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MOMBE and MOVPE—A comparison of growth techniques
Pmg. C~.stal
014m63sta 8.m + .x) Copyright Q 1989 Pergamon Press plc
Gmwth and Charact. 1999. Vol. 19.PP. 83-96 Printed in Great Erwin. All rights rawwed
MOMBE AND MOVPE-A COMPARISON OF GROWTH TECHNIQUES M.
Weyers
Institute of Semiconductor Electronics, Technical D-5100 Aachen, F.R.G.
University of Aachen,
1. INTRODUCTION Since its inception in 1968 /l/ MOVPE(metal organic vapor phase epitaxy) or NOCVD (metal organic chemical vapor deposition) has undergone a steady development. Operation at reduced pressures /2/ enables better homogeneity of deposited layers /3/ and sophisticated gas manifolds allow fast switching and abrupt changes of composition in the deporited films /J/. Especially for III-V semiconductor layers and layered structures HOVPE now has become a widespread growth technique for a range of electronic and optoelectronic devise structures like RENTS/5/, lasers /6/, photodetectors /7/ or complete integrated circuits /8/. Although it is being successfully used for production of layers, the MOVPE process is only partly understood since the interplay of gas phase and surface reactions makes an investigatian of this growth system difficult. To shed more light on the surface reactions in IIOVPE, in our laboratory a study was started to investigate these processes under uhv conditions /9/. This approach, which we called metalorganic molecular beam epitaxy (HOIIBE) has developed into a powerful growth technique in its own right. It was subsequently named chemical beam epitaxy (CBE) by others using a slightly modified version /lo/. Although originally very closely related to MOVPEit is widely considered as a special form of HBE. The purpose of this paper is to compare deposition eauipment, growth kinetics and the aualitv of the- deposited layers and to di;cuss the sinilia&ties and dissiailiarities of ROW% and is dealinD UOUBE. The focus will be on the III-V materials since nearly all the work on HOHBE with these compounds. Growth kinetics will mainly be discuss;d for Gab which still is the besi investigated III-V material. Although MOHEJE is a relatively new technique its rapid development raises high expectations for the quality of the material that can be obtained by this approach in the near future. It will be argued in this paper, that it has the potential to become a fabrication process for applications where a high level of control is required. 2. DEPOSITION SYSTEM 2.1
Reactors
MOVPEreactors are usually made of an outer quartz tube equipped with an inner liner tube to facilitate the cleaning of the system. The substrate is placed on a graphite aubstrate holder which is externally heated either by radiation from quartz halogen lamps or by rf induction. The latter method is limited to pressures of above lpprox. 1000 Pa since the unintentional formation of an rf plasma may lead to irreproducibilities in the growth behavior at lower total pressures /ll/. The reactor is a cold wall system although extended hot zones upstream and above the ousceptor cannot be totally avoided and may give rise to prodeposition and loss of reactants. Rorizontal /3/ and vertical flow reactors /12/ are both being used, but have a basic feature in common: they exhibit distinct profiles in flow velocity, concentration of the reactants and temperature which are strongly dependent on the reactor geometry used (fig. 1). To achieve homogeneous deposition the conditions over the entire substrate esrentially should be constant. This implies that the gas phase composition should not change siipnificantly over the length of the substrate, especially for the growth of ternary or quaterr+ry layers. This in turn means that most of the starting materials leave the reaction zone unused. In contrast to the somewhat fragile quartzware used in NOVPE, an XOMBE deposition apparatus is a3
84
M. ‘Weyers
reactants
-V
carrier (Hz) quartz tube, (cold wall) Fig.
1: Schematic
of horizontal
EOVPE reactor.
a sturdy stainless steel MBE chamber /13/. It is a real cold wall system since the growth environment is LNz shrouded to reduce the partial pressure of contaminants like CO and other residual gases (fig. 2). For the same reason the systems are equipped with load lock chambers to keep atmospheric contaminants like oxygen or water out of the system (and toxic substances confined to it). The same technique has been adapted to MOVPEreactors in recent years. The substrates are either glued to a molybdenum holder with In, a technique which surely is only tolerable in research and not for production applications, or they are held by some support usually made also from molybdenum for reasons of purity. The substrate is radiatively heated from the backside and aay be rotated to achieve better temperature uniformity and to average over possible inhomogeneities in the flux distribution. As the thermocouple is only radiatively coupled to the backside of the rotating substrate or substrate holder, accurate temperature measurement is difficult to achieve /14/.
\
steel chamber Fig.
2: Schematic
of LNs shrouded
MOBBEgrowth
chamber.
The molecular beams of the different starting materials are impinging at different angles onto the substrate. By tailoring the beam profile such that it is homogeneous over the entire substrate the consunption of the starting materials, which depends on the sticking coefficients of the precursors on the substrate surface, may be kept low. Unintentional pyrolytic predecomposition of reactants like in MOVPEis eliminated since no extended hot zones are The use of mechanical shutters between the cell and the substrate allows present. fast switching by interrupting the molecular beams. The main difference from KOVPC is that MOBBEis a uhv process. It thus allows integration of in situ analytical tools like RHEED which has proved to be a powerful method for characterization of the growth process on a monolayer scale /15/. Bass spectrometry may provide information not only on the composition of the residual gas but also on cracking of starting materials and byproducts desorbing from the growing layer. Eaterials which are not readily available in gaseous form (for example, dopants like Be) may be introduced by evaporation, as in conventional MBE. Transfer to additional chambers of a multichamber system makes the entire range of uhv analytical tools (UPS, AES, SIES etc.1 available to characterize freshly prepared surfaces. It subsequent uhv compatible processing steps also allows (tocussed ion beam implantation, metallisation) without exposing the layer to atmosphere.
A comparison of MOMBEand MOVPE
85
2.2 Pumpiaa systems Although XOVPEat atmospheric pressure is still widespread, the modern approa:h to a:hi~ce good homogeneity in layer composition and thickness is to lower the total pressure. In this manner the the local variation in the composition of the gas phase is reduced by increasing To this end a two stage rotary pump is normally used. To protec: chip pump diffusivity /3/. accumulate in the puapiag oil or reduce (and the environment) from gases that are toxic, sometimes a thermal cracker together with a cold trap is ldde4 betvfen reactor pumping speed, and pump; to treat the exhaust gases of the pump a scrubbing system may be employed (fig. 3).
Fig. 3: Low pressure BOVPEpumping arrangement. Even though the base pressure (without gas flow) of this pumping equipment may be below 0.1 Pa, operating pressures are well above 100 Pa. This means that the mean free path of a molecule between two collisions is less than 0.1 pa and gas phase reactions are readily possible. As MJHBE takes place at uhr conditions a corresponding pump has to be employed. Due to the high of the starting materials and their decomposition products vapor pressure (hydrocarbons and especially hydrogen) ion getter pumps normally used in an BBEsystrn are not satisfactory and other types of high vacuum pumps are necessary. Some laboratories employ oil diffusion pumps with appropriate LEr traps to protect the vacuum: in our case we have good experience over several years with turbomolecular pumps, again with two stage rotary pumps as fore pumps (fig. 41. During the deposition process a cryopuap provides additional pumping capacity especially
turbopump
trap
Fig. 4:.BOBBEpumping system with turbomolecular pump. condenses undecoaposed aetalorganics. With this configuration for Bz, whilst the cryoshroud a base pressure of 10-v Pa (pressure limit of the turbo pump) is achieved. After bakeout of the system the pressure may be further lowered using an additional ion pump, but in our opinion there is no real need to do so. Depending on the pumping speed and on the Ha load from the hydride decomposition or the carrier gas a process pressure of lO_’ to 10-a Pa is attained. While at 10-• Pa the mean free path is approx. 50 a and consequently molecular beam conditions are given, at 10-I Pa it is only 5 cm, half the usual distance between cell and substrate; in this case occasional gas phase collisions will occur. The exhaust gases may be treated in a similiar way as in KOVPE. However, since the highly toxic hydrides are efficiently cracked in the cell and their consumption is low, the concentration of toxic gases in the exhaust is much lower than in EOVPE. 2.3. Gas inlet
SVStemS
accurate and reproducible control of the fluxes of the starting materials is essential for the deposition of heterostructures; especially if lattice matching has to be achieved. For this reason not only a description of the different gas inlet systems but additionally some regarding accuracy will be given. considerations In a modern sophisticated BOVPE system, pressure balanced run- vent lines /I/ allow fast switching of the reactants into the reactor without pressure or flux transients (fig. 5). This technique ensures sharp interfaces when changing from one composition to the other during growth of heterostructures /16/. The fluxes of the gases having high vapor pressures (Ash, PHJ, Br, diluted dopants) are controlled by electronic mass flow controllers (KFC). Assuming a moderate temperature drift of 1 A in the gas cabinet this gives a relative accuracy of 1% at full scale (f.s.1 and 9%,of the reading at of the HFC/17/. 10% of f.s. This indicates the limited dynamic range of todays HFCswhich in An
M. Weyers
vent run
hydride
fun ver.t
Fig ‘. 5: Schematic
of MOVPEgas supply
system.
the accuracy in growing graded structures. turn limits For the low vapor pressure metalorganics usually hydrogen is used in MOVPEto transport them from their stainless steel container, kept in a thermostated bath, into the reactor. The flux of hy?ro;e:t 1s controlied ir:e;;ective of upstream of this container. To keep the pressure inside the bottle constant, the total pressure inside the reactor, a pressure control circuit consisting of a pressure transducer (usually a capacitance manometer) and a servovalve is used. In this case the flux of metalorganic components is given by Qwo = Qe
where
PV
= A
(1)
Pv/Pb
exp (-B/T
ln101
(2)
is the vapor pressure at the temperature of the container and pb is the pressure inside the bottle. Assuming a temperature accuracy of 0.1 K at 273 K in the thermostated bath, a constant the accuracy of the vapor pressure adjustment is 2 of typically 1500 to 2000 E IN, B in eq. approx. 0.82. A pressure transducer working in the upper decade of its range may add an error of < 0.61 /17/. The largest inaccuracy again is due to the XFC. cracbr
hydride
cell
I
MFC _
=I
Cell
shutter
a)
hydride vent
bl
Fig. 6: Schematic of XOHBEgas supply system b) without gas and IFC a3 with carrier
carrier
gas and with pressure
control.
