (1957). 7. F. H. Doleiden, P r iva te communicat ion. 8. F. J. Mor in and J. P. Maita, Phys. Rev., 94, 1525

(1954). 9. F. J. Morin, ibid., 93, 62 (1954).

10. E. Conwell and V. F. Weiskopf, ibid., 77, 388 (1950). 11. R. P. Ruth, J. C. Marinace, and W. C. Dunlap, J.

Appl. Phys., 31, 995 (1960). 12. C. S. Fu l l e r and F. H. Doleiden, Physics. and Chem.

Solids, 19, 251 (1961). 13. G. A. Kurov , S. A. Semiletov, and Z. G. P insker ,

Kristallografiya, 2, 59 (1957). [Trans la ted in Soviet Physics--Crystallography, 2, 53].

14. F. Reizman, Proc. of AIMME Conference on Meta l - l u rgy of E lementa l and Compound Semiconduc- tors, Augus t 1961, to be published.

The Preparation and Properties of Vapor-Grown GaAs-GaP Alloys

San-Mei Ku General Telephone & Electronics Laboratories Inc., Bayside, New York

ABSTRACT

A method for the p repa ra t ion and doping of var ious composi t ion of s ingle- c rys ta l G a A s - G a P is described. By measur ing the opt ical energy gap and the la t t ice p a r a m e t e r of the synthesized mixed c rys ta l the iodine vapor ep i tax ia l g rowth technique is found to give a one- to -one ra t io of t r anspor t of GaAs and GaP f rom source to substrate . The var ia t ion of Hal t mobi l i ty wi th composit ion f rom 5 to 50% of GaP mix tu re has been invest igated. The edge emission peak of diodes made wi th al loys or less than 50% GaP var ies l inea r ly be tween the d i rec t energy gap of GaAs and GaP.

The o r i g i n a l r e p o r t on the m i x i n g of two s e m i - conduc to r s is t h a t of S t 5 h r a n d K l e r n m (1) , w h o i n v e s t i g a t e d the p h a s e d i a g r a m of G e - S i b y a p o w - d e r a n n e a l i n g t echn ique . F o l l o w i n g th i s i n i t i a l w o r k , f u r t h e r i n v e s t i g a t i o n s w e r e m a d e b y m a n y a u t h o r s (2 -4 ) on th i s t y p e of a l loy. I t w a s s h o w n t h a t con- t inuous sol id s o l u b i l i t y a n d v a r i a t i o n of e n e r g y gap ex is t s ove r t he en t i r e r a n g e of compos i t i on f r o m p u r e Ge to p u r e Si. W o r k on s e m i c o n d u c t o r a l loys has b e e n e x p a n d e d to m i x t u r e s of t h e I I I - V c o m - pounds for f u r t h e r e x t e n s i o n of t he r a n g e of a v a i l - ab l e e n e r g y gap ( 5 - 9 ) . Recen t p r o g r e s s in d e v e l o p -

ing h igh-e f f i c i ency i n f r a r e d - e m i t t i n g G a A s d iodes has s t i m u l a t e d f u r t h e r i n t e r e s t in o b t a i n i n g a h i g h - eff iciency l igh t source in t he v i s ib l e r e g i o n a n d t hus p r o m p t e d a d d i t i o n a l s tud ies on m i x i n g a h i g h e r - b a n d - g a p m a t e r i a l such as G a P (Eg = 2.2 ev) w i t h GaAs . The i n i t i a l r e su l t s on t h e G a A s - G a P s y s t e m h a v e a l r e a d y b e e n p u b l i s h e d (5, 10, 18). I t was f o u n d t h a t con t inuous so lu t ion was o b t a i n e d t h r o u g h o u t t he en t i r e a l l oy range . F o l b e r t h (5) m e a s u r e d the v a r i a t i o n of e n e r g y gap w i t h compos i - t ion b e t w e e n 50 a n d 100% of G a P fo r th i s sys tem, a n d m o r e r e c e n t l y P i zza re l l o (11) m e a s u r e d t h e l a t -

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.130.252.222Downloaded on 2014-07-12 to IP

http://ecsdl.org/site/terms_use

992

tice p a r a m e t e r and energy gap for var ious compositions.

