Journal oJ Electronic Materials, Vol. 2, No. 2, 1973
SEMICONDUCTOR MATERIALS FOR MICROWAVE DEVICES
Don W. Shaw
Abstract Microwave device performances have been r a p i d l y improving due to recent developments i n the areas of d e v i c e c o n s t r u c t i o n and m a t e r i a l preparation. In t h i s paper emphasis is placed on m a t e r i a l s e l e c t i o n and prep a r a t i o n f o r several devices which are c u r r e n t l y r e c e i v i n g a t t e n t i o n t h r o u g h o u t the i n d u s t r y . These i n c l u d e t r a n s f e r r e d e l e c t r o n d e v i c e s , avalanche diodes~ b i p o l a r t r a n s i s t o r s ~ v a r a c t o r and mixer diodes, and field effect transistors. The s t r u c t u r e s of these devices are considered w i t h reference to the requirements which they place on the e p i t a x i a l l a y e r s . Various techniques of e p i t a x i a l growth are described t o g e t h e r w i t h the r e l a t i v e advantages and disadvantages of each.
Physical Sciences Research Laboratory a Texas Instruments~ I n c o r p o r a t e d , D a l l a s , Texas 75222.
Received29 August 1972; revised29 January 1973.
25$ 9 1973 by the American Institute of Mining, Metallurg/cal, and PetrOleum Engineers, Inc.
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Tntroduction Hodern microwave systems c o n t a i n a number of d i s c r e t e semiconductor devices which c o n t r i b u t e s i g n i f i c a n t l y to the o v e r a l l o p e r a t i n g perfonnance. Continued improvement of these components has c o n s i d e r a b l y extended the technology f o r d e v i c e f a b r i c a t i o n and m a t e r i a l p r e p a r a t i o n i n recent times. Hicrowave devices place severe requirements on the semiconductor m a t e r i a l and the method f o r i t s p r e p a r a t i o n . In order to minimize p a r a s i t i c r e s i s t a n c e s the devices are g e n e r a l l y f a b r i c a t e d in e p i t a x i a l l a y e r s w i t h c a r e f u l l y c o n t r o l l e d thicknesses. Operation at microwave f r e q u e n c i e s r e q u i r e s high s a t u r a t i o n d r i f t v e l o c i t i e s and consequently high c a r r i e r m o b i l i t i e s . The doping p r o f i l e s w i t h i n the e p i t a x i a l l a y e r s must f a i l w i t h i n w e l l d e f i n e d s p e c i f i c a t i o n s . Some devices, such as microwave t r a n s i s t o r s , r e q u i r e high r e s o l u t i o n photolithography. Here the e p i t a x i a l surfaces must be smooth, free of d e f e c t s , and the o v e r a l l s l i c e must be very f l a t . These and other l e s s e r but s t i l l important reasons make p r e p a r a t i o n of m a t e r i a l f o r microwave devices a very demanding task. Hicrowave device performances have been r a p i d l y improving due in p a r t t o d e v e l o p m e n t s in f a b r i c a t i o n t e c h n i q u e s and t o improvements in material preparation. I t i s t h e p u r p o s e o f t h e p r e s e n t paper t o d e s c r i b e some o f t h e c u r r e n t d e v e l o p m e n t s and a c t i v i t i e s r e l a t e d t o m a t e r i a l s e l e c t i o n and p r e p a r a t i o n . Several devices which are c u r r e n t l y r e c e i v i n g a t t e n t i o n and i n v e s t i g a t i o n w i l l be considered. These w i l l i n c l u d e Gunn o s c i l l a t o r s and o t h e r t r a n s f e r r e d e l e c t r o n devices, avalanche diodes, b i p o l a r t r a n s i s t o r s ~ f i e l d e f f e c t t r a n s i s t o r s , v a r a c t o r diodes, and mixer and d e t e c t o r diodes. No e f f o r t w i l l be made to present the p r i n c i p l e s of a p p l i c a t i o n and o p e r a t i o n of these devices as t h e r e are several good reviews ( 1 - ] 1 ) i n the area. Instead the d i s c u s s i o n w i l l be d i r e c t e d toward the s c i e n t i s t i n t e r e s t e d i n the area of m a t e r i a l s p r e p a r a t i o n . Device s t r u c t u r e s and f a b r i c a t i o n techniques w i l l be described w i t h emphasis on the requirements they place on the m a t e r i a l p r e p a r a t i o n process. F i n a l l y , the most important semiconductor m a t e r i a l s w i l l be discussed w i t h respect to methods f o r t h e i r preparation. E p i t a x i a l l a y e r growth w i l l be emphasized as d e v i c e requirements such as low capacitance, minimum s e r i e s r e s i s t a n c e , and high c a r r i e r m o b i l i t y demand e p i t a x i a l s t r u c t u r e s . For each m a t e r i a l the various methods f o r p r e p a r a t i o n w i l l be presented together w i t h t h e i r r e l a t i v e advantages and disadvantages. Hicro~ave Devices T r a n s f e r r e d E l e c t r o n Devices, These microwave devices make use of a n e g a t i v e conductance phenomenon which r e s u l t s from t r a n s f e r of c a r r i e r s from a low energy, high m o b i l i t y conduction v a l l e y to higher energy s u b s i d i a r y v a l l e y s w i t h lower m o b i l i t i e s under the i n f l u e n c e of an a p p l i e d e l e c t r i c f i e l d . They i n c l u d e Gunn e f f e c t devices or t r a n s f e r r e d e l e c t r o n o s c i l l a t o r s (TEO) and t r a n s f e r r e d e l e c t r o n a m p l i f i e r s (TEA). Bulk n e g a t i v e d i f f e r e n t i a l c o n d u c t i v i t y as a r e s u l t of i n t e r v a l l e y t r a n s f e r was e x p e r i m e n t a l l y detected by Gunn (12) w h i l e working w i t h g a l l i u m a r s e n i d e and indium phosphide. Although several other
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m a t e r i a l s have been proposed f o r t r a n s f e r r e d e l e c t r o n devices 3 by f a r most o f the work has been conducted w i t h GaAs~ and i t remains the o n l y m a t e r i a l which has made the t r a n s i t i o n from the l a b o r a t o r y t o a commerc i a l l y a v a i l a b l e device. Uenohara (13) has l i s t e d the requirements which a material must meet to e x h i b i t the transferred electron phenomenon. .Firstj a r e l a t i v e l y low l a t t i c e temperature must prevail so that most of the carriers are in the lower conduction valley in the absence of an applied e l e c t r i c f i e l d . The carriers in this valley should have a low e f f e c t i v e mass which corresponds to a low density of states and a high m obi l i t y. The opposite should be true of carriers in a subsidiary valley. Here a r e l a t i v e l y high e f f e c ti v e mass is desired to produce a low mobility and high density of states. Finally~ the energy separation between the conduction valleys must be less than the energy gap separating the valence and conduction bands so that a negative conductance occurs prior to avalanche breakdown as the applied f i e l d is increased. All of these requirements are s a ti s fi e d by gallium arsenide. The basic s t r u c t u r e s o f t r a n s f e r r e d e l e c t r o n devices are shown s c h e m a t i c a l l y in Figure !. In p r i n c i p l e the devices can be f a b r i c a t e d by p l a c i n g two contacts onto a piece of GaAs. Howeverj in p r a c t i c e the s t r u c t u r e becomes much more complicated. Relatively low c a r r i e r d e n s i t i e s and high e l e c t r o n m o b i l i t i e s are r e q u i r e d . Soon a f t e r the discovery of the Gunn e f f e c t it became