A comparison
of MOMEE
87
and MOVPE
differing only in Whereas most ROVPE gas manifolds employed exhibit same basic features, detail, in HORSEtwo fundamentally different approaches for flux control are Med. In fact, the ROWBEgas handling systems currently operated are largely homebuilt since equipment suppliers only recently started to deliver commercial systems. The first approach /lo/ (fig. 6a) more or less adopts the method utilized in IIOVPE. For the hydrides an RX operating at reduced pressure (for example, 3000 Pa /19/) on its low pressure side controls the flux into the In the metalorganic line only the position of the HFChas been altered compared cracker cell. to HOVPE from upstream to downstream of the metalorganic container to reduce flux transients /20/. The metalorganics are then introduced into a (multiple) gas cell and directed towards the The beams may be substrate from the same geometric position to ensure homogeneous deposition. interrupted by shutters. Eowever, for his system Tsang observed RHEED oscillations indicating 7) with the HO fluxes turned on and the shutter closed /21/. This persistent growth (fig. behavior indicates the inability of the shutter to completely stop metalorganics from reaching the substrate surface.
3 0
7
SWITCH IN FLUX.
SHUTTER
OPENED.
: iii 3 t I
SWITCH OUT FLUX f
: w
Ts = 600-C
2
I 0
TEGa = 1.15 CClmin. ASH, = 3 CC/min.
CLOSE SHUTTER. FLUX STAYED IN. I
20
I
40
I
60
I
80
I
100
I
120
-I 140
TIME (SEC)
Fig. 7: REEFDoscillations
indicating
persistent
growth with shutter closed /al/.
/9,22/, one In a second approach, used in our laboratory since the early days of HOHRE does not use MFCsand a hydrogen carrier but pressure transducers and direct distillation from the container to keep the hydrogen load to the system and thus the pressure as low as possible (fig. 6b). Fluxes through the gas cells are controlled by adjusting the pressure upstream of an orif ice. Typically, this pressure is 100 to loo0 Pa for the hydrides and 10 to 100 Pa for the metalorganics. RRERD oscillation studies on the effectiveness of shutters in our system have not yet been performed. If necessary, fast switching may be accomplished by adding a run-vent facility to every gas line. Considerations regarding the accuracy and dynamic range of control of the system using KFC are basically the same as in ROVPE. In contrast, the use of pressure transducers allows an accuracy of better than 0.12 at full scale of the instrument and still better than 1% at It of f.s.; further improvement (better than 1% at 0.1% of f.s.) is possible using high precision pressure transducers /23/. At this point it may be remarked that where on first sight HOVPE and RORBE look totally different due to the different types of reactors employed, a closer look at the pumping systems and the gas manifolds shows a number of siniliarities in approach. Still, differences may occur in important details, like in the manner in which the fluxes are controlled. 3. GROWTH KINETICS Growth kinetics in ROVPEare far from totally understood. Nass spectrometric investigations indicate a change in the relative abundance of reaction products like alkylarsines when the pressure is lowered 1241. This points to the fact, that the importance of certain steps or reaction pathways in the growth process is different at different total pressures. Comparison of HOVPE(at different total pressures) to ROBE may provide information whether certain steps like decomposition of the precursors and interactions between different species mainly take place at the growing surface or whether reactions in the gas phase are involved. 3.1. In
Role of AsH3
ROVPEat lOgPa using TIC the growth rate is independent of the V/III ratio and thus of the supply above a value of 4-5 at a substrate temperature of 803 I( (fig. 8) /25,26/. Below this critical value, which may vary with reactor design and deposition parameters, the growth whiskers) are obtained. Similiar observarate drops off and Ga-rich conditions (Ga droplets,
AsHa
M. Weyers
$-
$
1
--l
=
V
58
L6
5;
P 101 = 500
L2-
Pa
= 69 = 783
hc T
3/>
5.8
l-
cmls
Pa K cm/s
$/’
. c
1 2
I L
6
1 8
1 10
1 12
Fig. 8: Dependence of growth rate on V/III ratio growth rate is As83 limited at 500 Pa.
1L 16 P-III ratio
at 10’ and 500 Pa:
tions were made at higher total pressures, where the critical V/III ratio approaches one. At low total pressure (500 Pa) the situation changes and the growth rate shows a dependence on V/III ratio over a much wider range. In this case even layers deposited at V/III = 2 show mirrorlike surfaces and no evidence of Ga-rich conditions is observed /27/. This points to the fact that the importance of certain steps of the growth process strongly depends on the total While at higher pressures gas phase processes like predecomposition of AsEa and THG, pressure. adduct formation between these compounds or homogeneous nucleation are predominant, at lower pressures surface reactions become important. The kinetic limitation of the growth by insufficient arsine decomposition already showing up in HOVPE at 500 Pa is even more pronounced in HOIIDE. Fig. 9a shows the linear dependence of the growth rate of GaAs on the ratio of cracked to untracked AsL and thus of the supply of elemental arsenic at the surface up to a saturation value determined by the TIC flux. The layers stay mirrorlike in this arsenic controlled growth regime, indicating that TIG At the same temperature decomposition is not complete if there is a lack of arsenic. Z
T
z
=zo. c
* &I)
p+ . pw’_* . PM’
7
K
_
T
z
20 .l6’ PO 22 “dP0
_.
32.0 L
*
. (IL.9 K
&, _
* la . 16‘Pa
p,.
s 5L . lo“ Pa
_
* I.L . cd’ PO .
0 10 .
1.0 . /i : /
:
:
.’
.*
,.*
,..’ IO
as Cl)”
2
‘-9
b)
Fig. 9: Dependence of growth rate on ASHI cracking ratio (and thus on arsenic supply) a) using TIC (dashed line: extrapolation to lower rates) b) using TEG (dashed line: breakdown of epitaxial growth).
A comparison
of MOMBE
89
and MOVPE
si;;?iy is decomposition of the thermally less stable TKGtakes place even if the arsoci: Below a certain cracking efficiency Ga droplets and whiskers x?:z~tt the insufficient. 9b) /28/. The above mentioned effects can nor tb ra:sed by breakdown of epitaxial growth (fig. blocking of surface sites by undecomposed AsEe since experiments using an As effur;zn ~11 led to the same observations /29/. The results for both Ga sources show that precrackiag of the hydride is necessary to establish epitaxial growth in MOBBE. There are still some uncertainties on the optimum des:;n of the cracker cell, especially on possible catalytic effects on As83 cracking and the prod-ztioa of the favored diaeric instead of the trtrueric species /30,31,32,33/. However. data published so far show that cracking efficiencies 1 99 S are easily obtained. Together w:th the low consumption of the hydrides (for example 3sccm As& for a GaAs growth rate of lpprox. 1.5 urn/h /21/l this leads to a low concentration of these compounds in the exhaust gases and makes BOHBE reduction of hydride consumption may be achieved in HOVPE less hazardous than MOVPE. Eoweoer, as well by precrackiag it in a plasma discharge /25/. 3.2.
Selective
growth
There is ample evidence that the surface plays an important role in the BOVPEof GaAs. Growth rate and morphology are strongly depoadeat on the orientation of the substrate /3/. By the same token, one expects on substrates partially covered with glassy materials, like SiOz. different It is indeed possible to establish growth rates on the mask (SiOe) and unmasked (GaAs) areas. conditions where growth takes place with complete selectivity on the unmasked substrate only.
10: Selective growth of GaAs a) IOVPE; 10’ Pa; TUG; 890 K b) IOVPE; 500 Pa; TEG; 1070 K cl MOMBE: TEG; 890 K (tip
Fig.
1100 -
10%
zz
900
-
c
800
0%
0%
1
TEG
11: Conditions
urn wide).
100%
1 TMG
1
TMG
overgrowth
=i
TMG MOMBE
Fig.
30% 1 TEG
I-
1000
is 0.8
for
selective
500 growth;
Pa overgrowth
10‘ Pa is given
in 0 of
layer
thickness.