This paper describes the p repa ra t ion of uni form s ing le -c rys ta l G a A s - G a P 1 by the ep i tax ia l growth technique at a r e la t ive ly low tempera tu re . The in- fluence of var ious types of dopants in the mixed alloys are discussed. Some electr ical and optical proper t ies of var ious composit ions of G a A s - G a P are also repor ted.

Preparation Previous works have proven the feas ibi l i ty of both

the closed and the open systems of growing indi- v idual GaAs or GaP by chemical react ion of the hal ides of the const i tuents or by other v a p o r - d e p o - sition techniques. The closed sys tem used in this invest igat ion to synthesize mixed b ina ry crysta ls is s imi lar to the one descr ibed by Hagenloeher (12) for the ep i tax ia l growth of GaAs. This method was chosen because it al lows uni form epi tax ia l growth of a mixed single c rys ta l at r e l a t ive ly low t e m p e r a - ture (as low as 650~ and s imul taneous ly permi t s doping dur ing growth.

Transpor t and crys ta l growth of G a A s - G a P alloys of var ious composit ion was car r ied out via a vapo r - phase react ion f rom a h i g h - t e m p e r a t u r e source zone to a lower t empera tu re subs t ra te zone using iodine as the t ranspor t agent. The proposed chemical r e - actions at the source end of the tube are:

2 x G a A s ( s ) -t- 2 ( 1 - - x ) G a P ( s ) + I2(g) kl 1 1

> - - x A s 4 ( g ) + (l--x) P4(g) + 2GaI (g )

and the competing react ion

1 1 3GaI (g ) + - - x A s 4 ( g ) + ( l - - x ) P4(g)

2 ks

----> GaI3 + 2x GaAs( s ) + 2 ( 1 - - x) G a P ( s )

and at the subs t ra te end

x As4(g) + ( l - - x ) P 4 ( g ) + 4GaI (g ) ks

> 4GaAsxP(1-x)(S) + 212(g)

Exper imenta l ly , the react ions are carr ied out in a sealed, evacuated quartz tube of 2 in. in d iamete r and 8 in. in length, p laced in a two-zone furnace. A typical the rmal d is t r ibut ion profile and sample tube p lacement for the furnace are shown schemat ica l ly in Fig. 1.

The source ma te r i a l consists of the calcula ted mole per cent of GaAs and GaP 2 (of same par t ic le size) and measured amount of dopant (such as Te, Se, and Sn) . The source mater ia l s are cleaned by etehants pr ior to being placed in the react ion tube. A small vacuum-sea led ampoule containing a meas - ured amount of iodine is also inser ted into the r e - action tube at the source end.

x T h e e p i t a x i a l l y g r o w n G a (AsxPl-x) l a y e r ha s l e ss t h a n 0.002 in. v a r i a t i o n i n t h i c k n e s s o v e r 80% of t h e to ta l s u r f a c e area. U n i f o r m i t y of c o m p o s i t i o n was conf i rmed b y e lec t r i ca l a n d op t i ca l m e a s u r e - ments . Less t h a n 8% l a t e r a l v a r i a t i o n o f f ree carr ier c o n c e n t r a t i o n was f o u n d o n t h e e p i t a x i a l l a y e r o f 0.010 in. t h i c k and 0.750 in. i n d i ame te r . Edge e m i s s i o n p e a k o n d i o d e s o f 0,003 i n . - d i f f u s i o n f r o m e i t h e r su r f ace s of t h e s a m e e p i t a x i a l l y g r o w n l a y e r of t he se m i x e d c rys t a l s h a v e i d e n t i c a l va lue s . There fo re , t h e g r o w n l aye r is n o t o n l y u n i f o r m i n c o m p o s i t i o n across t h e w a f e r s u r f a c e h u t a lso u n i f o r m t h r o u g h o u t the th i ckness .

C o m m e r c i a l l y a v a i l a b l e h i g h e s t p u r i t y G a A s a n d GaP .

J O U R N A L O F T H E E L E C T R O C H E M I C A L S O C I E T Y September 1963

SUBSTRATE WAFERS

78C /

76C ~ - ~ " ~ " ' - ~ DOPANT AND ~: SOURCE D 74(: / I-

ac 72(

70q F ILL

68( I I ~ I L I I l I I 3 5 S I0 DISTANCE ( INCHES)

Fig. 1. Furnace temperature profile for epitaxiol growth and relative position of the reaction tube in the furnace.