e v i d e n t t h a t b u l k GaAs was not s u i t a b l e f o r e f f i c i e n t o p e r a t i o n and s was necessary t o r e s o r t t o e p i t a x i a l m a t e r i a l (14) t o achieve success. The s u b s t r a t e upon which the e p i t a x i a l l a y e r is grown is g e n e r a l l y Te-~ Se-~ or Si-doped w i t h c o n c e n t r a t i o n s o f around lOl e cm-3o This serves both as a support f o r the e p i t a x i a l " a c t i v e '~ r e g i o n and as a low r e s i s t a n c e r e g i o n to f a c i l i t a t e making one o f the ohmic c o n t a c t s . The remaining c o n t a c t may be made by a l l o y i n g d i r e c t l y to the a c t i v e r e g i o n . Since l a r g e amounts of heat are generated d u r i n g o p e r a t i o n o f the d e v i c e s t h i s simple s t r u c t u r e is advantageous from the p o i n t of view o f heat t r a n s f e r s since a m i n i mum thermal impedance is presented when the heat s i n k i s attached d i r e c t l y t o the a c t i v e n - r e g i o n . However~ the c o n t a c t i n g procedure tends to i n t r o d u c e damage i n t o the a c t i v e r e g i o n j and i t is d i f f i c u l t to make ohmic c o n t a c t t o t h i s l i g h t l y doped r e gion~ Consequently, a second e p i t a x i a l l a y e r w i t h a r e l a t i v e l y high doping l e v e l (lO 1~ lOl e cm- ~ ) is o f t e n grown upon the a c t i v e l a y e r . Ohmic c o n t a c t is then made to t h i s regiono This r e s u l t s in the sandwich s t r u c t u r e shown in Figure i a . I t is a l s o p o s s i b l e to take advantage o f the s e m i - i n s u l a t i n g form of GaAs and c o n s t r u c t a " p l a n a r ' , or t r a n s v e r s e o s c i l l a t o r (15) such as t h a t shown in Figure i b . The a c t i v e r e g i o n is e p i t a x i a l l y grown upon a s e m i - i n s u l a t i n g (10a ohm cm)~ Cr-doped GaAs s u b s t r a t e and both contacts a re made to the upper surfac~ 3 e i t h e r d i r e c t l y or a f t e r growth of a second h e a v i l y doped e p i t a x i a l l a y e r . This method o f cons t r u c t i o n o f f e r s p o s s i b i l i t i e s f o r f a b r i c a t i o n o f microwave i n t e g r a t e d circuits. To date~ however~ most o f the a t t e n t i o n has been devoted t o sandwich s t r u c t u r e s because o f t h e i r s u p e r i o r thermal c h a r a c t e r i s t i c s and general a p p l i c a b i l i t y to many microwave systems.
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PLANAR STRUCTURE Basic S t r u c t u r e s o f Transs E l e c t r o n Devices. ao Sandwich S t r u c t u r e b. Planar S t r u c t u r e
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E f f i c i e n t operation of a transferred electron device requires that the a c t i v e n - r e g i o n be u n i f o r m l y doped a t a low c o n c e n t r a t i o n w i t h very l i t t l e compensation. U n f o r t u n a t e l y , high r e s i s t a n c e or p - t y p e regions are o f t e n found a t i n t e r f a c e s between the a c t i v e l a y e r and the adjacent material. These r e s i s t i v e notches~ or i - l a y e r s s are extremely d e t r i mental to high f i e l d t r a n s f e r r e d e l e c t r o n devices and have undoubtedly retarded the progress of t h e i r development. They have been the o b j e c t of i n t e n s i v e research i n many l a b o r a t o r i e s . In most cases e m p i r i c a l approaches have r e s u l t e d in techniques f o r e l i m i n a t i o n o f the i - l a y e r s ~ but as y e t the most s i g n i f i c a n t cause has not been c o n c l u s i v e l y determined. Most l i k e l y t h e r e are a number o f p o s s i b l e causes d i f f e r i n g i n r e l a t i v e importance among the v a r i o u s e p i t a x i a ] growth systems. Among these are a r s e n i c vacancies ( ] 6 - 2 0 ) formed i n the e a r l y sta9es o f growths g a l l i u m vacancies (21)j d i f f u s i o n o f i m p u r i t i e s from the s u b s t r a t e (22)j s i l i c o n c o n t a m i n a t i o n from the growth apparatus (23)j copper c o n t a m i n a t i o n on the s u b s t r a t e s u r f a c e (24,25)j v a r i a t i o n of the gas phase s t o i c h i o m e t r y i n the e a r l y stages of growth (22~26)j and c r y s t a l defects and i m p e r f e c t i o n s (27~28). I t i s i n t e r e s t i n g to note t h a t i - l a y e r s have been found w i t h both l i q u i d and vapor phase e p i taxial structures. Devices are o f t e n f a b r i c a t e d from t r i p l e e p i t a x i a l l a y e r s t r u c t u r e s . In a d d i t i o n to the top n + - e p i t a x i a l l a y e r s which serves as an ohmic contacts an a d d i t i o n a l n + - l a y e r is placed between the a c t i v e r e g i o n and the s u b s t r a t e (29)~ This i n t e r m e d i a t e or b u f f e r l a y e r serves to i s o l a t e the a c t i v e region from the r e l a t i v e l y d e f e c t i v e s u b s t r a t e and y e t i s h e a v i l y doped to make a low r e s i s t a n c e c o n t a c t to the a c t i v e r e g i o n . As y e t t h e r e have been no d e f i n i t i v e s t u d i e s of the r e l a t i v e p e r f o r mances of devices formed from m a t e r i a l w i t h and w i t h o u t the i n t e r m e d i a t e layer. Nevertheless s i t s presence appearss i n p r i n c i p l e s to have several advantages. As mentioned p r e v i o u s l y i t i s o l a t e s the a c t i v e region from the s u b s t r a t e defects whichs i f present a t the a c t i v e region i n t e r f a c e s can cause r e s i s t i v e notches or n o n - u n i f o r m i t i e s . The i n t e r m e d i a t e l a y e r can a l s o be doped at the same l e v e l as the top n+ layers thus forming a symmetrical s t r u c t u r e w i t h the a c t i v e r e g i o n sandwiched between two n+ regions o f i d e n t i c a l doping l e v e l . Finallyj i n c o n j u n c t i o n w i t h a p p r o p r i a t e processing techniques to be discussed below, the i n t e r m e d i a t e l a y e r permits f o r m a t i o n of a l l e p i t a x i a l devices. An example o f the cleaved and etched cross s e c t i o n through a t h r e e e p i t a x i a i l a y e r sandwich is shown i n Figure 2. The e t c h a n t (30) used reveals c r y s t a l defects and d i f f e r e n c e s in e l e c t r i c a l c h a r a c t e r i s t i c s o f d i f f e r e n t regions (31). The s u p e r i o r p e r f e c t i o n of the e p i t a x i a l l a y e r s w i t h respect to the s u b s t r a t e is e v i d e n t . Another m a t e r i a l parameter which must be c a r e f u l l y c o n t r o l l e d f o r these devices is the e p t i a x i a l l a y e r t h i c k n e s s . The frequency of o p e r a t i o n of a Gunn device or TE0 i s a f u n c t i o n o f the t h i c k n e s s (L) and the c a r r i e r d e n s i W (n) o f the a c t i v e r e g i o n . T y p i c a l val_ues f o r devices o p e r a t i n g a t X-band are E ~ 10 Hm and n ~ 2 x ]0 zs cm o In general h i g h e r frequency o p e r a t i o n r e q u i r e s t h i n n e r l a y e r s w i t h 9 r e a t e r carrier densities. The thickness of the n+ l a y e r or the l a y e r to which the heat s i n k is attached must be held to a minimum to reduce the thermal impedance. At h i g h e r f r e q u e n c i e s s p a r a s i t i c r e s i s t a n c e s
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Figure 2.