90
M. Weyers
At WPa selective growth is only possible at higher temperatures (fig. lOa) and still a considerable overgrowth of Gals on the mask is observed. At 500 Pa this overgrovth is reduced and selective growth is possible at lower temperatures (fig. lob). Lowering the pressure further to BOKBK conditions totally eliminates overgrowth and makes small structures with an height to width ratio ) 1 possible (fig. 10~). In KOBBE selective growth is obtained over the whole investigated temperature range using THG. For TEGselectivity is only achieved above 890 K. which again points to the fact that the lower stability of this compound facilitetes the decomposition without interaction with As, as discussed above. Fig. 11 summarizes the observed conditions for selective growth. They nay very somewhat with growth rate end V/III however, the trend reaains unaffected /34/. retio, Selective growth of InP her already been demonstrated both in KOVPS/35/ and KOEBE/36/. For this material the orientation dependence of the growth rate is very pronounced which gives rise to nonplenar growth fronts end orientetion dependent overgrowth. These effects make further detailed studies necessary. 3.3. Dependence of growth rate on temperature Fig. 12 shows the Arrhenius plot of the growth rate for the WOVPE of the three Ge compounds TUG, TEGend TIBG (Ga(C~liv)al at 104Pa /26/. TIC shows the well known behaviour: kinetically controlled growth et lower temperetures with en lctivetion enrrgy of t - l.leV, e constent growth rete limited by the supply of Ge to the surface in the diffusion controlled regime and a drop of the growth rete at higher temperatures. This drop is often ascribed to predeporition or nucleetion in the gas phare in the reactor upstream of the substrate, but mey also have other reasons, es will be discussed later on. For the two other compounds the onset of kinetic control is shifted to lower temperetures (for example, by 70 I( for TKG) mirroring the lower theraal stebility of those materials. In both cases the diffurion controlled growth regine is not very pronounced. A simple ectivetion energy in not readily estimated since the slope of the
1100 1000
I
900
800
750
700
T(K) 650
600
550
Ptot = 10‘Pa
g10.
0
oTMG L
5.
hG
= 11,7
P AsHJ
=
V
=
=@pa
126 Pa
=
5,8 cm/s
170 Pa
= 10 cmls
2. f
1.0.
l
+TIBG
‘++
= 1.7 Pa PAsH3= 17 Pa
Pn,
0.5.
V
=
10 cm/s
\
l
I
49
1.0
11
1.2
Fig. 12: Growth rate vs. reciprocal in HOVPE et 10’Pe.
1.3
1.L
1.5
1,6
1,7 1.8 l! T (109K')
temperature using three different
Ga sources
kinetic branch is not constant end different kinetic liaitetions appear to be involved. The growth rate curves in WOKBE (fig. 13) look very similiar to those for BOVPE. Again a kinetic brench end a Ga supply limited region can be distinguished end agein in the cese of TIC the activation energy is epprox. l.leV, while for TEG there is still some controversy on the activation energy: values of 0.65 /31/, 2.1 /lo/ or 1.05 eV /38/ may be obteined from the In the high temperature region (dashed curves) two mechanisms cause a drop of the litereture.
A comparison
1000
of MOMEE
and MOVPE
900
850
91
T IK)
800
g213 alkyl reevap.
5 -;:1.0 0.5t
.
,,
,
--
,
I -0 .o-
c-
, .-
**
TEG
0 o--Q--,,
0
r . _ _ _ - - - _- __ ___)cX
‘X
\ \
0
‘X \
xTMG
0.2
I
I
Fig.
13:
HOHB)E growth
rate
I
I
1.0
vs.
reciprocal
dashed curves indicate
estimates
I
1.2 l.3 l/T (d/K)
1.1
temperature for TRGand TEG; based on published results (/37/,/40/I.
One is the effect of Ga reeoaporation from the substrate which, as is well known Growth rate. The second has a lower activation from classical RRE, has an activation energy of 1.25 eV/39/. and is ascribed to the loss of Ga alkyls (TEGor DEG) from the surface /37,40/. The energy same effect may also contribute to the drop of the rate at high temperatures in ROVPE. These results obtained by measuring the layer thickness /38/ or RREED-oscillation frequencies /IS/ need further detailed studies, for example by modulated beam mass spetronetry /41/, to identify the derorbing species. Rowever, they already contribute new insights to the understanding of the kinetics of possible reactions in XOVPE.
0 TMG x TEG
T = 848
Fig.
14:
Background
doping
K
in IfOHBE using
TEGor TlfG.
92
M. Weyers
3.4.
Carbon
incorporation
If the decomposition of the metallocganics incorporated into the grcwing layer. Fig. 14 yields a high p-type background doping level radicals may reduce this background by an order
/ +
% I % lo’* d 5
5
15:
Carbon
____lj / +
+
doping
PTEG,beom
in MOEBE using
I
Car’son
doping
in
;at-~Al~As
-4 3.6x;O Pa
7.0x 16' Pa
TEG and TIC.
,
3.4 C0mposition.
15:
=
pAsHj+-=
02 Pig.
= 8L8 k
T
I
$ 1o16 ./+ k s
Fig.
l
!
.z 1o17 t E 8 I
does not go to completion, carbon may be shows that under EOBBX conditions the use of TBG due to carbon /28/. Introduction of hydrogen of magnitude /42/ but the impurity level still
*I6
I
0.8
x
in YOy?E using
TEA 2s a dopant
soxce.
A comparison
of MOMBE
and MOVPE
93
is too high for most practical applications. In case of TEC the so called S-eliz:zarion process /43,44/ facilitates the breaking of the Ca-C bond so that the background doprnq zay & rddu:ed In XOVPEhigh V/III ratios usually make a large aaoz~: :f actare to below 10’4 cm-) /45/. hydrogen available and layers with 77 K labilities above lOO.GOO cn*/Vs /46/ can :e FTW:: from TiG. -Bowever, reducing the V/III ratio from 20 to 3 at 500 Pa results in an incrsas; of :he ptype background from below 101’ to above 101’ cm-). Again, with TEG this increased carbon In the GaAs/GaAlAs system carbon is a convenient acceptor dopant uptake is not observed /41/. and the above mentioned effects may be used to advantage if C is introduced in a controlled manner. In HOWBEthis may be achieved by adding TIC to the main Ga source TEG (fig. 15) /48/. In IIOVPE carbon doping of GaAlAs by employing TKAhas been demonstrated; in this case the correlation of the Al and the carbom content in the layers puts some limitations on this approach (fig. 16) /49/. While carbon may be used as a dopant in GaAs its autocompensation in GaInAs makes TIG an unsuitable Ga source for MMBEgrowth of this material /SO/. In general, starting materials and capable of S-elimination seem to be advantgeous in HOmE (and MOVPEat low todal pressures low V/III ratios) for growing undoped films. This comparison of some basic features of the growth process again shows many similarities but also a number of differences between the growth processes in MOVPE and KOHBE. The uhv environment of IfOHBEseems eminently suitable for localized uv-stimulated growth, which has been demonstrated in principle for IIOVPE/51/ and HOMBK/52/. Mile MOHBE will profit of the development of efforts undertaken to improve IIOWK (fabrication of purer source materials, alternatives to the hazardous compounds AsEa /53/ and P&/54/) the understanding of the ffOVPE process in turn will profit of the insights obtained with the surface analytical tools Only combining the knowledge of the surfaae reactions with available in an NOXBEenvironment. the understanding of gas phase processes obtained for example by CARS /55/ or mass spectrometry /44,56/ will reveal the whole picture. 4. KATERIALSAND DEVICE STRUCTURESGROWNBY HOKBE 4.1
III-V
compounds
GaAs , the material first deposited by HOHBE, can be grown from TEG with a p-type background which is low enough for most device applications. A comparison of below lO”cm-s/45/ the electrical properties of IfOWE Gals to NOVPEmaterial is difficult as in KO’fPE usually an ntype background is obtained for the use of both TlfG and TEG. Rowever, the total impurity concentration in high purity HOVPElayers still appears to be somewhat lower than in MOMBE trace grown layers. Low temperature photoluminescence spectra /57/ show, in addition to contamination of the films with other acceptors like Zn and Kg on a level amounts of carbon, exceeding that of C. This indicates that the purity of the layers still is limited by the purity of the sources which requires further improvement. The quality of the surfaces is comparable to those of KOVPEsamples; in both cases oval defects typical for HBE grown layers are absent /58.59,60/. MOMBEdeposition of GaAlAs has up to now not been studied in detail: data on tbe purity of this material are not available. Keeping in mind the considerations on carbon uptake. sources like TEA or DEAR ((CzRa)zAlH) should bi uied ideally in this material system. How&er,.using TEG and TIA a GaAs/Gao. IAlo. sAs 08 laser with Jtt, =SOG A/cm2 could be fabricated from NOnBE layers /19/ and MQUstructures showed high photoluminescence intensity and sharp subband transitions in photoluminescence excitation indicating smooth interfaces and a high purity of the GaAs wells /61/. InP has been deposited both from TM1 /62/ and TEI /63/. In the latter case an electron mobility an excellent purity although not yet totally comparable to of P7’1 = 109.000 cm’/Vs indicates best HOVPEresults (plr= 264000 cmz/Vs /4/l. The ternary GaInAs has been grown from TEG and THI (n = 5x10*acm-J, with a good purity. ~77 = 57.000 cmz/Vs) /64/; the use of TUG in this case has to be ruled out /50/. However, in systems entailing a heterojunction (for example InP, substrate (or buffer layer) - ternary (or quaternary) epilayer) comparison of electrical data is difficult since a a-dimensional electron gas at the interface may contribute to the conduction. Especially at low temperatures data obtained by the van der Pauv method often provide information on the interface quality and not on the properties of the epilayer. In the GaInAs system HQY laser (1.47-l-72 pm) /65/, phototransistors /66/ and MISFETs with high transconductance (gm = 330 M/mm) /67/ were made from HOMBElayers. The quaternary GaInAsP grown from TEG and THI over the entire range of composition showed nwith a carrier concentration ( lOa CIII-~ and mobilities comparable to those type conduction In this system for example a 1.55 urn DA laser with a very low threshold obtained in HOVPE /66/. grading multiplication avalanche current density of Jtb = 1 kA/cm* /69/ and separate absorption photodiodes (SIGH APD) with a gain bandwidth product of 70 CHz /TO/ have been demonstrated.