The subs t ra te mate r ia l used in this exper imen t is e i ther GaAs or GaP, depending upon the percentage of the al loy composition; for al loys wi th 0.5 or l a rger mole fract ion of GaP, pure GaP is used as substrate , otherwise, GaAs is used as substrate. The subs t ra te wafers are pol ished to 1/4~ finish and chemical ly etched in a mix tu re of H2SO4 and H202 solution, and r insed in deionized w a t e r and then in methanol . These wafers are suspended in the center of the quartz capsule by quartz hooks. The ent i re capsule is baked at 150~ for 12 hr under vacuum (10 -5 Tor r ) . The iodine i s then re leased f rom the ampoule by a magnet ic hammer and the ent i re capsule is backfi l led wi th a few tenths of an a tmosphere of hy - drogen and sealed off. Jus t p r io r to growth the sub- s t ra te is fu r the r cleaned by reverse deposi t ion for a few minutes, i.e., by t ranspor t ing the subs t ra te to the source region. Af ter growth for a per iod of t ime, the capsule is pushed out of the furnace and al lowed to cool to room tempera ture . General ly , the source t empera tu re is about 50~176 above the subs t ra te t empera tu re . In addit ion, i t was found tha t un i form crys ta l l ine ep i tax ia l g rowth is influenced not only by the subs t ra te surface prepara t ion , but also by the or ientat ion of the subs t ra te and the growth rate.

Results and Discussion As one would have expected, the growth ra te was

found to be influenced by the or ienta t ion of the sub- s t ra te surface. Epi tax ia l G a A s - G a P layers of 250~ to 630# have been obtained on subst ra tes of several low index or ientat ions (100), (110), (111), and (112). In contrast to the hydrogen chlor ide t r ans - por t ing sys tem repor ted by Moest and Shupp (13) for GaAs or GaP, no growth was observed on the As

or P face of the (111) direction. However, our re - sults are in agreement wi th the ep i tax ia l growth of GaAs repor ted by Pizzarel lo (14) using iodine as the t ranspor t ing agent. Growth on the (111) planes was inva r i ab ly found to contain t r i angu la r p y r a - mids. The (110) direct ion was found to be the fas t - est growing or ienta t ion among the four pr inc ipa l c rys ta l lographic direct ions examined. The (100) plane was found to be the slowest, and the (112) p lane gave the most uni form surface and growth pat tern , as shown in Table I.

Except for an in i t ia l per iod of 15-20 min requ i red to establ ish the s t eady-s t a t e condit ion of the reaction, the growth ra te was found to be independent of t ime for a fixed set of var iab les (source t e m p e r a - ture, pressure, orientat ion, geomet ry of the react ion



Vol . 110, No . 9 V A P O R - G R O W N G a A s - G a P A L L O Y S 993

Table I. EpJtaxial growth rate of GaAs-GaP crystals at 742~ source temperature and 680~ substrate temperature

O r i e n t a t i o n of g r o w t h d i r e c t i o n G r o w t h r a t e (]~/hr)

100 1.8-2.4 110 7.2-8.5 111 5.1-6.0 111 zero 112 5.4-5.9

tube , etc.) u n t i l t he source m a t e r i a l is e x h a u s t e d . No d e t e c t a b l e a m o u n t bf iod ine was f o u n d to be i n - c o r p o r a t e d w i t h i n t he e p i t a x i a l l a y e r ; t h e r e f o r e i t is r e a s o n a b l e to a s sume t h a t on ly a s m a l l a m o u n t of iod ine is n e e d e d to t r a n s p o r t a c o m p a r a t i v e l y l a rge a m o u n t of r eac t an t s . This m a y also e x p l a i n t he i n d e p e n d e n c e of g r o w t h r a t e w i t h r e s p e c t to t ime .