Cleaved Cross S e c t i o n Through a Three E p i t a x i a i Layer S t r u c t u r e For T r a n s f e r r e d E l e c t r o n Devices. Etched i n the AB Etchant (30). X
77o.
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become a major factor and the r e s i s t i v i t y of a l l non-active regions of the device~ such as the n+ contacting layersj must be as low as possible. I t is possible as a result of new techniques in device fabrication (32) to form an intermediate layer with s u f f i c i e n t thickness (10-20 llrn) to allow removal of the e n t i r e subs t r a t e during processing. Contact is made to the intermediate e p i t a x i a l layer rather than the substrate and an a l l e p i t a x i a l device results with inherently greater r e l i a b i l i t y due to the superior crystal perfection. In generals transferred electron devices place more severe demands on the q u a l i t y of e p i t a x i a l GaAs than any other device. Fortunatelyj the major problems appear to be under control and r e l i a b l e high efficiency devices are available.
Several m a t e r i a l s o t h e r than GaAs have been i n v e s t i g a t e d f o r t r a n s f e r r e d e l e c t r o n o s c i l l a t o r s and a m p l i f i e r s . In 1970 Hilsum and Rees (33) proposed t h a t e l e c t r o n t r a n s f e r between t h r e e v a l l e y s i n s t e a d of two as in GaAs c o u l d produce enhanced n e g a t i v e conductance and lead to more e f f i c i e n t t r a n s f e r r e d e l e c t r o n o s c i l l a t o r s . Indium phosphide was suggested as a l i k e l y candidate f o r t h i s t h r e e l e v e l o s c i l l a t o r , I n i t i a l attempts to make InP o s c i l l a t o r s were somewhat d i s a p p o i n t i n g and recent c a l c u l a t i o n s (34-36) of the v e l o c i t y - f i e l d characteristics i n d i c a t e t h a t a two l e v e l model may be more a p p r o p r i a t e to InP. Neverthelessj InP o s c i l l a t o r s have been f a b r i c a t e d r e c e n t l y w i t h cw e f f i c i e n c i e s comparable to GaAs (37~38). These devices e x h i b i t anomalous c u r r e n t / v o l t a g e c h a r a c t e r i s t i c s which are b e l i e v e d to r e s u l t from non-ohmic c o n t a c t s . InP o s c i l l a t o r s are c u r r e n t l y r e c e i v i n g c o n s i d e r a b l e a t t e n t i o n at several l a b o r a t o r i e s . Gunn type o s c i l l a t i o n s have been observed i n CdTe and ZnSe which have band s t r u c t u r e s s i m i l a r to GaAs and i n InAs under u n i a x i a l pressure. GaxInz-xSb has been s t u d i e d as a p o t e n t i a l m a t e r i a l f o r t r a n s f e r r e d e l e c t r o n o s c i l l a t o r s (39~40). The p r e p a r a t i o n of these a l l o y s has been i n v e s t i g a t e d ; however~ o n l y p - t y p e m a t e r i a l of the d e s i r e d composition (x = 0.7 - 0.8) could be obtained (40). Since the o s c i l l a t o r s r e q u i r e low c a r r i e r d e n s i t y n - t y p e m a t e r i a l j f u t u r e development of GaxInz-xSb o s c i l l a t o r s must a w a i t m a t e r i a l s improvements which enable p r o d u c t i o n of the desired composition w i t h c o n s i d e r a b l y lower acceptor c o n c e n t r a t i o n s . Avalanche Diodes. Avalanche diodes generate microwave o s c i l l a t i o n s from the n e g a t i v e conductance which r e s u l t s from two f a c t o r s : ( l ) the time delay between the a p p l i e d v o l t a g e and the r e s u l t i n g c u r r e n t due to avalanche at a j u n c t i o n j and (2) the t r a n s i t time delay f o r t r a v e l of c a r r i e r s through the diode. These s o l i d s t a t e microwave sources are g e n e r a l l y c a l l e d IMPATT d i o d e s - - t h e acronym r e f e r r i n g to IMPact Avalanche ~ r a n s i t ~ime. They were f i r s t proposed by Read (41) i n 1958. This suggested s t r u c t u r e i n c l u d e d a p-n j u n c t i o n f o r the avalanche region w i t h a l i g h t e r doped r e g i o n f o r the t r a n s i t time d r i f t space such as shown i n F i g u r e 3a. I t was subsequently found t h a t simple p-n j u n c t i o n s where the avalanche and d r i f t p o r t i o n s o f the n - r e g i o n were not c l e a r l y separated appeared to be as e f f e c t i v e
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as the more complicated structure proposed by Read (at least for s i l i c o n IMPAI-Fs). For this reason most of the IMPAI-rs are fabricated with the structure shown in Figure 3b. IMPAI-F diodes have been formed in gallium arsenid% s i l i c o n , and germanium; however, from a theoretical point of view GaAs is superior in most respects. Germanium is superior to s i l i c o n with respect to noise performance at low power levels (42,43); however, i t is not widely used, p a r t i c u l a r l y at higher power levelsj because of i t s i n f e r i o r thermal properties and r e l a t i v e l y low breakdown voltage. The highly developed processing technology available for s i l i c o n has contributed to the widespread work with s i l i c o n IMPATTs and t h e i r a v a i l a b i l i t y as a commercial device. On the other hand GaAs IMPAI-Fs have a higher theoretical efficiency (23% compared with 15% for Si) (44) and as a result of the recent advances in GaAs material preparation and processing techniques, should predominate, p a r t i c u l a r l y for the more demanding applications. The noise figures of GaAs devices are theoret i c a l l y (42,43) lower than Si, and i t has been demonstrated that GaAs devices can simultaneously produce both greater conversion efficiencies and lower noise figures. As i l l u s t r a t e d i n F i g u r e 3, the avalanche region may be formed e i t h e r by a p-n j u n c t i o n or a Schottky b a r r i e r . In the former case the j u n c t i o n may be produced d i r e c t l y d u r i n g e p i t a x i a l growth or by d i f f u s i o n of acceptors i n t o an e p i t a x t a l n - r e g i o n . S i l i c o n IMPATTs are u s u a l l y formed w i t h p-n j u n c t i o n avalanche regions because of the instabilities associated w i t h Si Schottky b a r r i e r s . Epitaxial layers must be grown w i t h very c a r e f u l l y c o n t r o l l e d thicknesses to minimize the s e r i e s r e s i s t a n c e associated w i t h undepleted m a t e r i a l d u r i n g o p e r a t i o n . The thermal r e s i s t a n c e s of the device are reduced by attachment o f a heat s i n k onto the m e t a l l i z e d c o n t a c t to the p - r e g i o n e i t h e r by d i r e c t bonding or by p l a t i n g (45,LI6). Gallium arsenide IHPATrs are produced using both p-n j u n c t i o n and Schottky b a r r i e r t e c h n o l o g i e s f o r formation of the avalanche regions. The Schottky b a r r i e r approach has a number of advantages and is p a r t i c u l a r l y approp r i a t e to GaAs because the high s u r f a c e s t a t e d e n s i t y o f the m a t e r i a l permits f o r m a t i o n of h i g h l y r e p r o d u c i b l e b a r r i e r s (47). A heat s i n k may be p l a t e d (/48) or bonded d i r e c t l y to the Schottky b a r r i e r thus e l i m i n a t i n g the thermal impedance o f the p - r e g i o n and i t s c o n t a c t which is associated w i t h a p-n j u n c t i o n d e v i c e . As w i t h Si IMPAI"Fs e p i t a x i a l m a t e r i a l i s used to form the a c t i v e region of GaAs devices because o f the b e t t e r t h i c k n e s s c o n t r o l , lower background doping l e v e l , and g r e a t e r c r y s t a l p e r f e c t i o n of e p i t a x i a l m a t e r i a l . As w i t h t r a n s f e r r e d e l e c t r o n devices~ the layers must have no abrupt d i s c o n t i n u i t i e s i n doping l e v e l and the thicknesses must be c a r e f u l l y c o n t r o l l e d to e l i m i n a t e p a r a s i t i c r e s i s t a n c e s . A cleaved and etched cross s e c t i o n through a GaAs s l i c e s u i t a b l e f o r f a b r i c a t i o n of IMPATT devices is i l l u s t r a t e d i n Figure 4. Note again the presence o f an n + - I a y e r to i s o l a t e the a c t i v e n - l a y e r from the s u b s t r a t e . There are c o n f l i c t i n g reports on the importance o f t h i s i n t e r m e d i a t e l a y e r . I t has been reported t h a t the s u b s t r a t e q u a l i t y is an important f a c t o r in high power o p e r a t i o n (3). T h i s would i n d i c a t e t h a t the presence of the
Semiconductor Materials for Microwave Devices
263
[ x I018 cm-3
I
I
>..
I SUBSTRATE I
3xlO 16 U cm-3
I--
r
I OPTIONAL "~.~.__1NTERMEDIATE I EPITAXIAL I LAYER
I,Li I.iJ
I I I I I
IxlOIScm-3 -.~
IFm
2Fm
SCHOTTKY BARRIER OR p-REGION
. DISTANCE (a]
I x I018 cm-3 I
I I I
)k-
I
I SUBSTRATE
z
uJ f~
I I
2x 1016cm"3
Q::
I I
o:: n.-
5Fm
I ~
I
I
I I
I I
I SCHOTTKY BARRIER OR p-REGION Figure 3. Idealized a. Read S t r u c t u r e
OPTIONAL INTERMEDIATE EPITAXIAL LAYER
DISTANCE
(b) D o p i n g P r o f i ] e s f o r IMPATT D i o d e s . b, Simpler Structure Without Separate Drift Region
264
Shaw
Figure 4.
Cleaved and Etched Cross S e c t i o n Through a GaAs S l i c e f o r IMPATT Diodes. Etched i n the AB Etchant (30).
X 490.
Semiconductor Materials for Microwave Devices
265
intermediate layer would be very important. On the other hand, some experiments have indicated that there is no difference in performance of devices fabricated with and without an intermediate layer (49). In generalj as with transferred electron devices, i f the doping level of the intermediate layer is s u f f i c i e n t l y high to reduce the series resistance, the presence of such a layer should not be in any way d e t r i mental to the device and could s i g n i f i c a n t l y improve i t s r e l i a b i l i t y , p a r t i c u l a r l y i f the substrate were removed during processing. Although most GaAs IMPATTsare produced from slices similar to that shown in Figures 3b and 4 with the simple structure combining the avalanche and d r i f t region in the same e p i t a x i a l layer, recent studies (50) have indicated that the Read structure (Figure 3a) may produce more e f f i cient devices. Much of the e f f o r t to improve IMPATTperformance is currently centered around production of devices with specially tai]ored doping p r o f i l e s . Varactor Diodes. Varactor diodes e x h i b i t variations in junction capacitance with applied voltage and serve as units of variable reactance for harmonic generation, tuningj switching, and mixing. The requirements for a material for fabrication of varactor diodes have been discussed in detail ( 5 l ) . In general, the material should have a high carrier mobility to maintain minimum e l e c t r i c a l resistance, a low d i e l e c t r i c constant f o r minimum capacitance, a large energy gap to minimize saturation currents and for potential higher temperature operation, and a high thermal conductivity. An examination of Table I reveals that GaAs is superior to Si in a l l respects except thermal conductivity. Germaniumpossesses no clear cut advantage over Si and is more d i f f i c u l t to process. Thus Si and GaAs are the dominant varact o t materials with the l a t t e r being superior, p a r t i c u l a r l y for the more demanding applications. S i l i c o n varactors are generally formed with a diffused p-n junction. However with GaAs both diffused and Schottky barrier junctions are used with the l a t t e r becoming increasingly popu= far. As previously discussed, reproducible Schottky barrier junctions are easily formed w i t h GaAs while controlled diffusions are somewhat more d i f f i c u l t to obtain.