M. Weyers
94
4.2. Other ratarials II-VI
NORBE ::owth has been demonstrated recently for ZnSe and ZnS starting fro= XZn, DES and I:-VI HOVPEsuffers from the low thermal stability of the deposited compounds (in of the narrow-gap materials) or premature breakdown of the precursors ( in case of the
EZS /ll/. case
this makes low growth temperatures necessary. Bowever, at these low wide-gap cospaunds); of soae precursors is rather low. temperature3 the reactivity Precracking of stable starting naterials nakes lfOB8E growth at low substrate temperature possible in a clean growth system. The apparatus can be baked at high temperatures (in contrast to that for conventional IBE)
since no mercury containing effusion cells limit the bakeout temperature. Since in BOBBE cross contamination of different sources can be avoided, growth of different materials in the same apparatus is possible. This flexibility is of special interest for the deposition of metals and insulator in 3-dimensiooally integrated devices.The capability of HOBBR to deposit insulators has been demonstrated for the growth of AlaOv from THAand NaO on Si substrates /fl/; deporition of metals at lea3t eppears to be possible. 5. SUNMARYANDOUTLOOK Comparison of the growth kinetics shows that HOEBE is the straightforward extension of EOVPEto very low pressures (uhv). Only in EOE8Efull advantage cao be taken of the integration of analytical tools, independent precracking of stable materials or the use of shutters to interrupt the flux. For production applications different factors like required material accuracy, flexibility, quality, safety requireaents and costs influence the choice of the appropriate deposition system. EOVPE,which already is used for fabrication, will be the system of choice for the mass production of thick layers (solar cells, photodiodes) due to the high growth rates attainable in this system, the possibility of scaling up of a flow reactor for use with several wafers sinultaoeously and the lower cost of the growth apparatus. For complicated multilayer devices usually fabricated only in 3mall quantities, not only EOVPEand MONBE but classical BBE as well have to be considered as potential candidates for fabrication purposes. Of these three, EOMBEappears to have some distinct advantages due to its high level of control. This is especially important for the growth of graded structures, where MBE suffers from difficulties in precisely controlling the change of the fluxes froa the effusion cells. The flexibility of an EOEBEsystem allows subsequent growth of different materials from different sources without downtime. Selective growth capability generates new concepts for device production by WOMBE,like the fabrication of waveguide structures without subsequent Safety considerations favor EBEas in this care sources and products are etching steps. confined to- the system. However, due to its lower consumption of toxic hydrides and the neglegible risk of reactor breakage f!OHBEpresents a smaller hazard than NOVPE. Utile it is too early to judge on the prospects of KOEBEfor the CaAs/AlGaAs system due to the limited data for the InP based materials HONBEappears to have a large potential. Because of the mentioned above, it may become the system of choice for the production of these In this case the competitor MBEsuffers froa the difficulty of obtaining a high evaporation of purity elemental phosphorus source and the problems in the controlled this element. Weighing the advantages and disadvrotages of NORBEgives reason for the allotropic
available, advantages materials.
expectation that the step out of the research laboratory into production facility, possibly as is definitely feasible in the not too distant a part of an integrated uhv fabrication line, This is especially the case since recent progress in system design (seven 2” wafers in future. one run /73/j may solve the problem of limited throughput. Acknowledgement The author on
would like to thank R.Beinecke (Siemens Res. Lab., Munich) for BOVPE and MOHBE: he is indebted to A.Brauers and P.Balk for critical
manuscript.
fruitful discussions comments on the
References /l/
H.M. Banasevit,
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/8/ /9/ /lo/ /ll/ /12/ /13/ /14/ /15/ /16/ /17/ /18/ /19/ /20/ /21/ /22/ /23/ /24/ /25/ /26/ /27/ /28/ /29/ /30/ /31/ /32/ /33/ /34/ /35/ /36/ /37/ /38/ /39/ /40/ /41/ /42/ /43/ /44/ /45/ /46/ /47/ /48/ /49/ /50/ 1511 /52/ /53/ /54/ /55/
35
R.D. Dupuis, J.C. Canpbell and J.R.Velebir, J. Crystal Growth 77, 598 (1986) T. Ohori, Ii. Take&i, W. Suzuki, H. Takikawa and J. Koaeno, J. Crystal Growth 91, 905 (1988) E. Veuhoff, Y. Pletschen, P. Balk and B.LQth. J. Crystal Growth 55, 33 (1981) Y.T.Tsang, Appl. Phys. Lott OS, 1234 (1984) Y. Segui, P. Carrere and A. Bui, Thin Solid Films 92, 303 (1982) l4.R. Leys and I!. Veenoliet, J. Crystal Growth 55, 145 (1981) J.C.H. Huang, T.B. Brennan and A.Y. Cho, J. Electrochem. Sot. 1)0, 493 (1983) E.S. Bellman and J.S. Earris,Jr., J. Crystal Growth 8l, 38 (1987) A.Y. Cho, J. Appl. Phys. a, 2780 (1970) 0. Crdtzaacher. K. Uolter. E. Jorgensen, C.U.T. Bulle Lieurma and P. Balk Appl. Phys. Lett. 52, 872 (1988) IKS product information on HFC 1159 and 1259 and on Baratron 122 prtssure transducer 0. Kayser, H. Heinecke, A. Brauers, 8. Lath and P. Balk, Chemtronics 1, 90 (1988) Y.T. Tsang, Appl. Thys. Lett. $3, 511 (1986) V.T. Tsang, private communization H.T. Tsang, T.E. Cbiu, J.E. Cunningham and A. Robertson, Appl. Phys. Lett. 50, 1376 (1987) N. Piitr, E. Veuhoff, II. Heinecke, K. Heyen, B. L8th and P. Balk, J. Vat. Sci. Technol. Bl, 671 (1985) RKS product information on Baratron 127 and 390 S. Eoriguchi, K. Kimura, S. Takagishi, K. Kamon, If. Bashita, B. Bihara and K. I&ii, Jpn. J. Appl. Phys. X, 2002 (1987) A. Brauers, F. Grafahrend, 8. Heinecke, 8. Lilth and P. Balk, in E-IRS Europe Proceedings, Symposium A, Code R-13, 231 (1986) C. Plass, A. Reinecke, 0. Kayser, 8. Liith and P. Balk, J. Crystal Growth 88, 455 (1988) 8. Beinecke, A. Brauers, R. Liith and P. Balk, J. Crystal Growth II, 241 (1986) N. PQtz, II. Heinecke, H. Heyen, P. Balk, B. Weyers and Ii. Lilth, J. Crystal Growth 74, 292 (1986) K. Nagata, Y. Iimura, Y. Aoyagi, S. Namba, S. Den and A. Koritani, J. Crystal Growth 93, 265 (1988) C. Lohe and C.D. Kohl, accepted for publication in J. Vat. Sci. Technol. A.R. Calawa, Appl. Phys. Lett. S, 701 (1981) R. Chow and Y.G. Chai, J. Vat. Sci. Technol. A&, 49 (1983) M.B. Panish and R.A. Xamm, J. Crystal Growth 78, 445 (1986) H. Neinecke, A. Brauers, P. Grafahrend, C. Plass, N. Pfltz, K. Werner, M. Meyers, Ii. Luth and P. Balk, J. Crystal Growth 11, 303 (1986) A.R. Clawson, C.R. Hanson and T.T. Vu, J. Crystal Growth 77, 334 (1986) D.A. Andrews, N.A.Z. Rejman-Greene, B. Wakefield and G.J. Davies, ~ppl. Phys. Lett. 53, 97 (1988) N. Kobayasbi, J.L. Benchimol, F. Alexandre and Y. Gao, Appl. Phys. Lett. 5& 1907 (1987) K. Kimura, S. iioriguchi, K. Kamon, If. Hashita. If. Xihara and If. Ishii, Jpn. J. Appl. Phys; 26, 419 (1987) S. Chika, II. Kato, N. Nakayua and Y. Sano, Jpn. J. Appl. Phys. 25, 1441 (1986) A. Robertson Jr.. T.E. Chiu, H.T. Tsang and J.E. Cunningham, J. Appl. Phys. G, 877 (1988) B.A. Joyce and C.T. Foxon, J. Crystal Growth 3l. 122 (1975) E. Tokumitsu. Y.Kudou, If. Konagai and K. Takahashi, Jpn. J. Appl. Phys. 24, 1189 (1985) D.G. Tuck, in: Comprehensive Organometallic Chemistry, eds. G. Wilkinson, G.A. Stone and E.W. Abel, Pergamon Press, Oxford (1982) K. Yoshida, Ii. Watanabe and F. Uesugi, J. Electrochem. Sot. 132, 677 (1985) T.E. Chiu, U.T. Tsang, E.F. Schubert and E. Agyekum, Appl. Phys. Lett. 5& 1105 (1987) A.C. Jones, G. Yales, P.J. Yright and P.E. Oliver, Chemtronics 2, 83 (1987) A. Brauers, private COmUniCatiOII If. Heinecke, K. Werner, F!. Yeyers, A. LQth and P. Balk, J. Crystal Growth 81, 270 (1987) L.J. Guido, G.S. Jackson, D.C. Hall, W.E. Plano and N. Ralonyak,Jr., Appl. Phys. Lett. 52, 522 (1988) H. Kamp, R. Contini, K. Werner, II. Heinecke, M. Weyers, 8. Lath and P. Balk, accepted for publication in J. Crystal Growth R. Luckerath, Y. Richter, Ii. Lath and P. Balk D. Grundmann, J. Uisser, Inst. Phys. Conf. Ser. No. 91: Chapter 8, 797 (1987) V.H. Donnelly, C-V. Tu. J.C. Beggy, V.R. IfcCrary, H.G. Lamont, T.D. Harris, F.A. Baiocchi and R.C. Farrow, Appl. Phys. Lett. 52, 1065 (1988) R. Kall, A. Reinecke, P. Balk and 8. Hofmann, A. Brauers, 0. Kayser. J. Crystal Growth 93, 7 (1988) R.R. Saxena, J.E. Fouquet, V.K. Sardi and R.L. Moon, Appl. Phys. Lett. 53, 304 (1988) D. Grundmann, A. Hertling and Y. Richter, R. Liickerath, P. Balk, B. Fischer, Chemtronics 2, 199 (1988)