A ser ies of e x p e r i m e n t s w e r e m a d e to d e t e r m i n e t h e g r o w t h r a t e vs. i n i t i a l p r e s s u r e , source t e m p e r a - ture , and s u b s t r a t e t e m p e r a t u r e s w i t h a f ixed s u b - s t r a t e o r i e n t a t i o n (100) . As expec t ed , w e h a v e f o u n d t h a t t h e g e o m e t r y of t h e t u b e and the pos i t i on of t he s u b s t r a t e in t he t u b e h a v e a l a r g e effect on the u n i f o r m i t y of t h e depos i t l aye r , s ince t he t r a n s - p o r t of G a A s - G a P f r o m the source to t he s u b s t r a t e is con t ro l l ed b y convec t ion as w e l l as b y diffusion. I t was also no ted t h a t t he g r o w t h r a t e was r e l a - t i v e l y i n sens i t i ve to t h e s u b s t r a t e t e m p e r a t u r e . H o w e v e r , for s i m p l e G a A s - G a P t r a n s p o r t , t h e s u b - s t r a t e t e m p e r a t u r e s can be as low as 550~ b u t to ach ieve u n i f o r m c r y s t a l l i n e g r o w t h a n d p r e s e r v e the d e s i r e d compos i t ion , t he s u b s t r a t e t e m p e r a t u r e shou ld be s l i g h t l y b e l o w the ef fec t ive d i s p r o p o r - t i o n a t i o n - t e m p e r a t u r e . F o r a f ixed source t e m p e r a - t u r e of 742~ and an in i t i a l i od ine p r e s s u r e of 210 T o r t , a v a r i a t i o n of s u b s t r a t e t e m p e r a t u r e f r o m 600 ~ to 700~ caused the g r o w t h r a t e to v a r y b y less t h a n 0.1%. F i g u r e 2 shows tha t , a t a f ixed source t e m p e r a t u r e of 742~ a n d an iod ine p r e s s u r e in t he r a n g e of 80 to 250 Tor r , t h e r a t e of g r o w t h inc reases w i t h d e c r e a s i n g in i t i a l iod ine p r e s s u r e w h e r e t he t o t a l p r e s s u r e of t he s y s t e m r e m a i n e d cons tan t . This r e su l t a g a i n s u b s t a n t i a t e s t h e f inding t h a t i od ine

20

I0

5

=: =L v

2

C) ,~1.0

.2

.I 650

I I I I 700 750 800 850

TEMPERATURE (~

I 900

Fig. 3. Rate of growth vs. source temperature at initial 12 pressure of 210 mm Hg for (100) substrates.

acts on ly as a t r a n s p o r t a g e n t in th is process , and t h a t i t does no t e n t e r t he e p i t a x i a l l aye r . F i g u r e 3 shows t h a t t he g r o w t h r a t e i nc r ea se s w i t h i n c r e a s i n g source t e m p e r a t u r e at a cons t an t i n i t i a l i od ine p r e s - su re of 210 Torr . E v i d e n t l y s e v e r a l r e ac t i ons a r e c o m p e t i n g at t he source, w h e r e h igh source t e m - p e r a t u r e and low iod ine p r e s s u r e w i l l f a v o r k l ove r k2.

The compos i t i on of t he m i x e d a l loys was d e t e r - m i n e d b y two m e t h o d s : ( i ) A s s u m i n g t h a t V e g a r d ' s l a w of sol id so lu t ions ho lds fo r th is G a A s - G a P sys - tem, t h e r e b y m e a s u r i n g the l a t t i c e cons t an t of these a l loys , i t w i l l in t u r n d e t e r m i n e t h e compos i t ion . ( i t ) F r o m the e n e r g y gap vs. compos i t ion d a t a of t he m i x e d a l loys o b t a i n e d b y p r e v i o u s a u t h o r s (Fig . 4) , compos i t i on can be d e t e r m i n e d b y m e a s u r i n g the op t i ca l e n e r g y gap, Eg, va lues . The l a t t i c e cons tan t s of G a A s - G a P a l loys m a d e b y the m e t h o d m e n t i o n e d a b o v e w e r e m e a s u r e d a t r o o m t e m p e r a t u r e b y t h e Lau~ b a c k - r e f l e c t i o n t echn ique . T h e r e su l t s a r e

7

- I ~6

5

='i 0 I I L 80 I00 150 200

INITIAL "1" 2 PRESSURE (TORR)

I 250

Fig. 2. Rate of growth vs. initial 12 pressure at source temperature 742~ for (100) substrates.