,Table I .
Selected Properties of Semiconductor Materia]s Energy Gap (eV)
Mobility (300K) Thermal (cm=/vsec) Conductivity Electrons Holes (W/cm-K)
Dielectric Constant
Si Ge GaAs
I.I 0.6? 1.4
1500 3900 8500
600 1900 400
1.45 0.64 0.46
12 16 12
InP
1.3
4600
150
0.65
14
266
Shaw
Varactor diodes are fabricated in much the same manner as IMPATT diodes except that massive heat sinks are not required and the requirements of high crystal perfection and doping uniformity are not as severe as with devices serving as microwave power sources. Typically they are produced by diffusing acceptors into a single n-type epitaxial layer or by formation of a Schottky barrier upon its surface. The doping level of the epitaxial layer must be controlled to provide the proper capacitance and breakdown voltage in the device. Also the substrate r e s i s t i v i t y should be low and the epitaxial layer thickness c a r e f u l l y regulated to minimize excessive series resistance. Mixer I Detector 1 and PIN Diodes. Mixer and d e t e c t o r diodes are u s u a l l y formed w i t h metal-semiconductor j u n c t i o n s ~ as p-n j u n c t i o n s do not possess the desired low l e v e l o f m i n o r i t y c a r r i e r i n j e c t i o n . Included in t h i s group o f devices are Schottky b a r r i e r diodes formed from Ss and GaAs~ p o i n t c o n t a c t diodes from S i~ and back or backward diodes o f Ge and GaAs. The l a t t e r diodes r e s u l t from a l l o y i n g a p-type i m p u r i t y i n t o a very h e a v i l y doped n - r e g i o n (which may be e i t h e r bulk or e p i t a x i a i ) in order to produce an abrupt p-n j u n c t i o n by regrowth. The remaining devices are g e n e r a l l y produced e n t i r e l y from e p i t a x i a l m a t e r i a l - - n - t y p e f o r the Schottky b a r r i e r diodes and p - t y p e f o r the Si p o i n t c o n t a c t diodes. The a l l important noise performance of mixers can be improved c o n s i d e r a b l y when e p i t a x i a l layers are used (52) because of the s i g n i f i c a n t r e d u c t i o n s in s e r i e s r e s i s t a n c e that can be o b t a i n e d when the s u b s t r a t e j which supports the a c t i v e e p i t a x i a ] layer~ is very h e a v i l y doped. PIN diodes are w i d e l y used as switchesj a t t e n uators~ and phase s h i f t e r s . They c o n t a i n a r e l a t i v e l y t h i c k high resistivity ( i ) l a y e r sandwiched between h e a v i l y doped p and n regions. Although GaAs has received c o n s i d e r a t i o n (95) a l l commercial devices are con s t ru c t e d from s i l i c o n . B i p o l a r Microwave T r a n s i s t o r s ~ This f i e l d is c u r r e n t l y dominated by s i l i c o n w i t h o u t a p p r e c i a b l e c o m p e t i t i o n from any o t h e r m a t e r i a l . Germanium~s h i g h e r c a r r i e r m o b i l i t i e s are e f f e c t i v e l y o f f s e t by the more f a v o r a b l e d i e l e c t r i c constant~ h i g h e r breakdown v o l t a g e j g r e a t e r thermal c o n d u c t i v i t y ~ and s u p e r i o r device f a b r i c a t i o n technology of silicon. Gallium a r s e n i d e as a b i p o l a r t r a n s i s t o r m a t e r i a l s u f f e r s from a number o f problems. Although OaAs has a high e l e c t r o n m o b i l i t y the o v e r a l l performance of the t r a n s i s t o r is a f u n c t i o n of the product of the hole and e l e c t r o n m o b i l i t i e s r a t h e r than the e l e c t r o n m o b i l i t y a l o n e . For e f f i c i e n t o p e r a t i o n and high i n j e c t i o n e f f i c i e n c i e s the e m i t t e r r e 9 i o n of the t r a n s i s t o r must be c o n s i d e r a b l y more h e a v i l y doped than the base. With GaAs the p r a c t i c a l upper l i m i t o f the e m i t t e r doping l e v e l is r e l a t i v e l y low (~ 2 x l0 l e cm"a) r e q u i r i n g low base c o n c e n t r a t i o n s which are very d i f f i c u l t to achieve by d i f f u s i o n . F i n a l l y j the s h o rt c a r r i e r d i f f u s i o n lengths demand extremely t h i n base w i d t h s . The problem o f i n j e c t i o n e f f i c i e n c i e s may be p a r t i a l l y overcome in the f u t u r e by use o f a h e t e r o e p i t a x i a l s t r u c t u r e in which the e m i t t e r is formed in a w i d e r energy gap m a t e r i a l such as GaA~As. Nevertheless, the o t he r problems w i l l remain.
Semiconductor Materials |or Microwave Devtc~
267
Although material preparation is very important to the continued improvement of Si bipolar transistors~ the greatest impact is expected in the area of device fabrication technology. Improvementsin the near future are l i k e l y to result from smaller device geometries which can be obtained from electron beam or x-ray (53) exposureof photoresists and from more accurate control in the d i ffu s i o n steps. Ion implantation is also a promising approach to microwave transistor fabrication~ but this f a l l s out of the realm of the present paper.