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P.W. Lee, T.R. Omstead, D.R. HcRenna and K.F. Jensen, J. Crystal Growthx. 165 (1987) K. Volter, unpublished C.Y. Chang, Y.K. Su, E.K. Lee, L.G. Chen and !f.P. Houng, J. Crystal Growth 55, 1065 (19811 E. Ishikawa, K. Kondo, S. Sara. A. Tanaka and S. Eiyanizu, J. Crystal Growth 76, 521 (1986) K. Werner, Ii. Reinecke. If. Yeyers, 8. Liith and P. Balk, J. Crystal Growth 8l, 281 (1987) U.T. Tsang and R.C. Miller, Appl. Phys. Lett. 48, 1288 (1986) D.A. Andrews, S-T. Davey. C.G. Tuppen, 8. lfakefield and G.J. Davies, Appl. Phys. Lett. 52, 816 (1988) /63/ Y. Kawaguchi. II. Asahi and 8. Nagai, Inst. Phys. Conf. Ser. No. 79: Chapter 2, 79 (19861 /64/ Y.T. Tsang, A.H. Dayen, T.H. Chiu, J.E. Cunningham, E.F. Schubert, J.A. Ditzenberger, J. Shah, J.L. Zyskind and N. Tabatabaie, Appl. Phys. Lett.49, 170 (1986) /65/ 4.T. Tsang, Appl. Phys. Lett. 49, 1010 (19861 /66/ J.C. Canpbell. W.T. Tsang and G.J. Qua, EDL-8, 171 (19871 /67/ E.F. Schubert, Y.T. Tsang, M.D. Feuer and P.M. Xankiewich, EDL-), 145 (1988) /68/ U.T.Tsang, E.F. Schubert,T.A. Chiu, J.E. Cunningham, E.G. Burkhardt, J.A. Ditzenberger and E. Agyekum, Appl. Phys. Lett. 5& 761 (1987) /69/ U.T. Tsang, Appl. Phys. Lett. SO, 63 (1987) /70/ J.C. Campbell, W.T. Tsang, G.J. Qua and J.E. Bouwers, Appl. Phys.Lett. g. 1454 (1987) /71/ Ii. Ando, A. Taike, M. Konagai and K. Takahashi. J. APD~. Phvs. 62. 1251 (1987) I121 K. Sawada, M. Ishida, T. Nakamura and N. Ohtake, Appi: Phys: Lez; 52, 1672 (I9881 1731 T. Sonoda, U. Ito, K. Segawa, S. Takamiya and S. lfitsui, Jpn. J. Appl. Phys. 21. 337 (1988)
THE AUTHOR
MARKUS
WEYERS
Karkus Weyers received his Diploma in Physics from the Technical University presently he is a graduate student at the Institute of Aachen in 1986; During the past four years he was Semiconductor Electronics, TU Aachen. he is the author of several papers on this in MOMBEresearch: engaged subject, some of them comparing MOEBE and NOVPE results.
014m63sta 8.m + .x) Copyright Q 1989 Pergamon Press plc
Gmwth and Charact. 1999. Vol. 19.PP. 83-96 Printed in Great Erwin. All rights rawwed
MOMBE AND MOVPE-A COMPARISON OF GROWTH TECHNIQUES M.
Weyers
Institute of Semiconductor Electronics, Technical D-5100 Aachen, F.R.G.
University of Aachen,
1. INTRODUCTION Since its inception in 1968 /l/ MOVPE(metal organic vapor phase epitaxy) or NOCVD (metal organic chemical vapor deposition) has undergone a steady development. Operation at reduced pressures /2/ enables better homogeneity of deposited layers /3/ and sophisticated gas manifolds allow fast switching and abrupt changes of composition in the deporited films /J/. Especially for III-V semiconductor layers and layered structures HOVPE now has become a widespread growth technique for a range of electronic and optoelectronic devise structures like RENTS/5/, lasers /6/, photodetectors /7/ or complete integrated circuits /8/. Although it is being successfully used for production of layers, the MOVPE process is only partly understood since the interplay of gas phase and surface reactions makes an investigatian of this growth system difficult. To shed more light on the surface reactions in IIOVPE, in our laboratory a study was started to investigate these processes under uhv conditions /9/. This approach, which we called metalorganic molecular beam epitaxy (HOIIBE) has developed into a powerful growth technique in its own right. It was subsequently named chemical beam epitaxy (CBE) by others using a slightly modified version /lo/. Although originally very closely related to MOVPEit is widely considered as a special form of HBE. The purpose of this paper is to compare deposition eauipment, growth kinetics and the aualitv of the- deposited layers and to di;cuss the sinilia&ties and dissiailiarities of ROW% and is dealinD UOUBE. The focus will be on the III-V materials since nearly all the work on HOHBE with these compounds. Growth kinetics will mainly be discuss;d for Gab which still is the besi investigated III-V material. Although MOHEJE is a relatively new technique its rapid development raises high expectations for the quality of the material that can be obtained by this approach in the near future. It will be argued in this paper, that it has the potential to become a fabrication process for applications where a high level of control is required. 2. DEPOSITION SYSTEM 2.1
Reactors
MOVPEreactors are usually made of an outer quartz tube equipped with an inner liner tube to facilitate the cleaning of the system. The substrate is placed on a graphite aubstrate holder which is externally heated either by radiation from quartz halogen lamps or by rf induction. The latter method is limited to pressures of above lpprox. 1000 Pa since the unintentional formation of an rf plasma may lead to irreproducibilities in the growth behavior at lower total pressures /ll/. The reactor is a cold wall system although extended hot zones upstream and above the ousceptor cannot be totally avoided and may give rise to prodeposition and loss of reactants. Rorizontal /3/ and vertical flow reactors /12/ are both being used, but have a basic feature in common: they exhibit distinct profiles in flow velocity, concentration of the reactants and temperature which are strongly dependent on the reactor geometry used (fig. 1). To achieve homogeneous deposition the conditions over the entire substrate esrentially should be constant. This implies that the gas phase composition should not change siipnificantly over the length of the substrate, especially for the growth of ternary or quaterr+ry layers. This in turn means that most of the starting materials leave the reaction zone unused. In contrast to the somewhat fragile quartzware used in NOVPE, an XOMBE deposition apparatus is a3
84
M. ‘Weyers
reactants
-V
carrier (Hz) quartz tube, (cold wall) Fig.
1: Schematic
of horizontal
EOVPE reactor.
a sturdy stainless steel MBE chamber /13/. It is a real cold wall system since the growth environment is LNz shrouded to reduce the partial pressure of contaminants like CO and other residual gases (fig. 2). For the same reason the systems are equipped with load lock chambers to keep atmospheric contaminants like oxygen or water out of the system (and toxic substances confined to it). The same technique has been adapted to MOVPEreactors in recent years. The substrates are either glued to a molybdenum holder with In, a technique which surely is only tolerable in research and not for production applications, or they are held by some support usually made also from molybdenum for reasons of purity. The substrate is radiatively heated from the backside and aay be rotated to achieve better temperature uniformity and to average over possible inhomogeneities in the flux distribution. As the thermocouple is only radiatively coupled to the backside of the rotating substrate or substrate holder, accurate temperature measurement is difficult to achieve /14/.
\
steel chamber Fig.
2: Schematic
of LNs shrouded
MOBBEgrowth
chamber.