2.25 _..-----x z.2o f l ' ' ' ~ "

o 2. I0 x / / . 2.00

1.90

s tUujZ I.TO x/,/P A

o 1.60 / ~

1.5C o

1.4G / z~ L35 I I I I I I I I ( I

.I .2 .3 .4 ,S .6 .7 .8 ,9 LO MOLE FRACTION OF GaP IN GaAs

Fig. 4. Variation of energy gap vs. composition of GaAs-GaP alloys at 300~ o, 300~ transmittance vs. X data; X, 300~ Folberth, Pizzarello and other published data; ZX, 300~ emission peak vs. ~ data.



994 JOURNAL OF THE ELECTROCHEMICAL SOCIETY S e p t e m b e r 1963

5 . 6 5

o 5 . 6 0 ,1r

I--

I--

5 . 5 5

I- I -

_1

- !

5 . 5 0

5 . 4 5 0 .I .2 .5 .4 .5 .6 .7 .8 .9 1.0

M O L E F R A C T I O N OF GoP

i 0 4

i 0 -~

\

>o

> : I-- 1 _1

O

I 0 z I t I I I I 1 I t 0 .I .2 .3 .4 .5 .6 .7 ,8 .9 1.0 GaAs M O L E FRACTION GaP

Fig. 6. Mobility vs. composition of GaP in GaAs. X, carrier concentration ,~ 1.Sx10tT/cm3; o, carrier concentration ,~ 6x1017/cm 3.

Fig. 5. Lattice constant vs. various composition range in the GaP-GaAs.

shown in Fig. 5. The decrease in lattice constant with increase of mole fraction of GaP is found to be in good agreement with Vegard's law of solid solu- tions; also the sharpness of the Lau~ back-reflection peak fur ther indicates a high degree of perfection for these alloys. The energy gap values were determined by the linear extrapolation of the absorption edge in the transmittance vs. wavelength measurement. The compositions of these alloys determined from the Eg value vs . composition curve (Fig. 4) varied less than 2% with the predetermined source composition for the epitaxial growth. From the results of composition determination, by the above two methods, it seems that there is a one- to-one ratio of transport of GaAs and GaP from source to substrate.

All the synthesized alloys were doped with either selenium, tellurium, or tin. Each of these elements is an n - type dopant for the mixed crystals. Using the data from Hall-effect measurements, it was found that the highest f ree-carr ier concentration produced for these alloys was obtained by doping with selenium (3 x 10 TM carr iers/cc) . With tel lurium as a dopant, the highest carrier concentration ob- tainable was about 7.8 x 101S/cc, and 5 x 1018/cc was the highest impuri ty concentration achieved with tin doping. The lower free-carr ier concentration in t in-doped alloys may be attr ibuted to the relatively large size of the interatomic radii of tin (1.40A) in comparison to the other two types of dopants men- tioned (selenium--l .14A; te l lur ium-- l .32A) (15). Another plausible explanation for this result is that the tin atoms can be introduced into the GaAs-GaP alloys both as p- and n- type impurities.

From the room-tempera ture Hall effect and re- sistivity measurements, the simple Hall mobility calculated from the ~H = ~rRit relation, is plotted against the mole fraction of GaP in Fig. 6. In contrast to other quasi-binary alloys such as InSb-GaSb (8) system, the mobility in GaAs-GaP alloy does not fall off rapidly f rom the value of pure GaAs of equivalent carrier concentration; instead, it remains

2 l~ ~ ,8

lirect Gop

2.5 .5

Indirect Gap

2 .C ~ .0

~ Indirect m Gap

1.5 ! 5

Direct Gap

I 2 I I I I 1.2

M01e Fraction of GaP

Fig. 7 Variations of edge emission peak of GaAs-GaP diodes with composition at 31111 ~ and 77~ o, 3 ~ ~ transmittance data; A , 300~ edge emission data; e, 77~ edge emission data.

fairly constant up to 15% of GaP, and then de- creases steeply between 25-50% GaP.