Microwave b i p o l a r t r a n s i s t o r s are c o n s t r u c t e d e n t i r e l y in e p i t a x i a l l a y e r s in order to reduce the s e r i e s r e s i s t a n c e . The layers must possess a number of s p e c i a l i z e d p r o p e r t i e s . A high degree of doping l e v e l u n i f o r m i t y is essential without a long doping " t a i l " at the substrate epitaxial interface. Typically the e p i t a x i a l layers are n-typ% from I-5 IJm thick~ and are grown on a low r e s i s t i v i t y (~ O.Ol ohm cm)~ n-type substrate. Devices operating at r e l a t i v e l y high power levels demand a high degree of crystal perfection in the e p i t a x i a l layers and~ consequentlyj low dislocation density substrates are essential. Needless to sayj c a r e f u l l y prepared e p i ta x i a l layers with sharp doping p r o f i l e s should not be subjected to processing procedures such as high temperature diffusions or oxide growths which tend to make the doping p r o f i l e at the substrate-epitaxia] interface diffuse. Since the small geometries e s s e n t i a l to high performance microwave t r a n s i s t o r s r e q u i r e high r e s o l u t i o n p h o t o l i t h o g r a p h y ~ s l i c e c u r v a t u r e and e p i t a x i a i s u r f a c e defects which might prevent good c o n t a c t w i t h the photomask d u r i n g processing must be prevented. Microwave F i e l d E f f e c t T r a n s i s t o r s . Gallium arsenide possesses a unique combination of properties which make i t a suitable material for microwave FET's. These include a high mobility and saturation velocity so essential to the high frequency performance of these majority c a r r i e r devices, a well developedSchottky barrier technology for formation of reproducible gates~ and the existence of a semi-insulating (Cr-doped) form of GaAs which can act as a substrate for the active epitaxia] layer. I n i t i a l l y j these devices were constructed by di f f usi on of acceptors to form a p-type region for the gate (54~55); however~ use of a Schottky barrier (56-58) gate is currently more popular. A schematic diagram of the FET structure is shown in Figure 5. I t is constructed by formation of alloyed source and drain contacts and a Schottky barrier gate to a thin e p i ta x i a l GaAs layer grown on a semiinsulating substrate. In operation~ "pinch o f f " of the carriers occurs by extending the depletion layer beneath the gate to the high r e s i s t i v i t y substrate. Material Preparation Since most of the microwave devices discussed in the preceding s e c t i o n are f a b r i c a t e d w i t h i n e p i t a x i a l layers~ t h i s s e c t i o n on m a t e r i a l preparation w i l l emphasizee p i t a x i a l growth. Specific at t ent i on w i l l be given to the special features associated with growth of e p i t a x i a l layers designated for microwave applications.
268
Shaw
Silicon. Three vapor phase d e p o s i t i o n processes are c u r r e n t l y employed f o r growth o f e p i t a x i a i s i l i c o n l a y e r s f o r microwave purposes: hydrogen r e d u c t i o n o f s i l i c o n t e t r a c h l o r i d e ~ hydrogen r e d u c t i o n o f t r i c h l o r o s i l a n e ~ and p y r o l y t i c decomposition o f s i l a n e . The f i r s t two processes are very s i m i l a r and the remarks f o r one w i l l , in general~ apply to the o t h e r . Hydrogen r e d u c t i o n o f SIC14 is the o l d e s t and s t i l l the most w i d e l y used process. Hence c o n s i d e r a b l y more experience has been acquired f o r t h i s d e p o s i t i o n system. An open tube apparatus is employed w i t h a m i x t u r e of hydrogen and s i l i c o n t e t r a c h l o r i d e f l o w i n g over the substrates which are heated by i n d u c t i o n to a temperature near 1200~ On or near the s u b s t r a t e surface r e d u c t i o n takes place according to the reaction~ SiCl,(g) + 2H~(g) -* Si(s) + 4HCl(g)
[l]
w i t h the s i l i c o n being deposited e p i t a x i a l l y . There are two opposing factors which must be considered in the choice of a growth temperaturethe degree of crystal perfection and the doping p r o f i l e . Microwave devices require f l a t doping p r o f i l e s with a minimum doping t a i l or autodoping present at the substrate-epitaxial interface. Such sharp p r o f i l e s are promoted by growth at low temperatures so as to minimize the amount of dopant diffusion from the substrate. On the other hand superior crystal perfection is obtained at higher temperatures where the species being incorporated into the crystal are more mobile. These trade-offs result in rather well defined deposition temperatures. The s i l a n e process appears e s p e c i a l l y a t t r a c t i v e f o r microwave devices. Growth occurs by decomposition o f s i l a n e according to the reactionj
sire(g)
~ si(s)
+ 2H~(g)
[2]
The c a r r i e r gas may be hydrogen or helium or a mixture of these two. The deposition temperature is approximately 1000"C which is well below that required for the halide reduction process. In additionj the absence of halides precludes transfer of dopant as a v o l a t i l e halide from exposed areas of the substrate with subsequent incorporation into the growing layer. Generally the deposition rates are lower with the silane process~ a fact which becomes an advantage for growth of thin layersj since longer deposition times permit closer control over the layer thicknesses and device series resistance. In spite of the apparent advantages of silane epitaxy~ hydrogen reduction of s i l i c o n halides is s t i l l very widely used for growth of epitaxial layers for microwave devices. This is due in part to the greater experience and confidence with the l a t t e r process and in part to the achievement of good doping profiles with the higher temperature process through careful control of the growth parameters. Nevertheless, silane epitaxy is expected to become increasingly popular. Liquid phase epitaxial growth of s i l i c o n is not in use currently and l i t t l e or no work is in progress in this area.
Semiconductor Materials for Microwave Devices
269
SCHOTTKY
~
SOURCE
DRAIN . - - ~
BARRIER
IIIIll|
GATE
\\1
!i111I
L
SEMI-INSULATING GoAs
L
~--0,2
EPITAXIAL - O,5~m n=8xlO~scm'3
Figure 5.
SUBSTRATE
Gallium Arsenide Hicrowave Field Effect Transistor.