The molecular beams of the different starting materials are impinging at different angles onto the substrate. By tailoring the beam profile such that it is homogeneous over the entire substrate the consunption of the starting materials, which depends on the sticking coefficients of the precursors on the substrate surface, may be kept low. Unintentional pyrolytic predecomposition of reactants like in MOVPEis eliminated since no extended hot zones are The use of mechanical shutters between the cell and the substrate allows present. fast switching by interrupting the molecular beams. The main difference from KOVPC is that MOBBEis a uhv process. It thus allows integration of in situ analytical tools like RHEED which has proved to be a powerful method for characterization of the growth process on a monolayer scale /15/. Bass spectrometry may provide information not only on the composition of the residual gas but also on cracking of starting materials and byproducts desorbing from the growing layer. Eaterials which are not readily available in gaseous form (for example, dopants like Be) may be introduced by evaporation, as in conventional MBE. Transfer to additional chambers of a multichamber system makes the entire range of uhv analytical tools (UPS, AES, SIES etc.1 available to characterize freshly prepared surfaces. It subsequent uhv compatible processing steps also allows (tocussed ion beam implantation, metallisation) without exposing the layer to atmosphere.
A comparison of MOMBEand MOVPE
85
2.2 Pumpiaa systems Although XOVPEat atmospheric pressure is still widespread, the modern approa:h to a:hi~ce good homogeneity in layer composition and thickness is to lower the total pressure. In this manner the the local variation in the composition of the gas phase is reduced by increasing To this end a two stage rotary pump is normally used. To protec: chip pump diffusivity /3/. accumulate in the puapiag oil or reduce (and the environment) from gases that are toxic, sometimes a thermal cracker together with a cold trap is ldde4 betvfen reactor pumping speed, and pump; to treat the exhaust gases of the pump a scrubbing system may be employed (fig. 3).
Fig. 3: Low pressure BOVPEpumping arrangement. Even though the base pressure (without gas flow) of this pumping equipment may be below 0.1 Pa, operating pressures are well above 100 Pa. This means that the mean free path of a molecule between two collisions is less than 0.1 pa and gas phase reactions are readily possible. As MJHBE takes place at uhr conditions a corresponding pump has to be employed. Due to the high of the starting materials and their decomposition products vapor pressure (hydrocarbons and especially hydrogen) ion getter pumps normally used in an BBEsystrn are not satisfactory and other types of high vacuum pumps are necessary. Some laboratories employ oil diffusion pumps with appropriate LEr traps to protect the vacuum: in our case we have good experience over several years with turbomolecular pumps, again with two stage rotary pumps as fore pumps (fig. 41. During the deposition process a cryopuap provides additional pumping capacity especially
turbopump
trap
Fig. 4:.BOBBEpumping system with turbomolecular pump. condenses undecoaposed aetalorganics. With this configuration for Bz, whilst the cryoshroud a base pressure of 10-v Pa (pressure limit of the turbo pump) is achieved. After bakeout of the system the pressure may be further lowered using an additional ion pump, but in our opinion there is no real need to do so. Depending on the pumping speed and on the Ha load from the hydride decomposition or the carrier gas a process pressure of lO_’ to 10-a Pa is attained. While at 10-• Pa the mean free path is approx. 50 a and consequently molecular beam conditions are given, at 10-I Pa it is only 5 cm, half the usual distance between cell and substrate; in this case occasional gas phase collisions will occur. The exhaust gases may be treated in a similiar way as in KOVPE. However, since the highly toxic hydrides are efficiently cracked in the cell and their consumption is low, the concentration of toxic gases in the exhaust is much lower than in EOVPE. 2.3. Gas inlet
SVStemS
accurate and reproducible control of the fluxes of the starting materials is essential for the deposition of heterostructures; especially if lattice matching has to be achieved. For this reason not only a description of the different gas inlet systems but additionally some regarding accuracy will be given. considerations In a modern sophisticated BOVPE system, pressure balanced run- vent lines /I/ allow fast switching of the reactants into the reactor without pressure or flux transients (fig. 5). This technique ensures sharp interfaces when changing from one composition to the other during growth of heterostructures /16/. The fluxes of the gases having high vapor pressures (Ash, PHJ, Br, diluted dopants) are controlled by electronic mass flow controllers (KFC). Assuming a moderate temperature drift of 1 A in the gas cabinet this gives a relative accuracy of 1% at full scale (f.s.1 and 9%,of the reading at of the HFC/17/. 10% of f.s. This indicates the limited dynamic range of todays HFCswhich in An
M. Weyers
vent run
hydride
fun ver.t
Fig ‘. 5: Schematic
of MOVPEgas supply
system.
the accuracy in growing graded structures. turn limits For the low vapor pressure metalorganics usually hydrogen is used in MOVPEto transport them from their stainless steel container, kept in a thermostated bath, into the reactor. The flux of hy?ro;e:t 1s controlied ir:e;;ective of upstream of this container. To keep the pressure inside the bottle constant, the total pressure inside the reactor, a pressure control circuit consisting of a pressure transducer (usually a capacitance manometer) and a servovalve is used. In this case the flux of metalorganic components is given by Qwo = Qe
where
PV
= A
(1)
Pv/Pb
exp (-B/T
ln101
(2)
is the vapor pressure at the temperature of the container and pb is the pressure inside the bottle. Assuming a temperature accuracy of 0.1 K at 273 K in the thermostated bath, a constant the accuracy of the vapor pressure adjustment is 2 of typically 1500 to 2000 E IN, B in eq. approx. 0.82. A pressure transducer working in the upper decade of its range may add an error of < 0.61 /17/. The largest inaccuracy again is due to the XFC. cracbr
hydride
cell
I
MFC _
=I
Cell
shutter
a)
hydride vent
bl
Fig. 6: Schematic of XOHBEgas supply system b) without gas and IFC a3 with carrier
carrier
gas and with pressure
control.
A comparison
of MOMEE
87
and MOVPE
differing only in Whereas most ROVPE gas manifolds employed exhibit same basic features, detail, in HORSEtwo fundamentally different approaches for flux control are Med. In fact, the ROWBEgas handling systems currently operated are largely homebuilt since equipment suppliers only recently started to deliver commercial systems. The first approach /lo/ (fig. 6a) more or less adopts the method utilized in IIOVPE. For the hydrides an RX operating at reduced pressure (for example, 3000 Pa /19/) on its low pressure side controls the flux into the In the metalorganic line only the position of the HFChas been altered compared cracker cell. to HOVPE from upstream to downstream of the metalorganic container to reduce flux transients /20/. The metalorganics are then introduced into a (multiple) gas cell and directed towards the The beams may be substrate from the same geometric position to ensure homogeneous deposition. interrupted by shutters. Eowever, for his system Tsang observed RHEED oscillations indicating 7) with the HO fluxes turned on and the shutter closed /21/. This persistent growth (fig. behavior indicates the inability of the shutter to completely stop metalorganics from reaching the substrate surface.
3 0
7
SWITCH IN FLUX.
SHUTTER
OPENED.
: iii 3 t I
SWITCH OUT FLUX f
: w
Ts = 600-C
2
I 0
TEGa = 1.15 CClmin. ASH, = 3 CC/min.
CLOSE SHUTTER. FLUX STAYED IN. I
20
I
40
I
60
I
80
I
100
I
120
-I 140
TIME (SEC)
Fig. 7: REEFDoscillations
indicating
persistent
growth with shutter closed /al/.
/9,22/, one In a second approach, used in our laboratory since the early days of HOHRE does not use MFCsand a hydrogen carrier but pressure transducers and direct distillation from the container to keep the hydrogen load to the system and thus the pressure as low as possible (fig. 6b). Fluxes through the gas cells are controlled by adjusting the pressure upstream of an orif ice. Typically, this pressure is 100 to loo0 Pa for the hydrides and 10 to 100 Pa for the metalorganics. RRERD oscillation studies on the effectiveness of shutters in our system have not yet been performed. If necessary, fast switching may be accomplished by adding a run-vent facility to every gas line. Considerations regarding the accuracy and dynamic range of control of the system using KFC are basically the same as in ROVPE. In contrast, the use of pressure transducers allows an accuracy of better than 0.12 at full scale of the instrument and still better than 1% at It of f.s.; further improvement (better than 1% at 0.1% of f.s.) is possible using high precision pressure transducers /23/. At this point it may be remarked that where on first sight HOVPE and RORBE look totally different due to the different types of reactors employed, a closer look at the pumping systems and the gas manifolds shows a number of siniliarities in approach. Still, differences may occur in important details, like in the manner in which the fluxes are controlled. 3. GROWTH KINETICS Growth kinetics in ROVPEare far from totally understood. Nass spectrometric investigations indicate a change in the relative abundance of reaction products like alkylarsines when the pressure is lowered 1241. This points to the fact, that the importance of certain steps or reaction pathways in the growth process is different at different total pressures. Comparison of HOVPE(at different total pressures) to ROBE may provide information whether certain steps like decomposition of the precursors and interactions between different species mainly take place at the growing surface or whether reactions in the gas phase are involved. 3.1. In
Role of AsH3
ROVPEat lOgPa using TIC the growth rate is independent of the V/III ratio and thus of the supply above a value of 4-5 at a substrate temperature of 803 I( (fig. 8) /25,26/. Below this critical value, which may vary with reactor design and deposition parameters, the growth whiskers) are obtained. Similiar observarate drops off and Ga-rich conditions (Ga droplets,
AsHa
M. Weyers
$-
$
1
--l
=
V
58
L6
5;
P 101 = 500
L2-
Pa
= 69 = 783
hc T
3/>
5.8
l-
cmls
Pa K cm/s
$/’
. c
1 2
I L
6
1 8
1 10
1 12
Fig. 8: Dependence of growth rate on V/III ratio growth rate is As83 limited at 500 Pa.