It has been shown from the published absorption data (5, 11) and the transmittance vs. wavelength measurement of this experiment (Fig. 4) that the energy gap of Ga(AsxPl-x) does not vary linearly over the entire composition range. This was fur ther confirmed by the edge emission vs. wavelength measurement obtained on the forward-biased, zinc-dif- fused diodes made from t in-doped alloys of 10, 15, 29, 25, and 50% GaP compositions (17, 19). At room tempera ture , the edge emission peak of diodes made with alloys of less than 50% GaP follows the linear variation between the direct energy gap of GaAs (1.37 ev) and GaP (2.6 ev} (Fig. 7). As expected, at liquid nitrogen temperature the edge emission peak shifts to a shorter wavelength, but the increase in energy gap is found to be in good agreement with the estimated 0E~0T values for these alloys (assume that the 0E/0T for the alloys is somewhere between --5.4 x 10 -4 e v / ~ for GaP and --4.9 x 10 -4 e v / ~ for GaAs) (16).

Acknowledgment The author gratefully acknowledges the assistance

of A. Marx in the preparation of the material and



Vol. 110, No. 9 V A P O R - G R O W N G a A s - G a P A L L O Y S 995

P. L u b l i n for m a k i n g the l a t t i c e cons t an t m e a s u r e - m e n t on these a l loys . The a u t h o r is spec i a l l y i n - d e b t e d to J. B lack for m a n y h e l p f u l d i scuss ions and to Dr. S. M a y b u r g for his g u i d a n c e in a l l phase s of th is work .

Manuscr ip t rece ived Feb. 12, 1963; rev ised m a n u - script rece ived Apr i l 19, 1963.

Any discussion of this pape r wi l l appear in a Discus- sion Section to be publ i shed in the June 1964 JOURNAL.

REFERENCES 1. H. S tohr and W. Klemm, Z. anorg. Chem., 241,

305 (1939). 2. A. Levitas , C. C. Wang, and B. H. Alexander , Phys.

Rev., 95, 846 (1954). 3. M. Glicksman, ibid., 102, 1496 (1956). 4. F. Herman, M. Gl icksman, and R. H. Parmente r ,

"Progress in Semiconductors" (1957). 5. O. G. Folber th , Z. Naturforsch., 10a, 502 (1955). 6. W. Kos te r and B. Thoma, Z. Metallkunde, 46, 293

(1955). 7. J. C. Woolley, B. A. Smith, and D. G. Lee, Proc.

Phys. Soc., B69, 1339 (1956). 8. J. C. Wool ley and C. M. Gil let t , J. Phys. Chem. of

Solids, 17, 34-43 (1960).

9. F. A. Trumbore , P. E. Free land , and A. D. Mills, This Journal, 109, 645 (1962).

10. G. R. Antel l , ibid., 106, 509 (1959). 11. F. A. Pizzarel lo, ibid., 1,09, 226 (1962). 12. A. Hagenlocher , ibid., 198, 213C (1961). 13. R. R. Moest and B. R. Shupp, ibid., 109, 1061 (1962). 14. F. A. Pizzarello, The Elec t rochemical Socie ty Los

Angeles Meeting, Electronics Division Abstracts, l l , [1] 166 (1962).

15. L. Paul ing, "Nature of the Chemical Bond," p. 179, Cornel l Univ. Press.

16. H. Welker , "So l id -S ta t e Physics ," 3, 51, Academic Press Inc. (1956).

17. H. Lockwood, S. M. Ku, and J. Black, "Visible In - ject ion Elec t ro luminescence f rom G a A s - G a P Diodes," Bul le t in of A.P.S. Series II, 7, No. 8, 537 (Nov. 23, 1962).

18. N. Holonyak, Jr., D. C. Ji l lson, and S. F. Bevacqua, "Halogen Vapor Transpor t and Growth of Epi - t ax ia l Laye r s of In te rme ta l l i c Compounds and Compound Mixtures ," "Meta l lu rgy of Semicon- ductor Mater ia ls ," J. B. Schroeder , Editor , I n t e r - science Publ ishers , Aug. 30-Sept. 1, 1961.