270
Shaw
Gallium Arsenide. B u l k GaAs may be grown by either the Czochralski technique or by gradient freeze from a boat. Generally the lower l i m i t of electron concentration is around 5 x l ~ S cm-3 and the mobility is low in comparison with epitaxial GaAs. Since high m obi l i t i es are essential For most microwave devices and the device structures require well defined geometries, almost a l l active GaAs devices are formed in epit ax ia l layers. Nevertheless, bulk material forms the substrate for epitaxia] growth and consequently receives a considerable amount of attention. A number of n-type dopants are used in the substrates, including Sn, Si, Se, and Te. However, in some cases Si-doped substrates have been suspected as a potential cause of the high resistance layers which sometimes form between the substrate and the epitaxia] f i l m . Semi-insulating substrates may be obtained by chromium or oxygen doping. Unlike s i l i c o n , bulk GaAs with a high degree of crystal perfection is d i f f i c u l t to obtain. For power devices such as IMPATTdiodes the substrate may be a very important factor in the overall performance of the devices (3). For this reason the current trend in GaAs epi t axi al technology is toward growth of heavily doped intermediate or buffer layers to isolate the active portions of the device from the substrate as i l l u s t r a t e d in Figures 2 and 4. The intermediate layer than serves as a high quality "substrate" For additional e p i t a x i a l growth. A number of systems have been investigated for potential use in vapor phase e p i t a xi a l growth of GaAs for microwave purposes. These systems are l i s t e d in Table I I together with t h e i r r e l a t i v e advantages and disadvantages. Currently only the f i r s t two systems are in widespread use in the microwave area and the discussion which follows w i l l be limited to these. The AsCla system (59,60) is the most popular vapor phase process for high purity e p i t a x i a l layers. Only three reagents (H2, Ga, AsCI3) are required and each of these is readily available in a very high purity form. A schematic diagram of a typical growth system is shown in Figure 6a. In operation, hydrogen, purified by a palladium d i f f u sion, is admitted into an AsCl3 saturator and the resulting mixture passes into the hot region of the main reaction tube where i t is reduced according to the reaction, 6~(g) + L~sCl3(g) -- Asc(g) + 12HCI(g)
[33
As the resulting mixture passes over the source boat a v o l a t i l e gallium species is Formed according to the reaction, 4HCI(9) + 4GaAs(s) . 4GaCI(g) + 2H~(g) + As4(g)
[4]
Thus AsC]3 serves as the source of the gallium transport agent (HCl) as well as the arsenic source. The source reaction, [4], is shown as involving solid gallium arsenide. I n i t i a l l y the source is liquid gallium; however, any arsenic formed according to [3] tends to dissolve in the gallium source until saturation is achieved, at which time a crust or f i l m of gallium arsenide forms upon the surface. Prior to saturation there is no arsenic available for growth downstream on the
Semiconductor Materials [or Microwave Devices
271
s u b s t r a t e s . A f t e r s a t u r a t i o n the hydrogen stream l e a v i n g the source c o n t a i n s a m i x t u r e o f a r s e n i c and g a l l i u m monochloride (plus some unreacted hydrogen c h l o r i d e ) . This m i x t u r e then passes i n t o the subs t r a t e region which is maintained a t a lower temperature where the thermodynamics are such t h a t the reverse o f r e a c t i o n [ 4 ] occurs and GaAs e p i t a x i a l l y grows upon the s u b s t r a t a . The thermodynamics and k i n e t i c s o f t h i s growth process have r e c e n t l y been s t u d i e d i n d e t a i l (26,61,62). Due t o n u c l e a t i o n d i f f i c u l t i e s , l i t t l e or no GaAs forms on the quartz tube w a l l s or the s u b s t r a t e h o l d e r in t h i s r e g i o n . Thus, the process is very s e l e c t i v e in nature and is p a r t i c u l a r l y a p p r o p r i a t e f o r growth of p l a n a r microwave devices by s e l e c t i v e etch and r e f i l l techniques (15~63,64). Very pure GaAs l a y e r s w i t h high m o b i l i t i e s a r e produced by the AsCla process (65-67). G e n e r a l l y the background i m p u r i t y l e v e l is so low t h a t i t is necessary t o add dopants to the gas stream t o produce the l e v e l s r e q u i r e d f o r most devices. Gas phase dopants such as H~S~ H2Se~ and GeH4 are o f t e n used f o r n-type l a y e r s . Sometimes Te or Se is added to the source boat; however, t h i s technique is not very v e r s a t i l e and due t o s e g r e g a t i o n e f f e c t s does not always y i e l d r e p r o d u c i b l e doping levels. Gas phas% p - t y p e doping is a problem i n vapor phase d e p o s i t i o n due t o the l i m i t e d number of v o l a t i l e compounds of the acceptor i m p u r i ties. D i e t h y l z i n c (68) and dimethyl cadmium have been used withlsome success. The problem o f p - t y p e doping w i l l be the s u b j e c t of a d d i t i o n a l d i s c u s s i o n below. The general area o f i m p u r i t y i n c o r p o r a t i o n d u r i n g e p i t a x i a l growth o f GaAs has r e c e n t l y received the a t t e n t i o n o f a number o f i n v e s t i g a t o r s (69-74). These s t u d i e s have g r e a t l y increased the understanding o f the i n f l u e n c e of growth parameters such as the Ga t o As vapor phase r a t i o on the e l e c t r i c a l c h a r a c t e r i s t i c s o f e p i t a x i a l l a y e r s and have provided a basis f o r improving the r e p r o d u c i b i l i t y o f the AsCl~ process. The o t h e r major vapor system f o r growth o f GaAs f o r microwave a p p l i c a t i o n s is the AsHs/Ga/HCI/H2 system (?5)3 sometimes c a l l e d the a r s i n e process. This system~ which is i l l u s t r a t e d i n F i g u r e 6b~ p e r mits a high degree of v e r s a t i l i t y in s e l e c t i o n of the gas phase s t o i c h i o m e t r y . The r a t i o of a r s e n i c t o g a l l i u m c o n t a i n i n g species can be e a s i l y a l t e r e d by v a r i a t i o n s in the r a t i o o f the Hg-AsHs f l o w t o t h e H~-HCI f l o w which passes over the Ga source. U n l i k e the AsClg process~ no s a t u r a t i o n o f the source is r e q u i r e d p r i o r t o e p i t a x i a ] growth. E s s e n t i a l l y the same growth temperatures are employed f o r both the AsCla and Asl~ processes. The a b i l i t y t o c o n t r o l the gas s t o i c h i e m e t r y is h i g h l y d e s i r a b l e j since i t has been shown t h a t the e l e c t r i c a l c h a r a c t e r i s t i c s of the e p i t a x i a l l a y e r s are very s e n s i t i v e to the Ga to As r a t i o in the vapor phase (64). This system has been used e x t e n s i v e l y f o r e p i t a x i a l growth o f p-n j u n c t i o n s where the p - r e g l o n is produced by Zn doping (75j76). A s o l i d Zn source is used w i t h t r a n s p o r t o b t a i n e d by passing a hydrogen stream over the Zn which is heated t o an a p p r o p r i a t e temperature. U n l i k e gaseous dopants a s o l i d dopant source r e q u i r e s the f u r t h e r c o m p l i c a t i o n o f an a d d i t i o n a l temperature zone w i t h i n the r e a c t i o n system. The AsHs system tends to produce background doping l e v e l s which are somewhat h i g h e r than the AsC13 process,
272
Shaw
Table I I .
GaAsVa or Grow h S s ems
System (Classified According to Reagents)
Advantages
Disadvantages
s i m p l i c i t y , minimum number of reactants a l l of which are available in very high purities
little control over vapor composition
AsH3-Ga-HCI-H2
ease of control of vapor stoichiometry
limited availability of high p u r i t y reagents, c o m p l e x i t y of 4 component system
As-Ga-HCl-ffa
ease of control of vapor stoichiometry
reagents e x i s t in three phases, complex reactor design
d e p o s i t i o n c a r r i e d out in cold wall reactorj may be RF heatedj good
d i f f i c u l t y of handl i n g reagentsj limited availability of high p u r i t y reagents~ growth process f a r removed from equi I ibrium
AsCI3-Ga-H2
Asl~ -Ga (CH~).~-I~
or Ga(C2~)3
control
over vapor
Semiconductor Materials |or Microwave Devices
273
//~- FURNACES o o
800-850~
/
'~
o o
SUBSTRATE o
o o o10 o o 'o/o/--1"-'--725- 775 C
: ....
o0:oo \,22s o (a)
H 2 /r + AsH3 H 2 + DOPANT 825 - 9 0 0 ~ 700-800 ~ I o o o o o I o o o o o I--.---FURNACES
.
.