1L 16 P-III ratio
at 10’ and 500 Pa:
tions were made at higher total pressures, where the critical V/III ratio approaches one. At low total pressure (500 Pa) the situation changes and the growth rate shows a dependence on V/III ratio over a much wider range. In this case even layers deposited at V/III = 2 show mirrorlike surfaces and no evidence of Ga-rich conditions is observed /27/. This points to the fact that the importance of certain steps of the growth process strongly depends on the total While at higher pressures gas phase processes like predecomposition of AsEa and THG, pressure. adduct formation between these compounds or homogeneous nucleation are predominant, at lower pressures surface reactions become important. The kinetic limitation of the growth by insufficient arsine decomposition already showing up in HOVPE at 500 Pa is even more pronounced in HOIIDE. Fig. 9a shows the linear dependence of the growth rate of GaAs on the ratio of cracked to untracked AsL and thus of the supply of elemental arsenic at the surface up to a saturation value determined by the TIC flux. The layers stay mirrorlike in this arsenic controlled growth regime, indicating that TIG At the same temperature decomposition is not complete if there is a lack of arsenic. Z
T
z
=zo. c
* &I)
p+ . pw’_* . PM’
7
K
_
T
z
20 .l6’ PO 22 “dP0
_.
32.0 L
*
. (IL.9 K
&, _
* la . 16‘Pa
p,.
s 5L . lo“ Pa
_
* I.L . cd’ PO .
0 10 .
1.0 . /i : /
:
:
.’
.*
,.*
,..’ IO
as Cl)”
2
‘-9
b)
Fig. 9: Dependence of growth rate on ASHI cracking ratio (and thus on arsenic supply) a) using TIC (dashed line: extrapolation to lower rates) b) using TEG (dashed line: breakdown of epitaxial growth).
A comparison
of MOMBE
89
and MOVPE
si;;?iy is decomposition of the thermally less stable TKGtakes place even if the arsoci: Below a certain cracking efficiency Ga droplets and whiskers x?:z~tt the insufficient. 9b) /28/. The above mentioned effects can nor tb ra:sed by breakdown of epitaxial growth (fig. blocking of surface sites by undecomposed AsEe since experiments using an As effur;zn ~11 led to the same observations /29/. The results for both Ga sources show that precrackiag of the hydride is necessary to establish epitaxial growth in MOBBE. There are still some uncertainties on the optimum des:;n of the cracker cell, especially on possible catalytic effects on As83 cracking and the prod-ztioa of the favored diaeric instead of the trtrueric species /30,31,32,33/. However. data published so far show that cracking efficiencies 1 99 S are easily obtained. Together w:th the low consumption of the hydrides (for example 3sccm As& for a GaAs growth rate of lpprox. 1.5 urn/h /21/l this leads to a low concentration of these compounds in the exhaust gases and makes BOHBE reduction of hydride consumption may be achieved in HOVPE less hazardous than MOVPE. Eoweoer, as well by precrackiag it in a plasma discharge /25/. 3.2.
Selective
growth
There is ample evidence that the surface plays an important role in the BOVPEof GaAs. Growth rate and morphology are strongly depoadeat on the orientation of the substrate /3/. By the same token, one expects on substrates partially covered with glassy materials, like SiOz. different It is indeed possible to establish growth rates on the mask (SiOe) and unmasked (GaAs) areas. conditions where growth takes place with complete selectivity on the unmasked substrate only.
10: Selective growth of GaAs a) IOVPE; 10’ Pa; TUG; 890 K b) IOVPE; 500 Pa; TEG; 1070 K cl MOMBE: TEG; 890 K (tip
Fig.
1100 -
10%
zz
900
-
c
800
0%
0%
1
TEG
11: Conditions
urn wide).
100%
1 TMG
1
TMG
overgrowth
=i
TMG MOMBE
Fig.
30% 1 TEG
I-
1000
is 0.8
for
selective
500 growth;
Pa overgrowth
10‘ Pa is given
in 0 of
layer
thickness.
90
M. Weyers
At WPa selective growth is only possible at higher temperatures (fig. lOa) and still a considerable overgrowth of Gals on the mask is observed. At 500 Pa this overgrovth is reduced and selective growth is possible at lower temperatures (fig. lob). Lowering the pressure further to BOKBK conditions totally eliminates overgrowth and makes small structures with an height to width ratio ) 1 possible (fig. 10~). In KOBBE selective growth is obtained over the whole investigated temperature range using THG. For TEGselectivity is only achieved above 890 K. which again points to the fact that the lower stability of this compound facilitetes the decomposition without interaction with As, as discussed above. Fig. 11 summarizes the observed conditions for selective growth. They nay very somewhat with growth rate end V/III however, the trend reaains unaffected /34/. retio, Selective growth of InP her already been demonstrated both in KOVPS/35/ and KOEBE/36/. For this material the orientation dependence of the growth rate is very pronounced which gives rise to nonplenar growth fronts end orientetion dependent overgrowth. These effects make further detailed studies necessary. 3.3. Dependence of growth rate on temperature Fig. 12 shows the Arrhenius plot of the growth rate for the WOVPE of the three Ge compounds TUG, TEGend TIBG (Ga(C~liv)al at 104Pa /26/. TIC shows the well known behaviour: kinetically controlled growth et lower temperetures with en lctivetion enrrgy of t - l.leV, e constent growth rete limited by the supply of Ge to the surface in the diffusion controlled regime and a drop of the growth rete at higher temperatures. This drop is often ascribed to predeporition or nucleetion in the gas phare in the reactor upstream of the substrate, but mey also have other reasons, es will be discussed later on. For the two other compounds the onset of kinetic control is shifted to lower temperetures (for example, by 70 I( for TKG) mirroring the lower theraal stebility of those materials. In both cases the diffurion controlled growth regine is not very pronounced. A simple ectivetion energy in not readily estimated since the slope of the
1100 1000
I
900
800
750
700
T(K) 650
600
550
Ptot = 10‘Pa
g10.
0
oTMG L
5.
hG
= 11,7
P AsHJ
=
V
=
=@pa
126 Pa
=
5,8 cm/s
170 Pa
= 10 cmls
2. f
1.0.
l
+TIBG
‘++
= 1.7 Pa PAsH3= 17 Pa
Pn,
0.5.
V
=
10 cm/s
\
l
I
49
1.0
11
1.2
Fig. 12: Growth rate vs. reciprocal in HOVPE et 10’Pe.
1.3
1.L
1.5
1,6
1,7 1.8 l! T (109K')
temperature using three different
Ga sources
kinetic branch is not constant end different kinetic liaitetions appear to be involved. The growth rate curves in WOKBE (fig. 13) look very similiar to those for BOVPE. Again a kinetic brench end a Ga supply limited region can be distinguished end agein in the cese of TIC the activation energy is epprox. l.leV, while for TEG there is still some controversy on the activation energy: values of 0.65 /31/, 2.1 /lo/ or 1.05 eV /38/ may be obteined from the In the high temperature region (dashed curves) two mechanisms cause a drop of the litereture.
A comparison
1000
of MOMEE
and MOVPE
900
850
91
T IK)
800
g213 alkyl reevap.
5 -;:1.0 0.5t
.
,,
,
--
,
I -0 .o-
c-
, .-
**
TEG
0 o--Q--,,
0
r . _ _ _ - - - _- __ ___)cX
‘X
\ \
0
‘X \
xTMG
0.2
I
I
Fig.
13:
HOHB)E growth
rate
I
I
1.0
vs.
reciprocal
dashed curves indicate
estimates
I
1.2 l.3 l/T (d/K)
1.1
temperature for TRGand TEG; based on published results (/37/,/40/I.
One is the effect of Ga reeoaporation from the substrate which, as is well known Growth rate. The second has a lower activation from classical RRE, has an activation energy of 1.25 eV/39/. and is ascribed to the loss of Ga alkyls (TEGor DEG) from the surface /37,40/. The energy same effect may also contribute to the drop of the rate at high temperatures in ROVPE. These results obtained by measuring the layer thickness /38/ or RREED-oscillation frequencies /IS/ need further detailed studies, for example by modulated beam mass spetronetry /41/, to identify the derorbing species. Rowever, they already contribute new insights to the understanding of the kinetics of possible reactions in XOVPE.
0 TMG x TEG
T = 848
Fig.
14:
Background
doping
K
in IfOHBE using
TEGor TlfG.
92
M. Weyers
3.4.
Carbon
incorporation
If the decomposition of the metallocganics incorporated into the grcwing layer. Fig. 14 yields a high p-type background doping level radicals may reduce this background by an order
/ +
% I % lo’* d 5
5
15:
Carbon
____lj / +
+
doping
PTEG,beom
in MOEBE using
I
Car’son
doping
in
;at-~Al~As
-4 3.6x;O Pa
7.0x 16' Pa
TEG and TIC.