19. S. M. K u and J. F. Black, " Inject ion E lec t ro lumi - nescence in G a A s - G a P Diodes," To be publ ished.

Effect of Electrolyte and Solvent on Polarographic Reductions of Nitrobenzene Derivatives

Donald J. Pietrzyk, z R. F. Breese, 2 and L. B. Rogers 8

Department of Chemistry and Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, Massachusetts

ABSTRACT

Anodic shifts of h a l f - w a v e potent ia ls for n i t robenzene der iva t ives on add i - t ion of calcium ion are usua l ly grea tes t for substances having the most nega- t ive reduct ions on the negat ive side of the e lec t rocapi l la ry max imum. How- ever, a smal l anodic shift has also been detected for reduct ions on the posi t ive side of the e lec t rocapi l la ry max imum. An increase in pH or in percentage of methanol leads to a grea ter shift in al l cases, p robab ly as the resul t of g rea te r in terac t ion be tween calcium ion and the carbanion in te rmedia te .

I r r e v e r s i b l e p o l a r o g r a p h i c r eac t i ons a r e f r e q u e n t l y sens i t ive to t he p r e s e n c e of s u r f a c e - a c t i v e agen t s a n d t h e ca t ion of the " i n e r t " b a c k g r o u n d e l e c t r o l y t e (1 -6 ) . C a l c i u m ion has been r e p o r t e d to sh i f t a n o d i - ca l l y t h e a l k a l i n e r e d u c t i o n of p - d i n i t r o b e n z e n e wh ich fa l l s on the n e g a t i v e s ide of t he e l e c t r o c a p i l - l a r y m a x i m u m (2) . A s im i l a r sh i f t for t h e first wave , w h i c h fa l l s on the pos i t i ve s ide of t he m a x i - m u m , has n o w b e e n s t u d i e d as a func t ion of p H a n d concen t r a t i ons of m e t h a n o l , and the s u r v e y has been e x t e n d e d to o t h e r n i t ro a roma t i c s .

Experimental E x c e p t for p - n i t r o t r i m e t h y l a n i l i n i u m ch lo r ide

w h i c h w e p r e p a r e d (7 ) , t h e n i t ro c o m p o u n d s w e r e E a s t m a n K o d a k C o m p a n y p r o d u c t s t h a t h a d been

Z Presen t address : Chemis t ry Depa r tmen t , S ta te Unive r s i ty of Iowa, I o w a City, Iowa.

P resen t address : Chicago, Rock Is land and Pacific Railroad, Re- search and Deve lopmen t Laboratory, Chicago g, Illinois.

s P re sen t address : Chemis t ry Depa r tmen t , P u r d u e Univers i ty , La- fayette, Ind iana .

f u r t h e r pur i f i ed b y d i s t i l l a t i o n or c rys t a l l i za t ion . L i t h i u m ch lo r ide was p r e p a r e d f r o m l i t h i u m c a r - b o n a t e a n d h y d r o c h l o r i c ac id ; a l l o t h e r i n o r g a n i c c o m p o u n d s w e r e c o m m e r c i a l l y a v a i l a b l e a n a l y t i c a l r eagen t s .

M o d e r a t e c o n c e n t r a t i o n s of h y d r o c h l o r i c ac id a n d s o d i u m h y d r o x i d e w e r e used for s m a l l a n d l a r g e p H va lues , r e spe c t i ve ly . I n t e r m e d i a t e v a l u e s w e r e m a i n t a i n e d us ing buffers , 0.01M in ion ized form, w h i c h w e r e p r e p a r e d f r o m a m m o n i a o r p y r i d i n e m i x e d w i t h h y d r o c h l o r i c acid.

P o l a r o g r a p h i c so lu t ions u s u a l l y c o n t a i n e d 1 . 0 0 x 10-4M n i t r o b o d y and 0.005% f r e s h l y p r e p a r e d g e l a - t in.

The a p p a r a t u s a n d p r o c e d u r e has been d e s c r i b e d (2, 3) . M e a s u r e m e n t s w e r e m a d e a t 25~ us ing a c a p i l l a r y w i t h a 0.03 r am bore , w h i c h h a d a d rop t i m e of 5 sec a n d a v a l u e of m218t II6 at - - 0.5 v (vs. S.C.E) of 1.477 m g ~/8 sec - l /~. D a t a for h a l f - w a v e p o t e n t i a l s u s u a l l y h a d a s p r e a d of less t h a n _--+- 3 m v w h e n r u n in t r i p l i c a t e r u n s on each of two solut ions .



the preparation and properties of vapor-grown gaas-gap alloys

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