1I
"
Io o o\o ol,o o o
H2+ HCl
\
SUBSTR
L_
G0
(b) Figure 6.
Schematic Diagrams of GaAs Epitaxial Deposition Systems. a. Ga/AsCI3/H2 b. Ga/HC 1/Asli3/H2
274
Shaw
primarily because of the d i f f i c u l t y of obtaining high purity HCl and AsHa. However~ Minden (77) has suggested that the higher purity levels associated with the AsCla process are related to the arsenic dissolution-gallium arsenide crust formation which takes place during source saturation. Solution growth or l i q u i d phase epitaxy of GaAs has been known for some time (78)~ but only in the l a s t few years has i t been widely applied to growth of GaAs for microwave purposes. I t s principal advantage l i e s in the fact that the d i s t r i b u t i o n coefficients for metallic impurities~ which are contaminants in GaAsj are favorable for growth from a solution of liquid' gallium saturated with arsenic. For this reason layers have been grown by l i q u i d epitaxy with very high p u r i t i e s (79-81). There are a number of d i f f e r e n t approaches to l i q u i d epitaxy but in each case the substrate is brought i n t o contact with a saturated solution of arsenic in gallium. This is then subjected to a controlled cooling cycle during which GaAs deposits e p i t a x i a l l y onto the substrate. The approaches may be c l a s s i f i e d according to the method by which the substrate crystal is presented to the solution. Three basic techniques (tipping~ dipping~ and the s l i d i n g boat) are i l l u s t r a t e d in Figure 7. The tipping technique is best known and was described in some detail by Nelson (78). I n i t i a l l y the source l i q u i d is held at one end of a horizontal tube with the substrate at the other. When the operating temperature is reached and the source is properly saturatedj the tube is t i l t e d ~ causing the source l i q u i d to cover the substrates. The cooling regime is then i n i t i a t e d and growth is stopped by t i l t i n g the tube in the other direction to remove the source l i q u i d . The d i p p i n g technique is somewhat simpler. In this case the source l i q u i d remains in a fixed position while growth is i n i t i a t e d by lowering (dipping) the substrate into the source l i q u i d and beginning the cooling cycle. At the end of the growth period the substrate is extracted from the l i q u i d . This method f a c i l i t a t e s control over the layer thickness. The s l i d i n g boat technique offers the p o s s i b i l i t y of multilayer or junction growth. Here the substrate is positioned in a c a r e f u l l y machined graphite holder which can slide beneath one or more source regions. This approach has been used for growth of w e l l defined~ multilayer laser structuresj and there seems to be no reason why i t should not be applicable to growth of multilayer microwave structures such as three layer (n+ n- n+) structures for Gunn effect devices. Typically Sn and Ge are used as n and p type dopants respectively. The theoretical aspects of l i q u i d phase epitaxial growth~ including the problems of constitutional super-cooling and optimum apparatus design~ are rather well deveI oped (82-8G). It is perhaps indicative of the immaturity of GaAs epitaxial growth technology that such widely diverse techniques for epitaxia] growth as liquid phase and vapor phase approaches are currently both being used to provide material for identical devices. It might be expected that within the last few years one of these systems would prove to have a decided advantage and would replace the other for growth of microwave quality GaAs. From this point of view it is perhaps worthwhile to discuss the relative merits of the two approaches.
Semiconductor Materials for Microwave Devices
275
L i q u i d phase growth is considerably simpler than vapor phase both in theory and in practice. Vapor phase techniques require sophisticated apparatus and gas handling equipment. In addition s considerable care is necessary in order to obtain the low background c a r r i e r concentrations required for some microwave devices. For these reasons l i q u i d phase growth i s more a p p e a l i n g to the novice i n e p i t a x i a l growth s i n c e he can begin producing r e l a t i v e l y high p u r i t y GaAs i n a s h o r t e r period of time w i t h less investment. On the o t h e r hand s as devices become more complicated and m u l t i - l a y e r s t r u c t u r e s are r e q u i r e d j the degree of c o n t r o l c h a r a c t e r i s t i c of vapor phase techniques becomes very d e s i r a b l e . The dopant c o n c e n t r a t i o n can be e a s i l y changed d u r i n g the growth c y c l e to produce layers of d i f f e r e n t doping l e v e l s or doping p r o f i l e s w i t h special characteristics. Vapor phase growth generally produces the bright smooth surfaces so essential to subsequent device processing and especially suitable for formation of Schottky barrier interfaces. Uncontrolled thicknesses and poor surfaces are the principal problems associated with l i q u i d phase epitaxy. In general s as more experience is developed3 i t appears that the l i q u i d phase technique w i l l be most useful for growth of r e l a t i v e l y thick~ l i g h t l y doped layers unless the s l i d i n g boat method lives up to i t s expectations. Indium Phosphide. Epitaxial growth of indium phosphide is similar in many respects to gallium arsenide and w i l l be b r i e f l y discussed. Development of methods for growth of InP bulk crystals has now progressed to the point that i t is no longer necessary to grow InP heteroe p i t a x i a l l y on GaAs or InAs substrates. Indium phosphide substrates can be cut from ingots grown from a pressurized crystal puller using l i q u i d encapsulation techniques (87s88). Alsos as with GaAs, semiinsulating InP substrates can be obtained by Cr-dopings although the r e s i s t i v i t y (~104 ohm cm) is not as high. B o t h l i q u i d (89) and vapor phase techniques have been used for epitaxial growth. The vapor process (90-93) is essentially identical with the AsCla process described previously for GaAs growth except that the Ga and AsCla are replaced by In and PCIa and lower temperatures are used. InP has also been grown e p i t a x i a l l y from an In/HCl/PHa/H2 system (94). The PCla method has been demonstrated to y i e l d r e l a t i v e l y high p u r i t i e s (92s93) and has made available high q u a l i t y material for investigation of the potential of InP as a material for microwave device fabrication. Conclusion In summarys preparation of semiconductor materials for microwave applications is a very specialized and challenging endeavor. For many devices s improvements in s t a r t i n g material can be translated d i r e c t l y i n t o better microwave performance. Due to the special properties required f o r these devices innovative approachess p a r t i c u l a r l y in the area of epitaxial growths have been developed. These deve|opmentss together with advances in device fabrication techniquess are contributing to the growing application of solid state components to microwave systems.
276
Shaw
AcknowledFFnents
The author wishes to acknowledge several helpful discussions with Drs. D. R. Chen, F. Doerbeck, T. G. Blocker, and D. N. McQuiddy. He also wishes to express his appreciation to Drs. T. E. Hasty, R. E. Johnso% and W. R. Wisseman for reviewing the manuscript.
A SOURC TIPPING
SORUC E~ DIPPING
I
*-*
KATE
SLIDING BOAT Figure 7-
Liquid Phase Epitaxy Te6hniques.
Semicondu~orMatefialsforMicrowave Devices
2.77
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