,
3.4 C0mposition.
15:
=
pAsHj+-=
02 Pig.
= 8L8 k
T
I
$ 1o16 ./+ k s
Fig.
l
!
.z 1o17 t E 8 I
does not go to completion, carbon may be shows that under EOBBX conditions the use of TBG due to carbon /28/. Introduction of hydrogen of magnitude /42/ but the impurity level still
*I6
I
0.8
x
in YOy?E using
TEA 2s a dopant
soxce.
A comparison
of MOMBE
and MOVPE
93
is too high for most practical applications. In case of TEC the so called S-eliz:zarion process /43,44/ facilitates the breaking of the Ca-C bond so that the background doprnq zay & rddu:ed In XOVPEhigh V/III ratios usually make a large aaoz~: :f actare to below 10’4 cm-) /45/. hydrogen available and layers with 77 K labilities above lOO.GOO cn*/Vs /46/ can :e FTW:: from TiG. -Bowever, reducing the V/III ratio from 20 to 3 at 500 Pa results in an incrsas; of :he ptype background from below 101’ to above 101’ cm-). Again, with TEG this increased carbon In the GaAs/GaAlAs system carbon is a convenient acceptor dopant uptake is not observed /41/. and the above mentioned effects may be used to advantage if C is introduced in a controlled manner. In HOWBEthis may be achieved by adding TIC to the main Ga source TEG (fig. 15) /48/. In IIOVPE carbon doping of GaAlAs by employing TKAhas been demonstrated; in this case the correlation of the Al and the carbom content in the layers puts some limitations on this approach (fig. 16) /49/. While carbon may be used as a dopant in GaAs its autocompensation in GaInAs makes TIG an unsuitable Ga source for MMBEgrowth of this material /SO/. In general, starting materials and capable of S-elimination seem to be advantgeous in HOmE (and MOVPEat low todal pressures low V/III ratios) for growing undoped films. This comparison of some basic features of the growth process again shows many similarities but also a number of differences between the growth processes in MOVPE and KOHBE. The uhv environment of IfOHBEseems eminently suitable for localized uv-stimulated growth, which has been demonstrated in principle for IIOVPE/51/ and HOMBK/52/. Mile MOHBE will profit of the development of efforts undertaken to improve IIOWK (fabrication of purer source materials, alternatives to the hazardous compounds AsEa /53/ and P&/54/) the understanding of the ffOVPE process in turn will profit of the insights obtained with the surface analytical tools Only combining the knowledge of the surfaae reactions with available in an NOXBEenvironment. the understanding of gas phase processes obtained for example by CARS /55/ or mass spectrometry /44,56/ will reveal the whole picture. 4. KATERIALSAND DEVICE STRUCTURESGROWNBY HOKBE 4.1
III-V
compounds
GaAs , the material first deposited by HOHBE, can be grown from TEG with a p-type background which is low enough for most device applications. A comparison of below lO”cm-s/45/ the electrical properties of IfOWE Gals to NOVPEmaterial is difficult as in KO’fPE usually an ntype background is obtained for the use of both TlfG and TEG. Rowever, the total impurity concentration in high purity HOVPElayers still appears to be somewhat lower than in MOMBE trace grown layers. Low temperature photoluminescence spectra /57/ show, in addition to contamination of the films with other acceptors like Zn and Kg on a level amounts of carbon, exceeding that of C. This indicates that the purity of the layers still is limited by the purity of the sources which requires further improvement. The quality of the surfaces is comparable to those of KOVPEsamples; in both cases oval defects typical for HBE grown layers are absent /58.59,60/. MOMBEdeposition of GaAlAs has up to now not been studied in detail: data on tbe purity of this material are not available. Keeping in mind the considerations on carbon uptake. sources like TEA or DEAR ((CzRa)zAlH) should bi uied ideally in this material system. How&er,.using TEG and TIA a GaAs/Gao. IAlo. sAs 08 laser with Jtt, =SOG A/cm2 could be fabricated from NOnBE layers /19/ and MQUstructures showed high photoluminescence intensity and sharp subband transitions in photoluminescence excitation indicating smooth interfaces and a high purity of the GaAs wells /61/. InP has been deposited both from TM1 /62/ and TEI /63/. In the latter case an electron mobility an excellent purity although not yet totally comparable to of P7’1 = 109.000 cm’/Vs indicates best HOVPEresults (plr= 264000 cmz/Vs /4/l. The ternary GaInAs has been grown from TEG and THI (n = 5x10*acm-J, with a good purity. ~77 = 57.000 cmz/Vs) /64/; the use of TUG in this case has to be ruled out /50/. However, in systems entailing a heterojunction (for example InP, substrate (or buffer layer) - ternary (or quaternary) epilayer) comparison of electrical data is difficult since a a-dimensional electron gas at the interface may contribute to the conduction. Especially at low temperatures data obtained by the van der Pauv method often provide information on the interface quality and not on the properties of the epilayer. In the GaInAs system HQY laser (1.47-l-72 pm) /65/, phototransistors /66/ and MISFETs with high transconductance (gm = 330 M/mm) /67/ were made from HOMBElayers. The quaternary GaInAsP grown from TEG and THI over the entire range of composition showed nwith a carrier concentration ( lOa CIII-~ and mobilities comparable to those type conduction In this system for example a 1.55 urn DA laser with a very low threshold obtained in HOVPE /66/. grading multiplication avalanche current density of Jtb = 1 kA/cm* /69/ and separate absorption photodiodes (SIGH APD) with a gain bandwidth product of 70 CHz /TO/ have been demonstrated.
M. Weyers
94
4.2. Other ratarials II-VI
NORBE ::owth has been demonstrated recently for ZnSe and ZnS starting fro= XZn, DES and I:-VI HOVPEsuffers from the low thermal stability of the deposited compounds (in of the narrow-gap materials) or premature breakdown of the precursors ( in case of the
EZS /ll/. case
this makes low growth temperatures necessary. Bowever, at these low wide-gap cospaunds); of soae precursors is rather low. temperature3 the reactivity Precracking of stable starting naterials nakes lfOB8E growth at low substrate temperature possible in a clean growth system. The apparatus can be baked at high temperatures (in contrast to that for conventional IBE)
since no mercury containing effusion cells limit the bakeout temperature. Since in BOBBE cross contamination of different sources can be avoided, growth of different materials in the same apparatus is possible. This flexibility is of special interest for the deposition of metals and insulator in 3-dimensiooally integrated devices.The capability of HOBBR to deposit insulators has been demonstrated for the growth of AlaOv from THAand NaO on Si substrates /fl/; deporition of metals at lea3t eppears to be possible. 5. SUNMARYANDOUTLOOK Comparison of the growth kinetics shows that HOEBE is the straightforward extension of EOVPEto very low pressures (uhv). Only in EOE8Efull advantage cao be taken of the integration of analytical tools, independent precracking of stable materials or the use of shutters to interrupt the flux. For production applications different factors like required material accuracy, flexibility, quality, safety requireaents and costs influence the choice of the appropriate deposition system. EOVPE,which already is used for fabrication, will be the system of choice for the mass production of thick layers (solar cells, photodiodes) due to the high growth rates attainable in this system, the possibility of scaling up of a flow reactor for use with several wafers sinultaoeously and the lower cost of the growth apparatus. For complicated multilayer devices usually fabricated only in 3mall quantities, not only EOVPEand MONBE but classical BBE as well have to be considered as potential candidates for fabrication purposes. Of these three, EOMBEappears to have some distinct advantages due to its high level of control. This is especially important for the growth of graded structures, where MBE suffers from difficulties in precisely controlling the change of the fluxes froa the effusion cells. The flexibility of an EOEBEsystem allows subsequent growth of different materials from different sources without downtime. Selective growth capability generates new concepts for device production by WOMBE,like the fabrication of waveguide structures without subsequent Safety considerations favor EBEas in this care sources and products are etching steps. confined to- the system. However, due to its lower consumption of toxic hydrides and the neglegible risk of reactor breakage f!OHBEpresents a smaller hazard than NOVPE. Utile it is too early to judge on the prospects of KOEBEfor the CaAs/AlGaAs system due to the limited data for the InP based materials HONBEappears to have a large potential. Because of the mentioned above, it may become the system of choice for the production of these In this case the competitor MBEsuffers froa the difficulty of obtaining a high evaporation of purity elemental phosphorus source and the problems in the controlled this element. Weighing the advantages and disadvrotages of NORBEgives reason for the allotropic
available, advantages materials.
expectation that the step out of the research laboratory into production facility, possibly as is definitely feasible in the not too distant a part of an integrated uhv fabrication line, This is especially the case since recent progress in system design (seven 2” wafers in future. one run /73/j may solve the problem of limited throughput. Acknowledgement The author on
would like to thank R.Beinecke (Siemens Res. Lab., Munich) for BOVPE and MOHBE: he is indebted to A.Brauers and P.Balk for critical
manuscript.
fruitful discussions comments on the
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THE AUTHOR
MARKUS
WEYERS
Karkus Weyers received his Diploma in Physics from the Technical University presently he is a graduate student at the Institute of Aachen in 1986; During the past four years he was Semiconductor Electronics, TU Aachen. he is the author of several papers on this in MOMBEresearch: engaged subject, some of them comparing MOEBE and NOVPE results.