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Halogen VPE of AlGaAs for optoelectronic device applications
Journal of Crystal Growth 82 (1987) 628-638 North-Holland, Amsterdam
628
HALOGEN VPE OF AIGaAs FOR OPTOELECfRONIC DEVICE APPLICAnONS M. DESCHLER, M. COPPERS, A. BRAUERS, M. HEYEN and P. BALK Institute c/ Semiconductor Electronics, Technical Uniuersity Aachen, Sommerfeldstrasse, D·5JOO Aachen, Fed Rep.
0/ Germany
Received 6 August 1986; manuscript received in final form 3 November 1986
We report on the growth of the ternary Al x Gaj ; x As in a specially designed halide transport reactor. High quality layers with background doping levels in the 1016 cm - 3 range have been achieved. Furthermore. the small FWHM (for example. 4 meV measured on an Ala l,GaO.83As film) obtained from low temperature photoluminescence demonstrates the excellent optical quality of the rilms. By varying the growth temperature between 700 and 800°C, the AI concentration of the layers can be changed between < 1 and 70% whereas the AsH], HO and HCI-bypass partial pressures have hardly any effect on the composition of the films. These results are i~ accordance with a thermodynamic model which takes into account a diffusion step from the bulk gas phase to the substrate surface. The doping behaviour with DEZn and H 2S as sources for n and p-type doping respectively leads to similar results as have been reported for MOCVD, MBE or LPE layers. The fabrication of a light emitting d iode with satisfactory efficiency demonstrates the suitability of this method for growing device quality films.
1. Introduction The ternary semiconductor AIGaAs is one of the most important materials for the fabrication of microwave or optoelectronic devices. This material is commonly grown by liquid phase epitaxy (LPE) , metalorganic vapour phase epitaxy (MOCVD) or molecular beam epitaxy (MBE) [1-3]. It is also possible to deposit epitaxial films of this ternary in a chlorine transport system, as was reported in earlier publications from our laboratory by Bachem and Heyen [4,5]. Using a Ga-10% AI source, these authors were able to grow AIGaAs films with Al concentrations up to 40% at temperatures between 700 and 800°C. However, the deposition of high quality material was not achieved in their work since the highly corrosive nature of the reactants, especially the Al chlorides, at the typical growth temperatures lead to high unintentional doping of the films. In this paper we will describe the deposition of device quality AIGaAs films in a reactor specifically designed to avoid this contamination problem. Data will be presented on the effect of the deposition parameters on growth rate and film composition. We will also present an expanded
modelling treatment of these data including the diffusion of the reactants towards the substrate surface. We will show that the electrical and opu, cal characteristics of the material obtained in this study are comparable to those obtained by other growth techniques. The fabrication of a light emi tting diode will be used as an example of the viability of the method for growing device grade films of controlled doping.
2. Model considerations The deposition of AlxGa1_xAs on a GaAs substrate is governed by the following equilibriulll equations: I
GaCi + AIel +
l I
4
1
K1
AS4 +! H 2 ~ GaAs + HCI, 1
K2
AS4 + ! H 2 ~ AlAs + HCI,
(1)
(2)
The chlorides are provided by reaction of Hel with the Ga-AI source.
0022-0248/87/$03.50 e Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
629
M. Deschler et al. / Halogen VPE of AIGaAsfor optoelectronic device applications
The growth kinetics will be described for the case of a small deviation from equilibrium. At this condition the activities of GaAs and AlAs in the gas phase near the substrate surface are close to their values in the solid phase. Assuming that AlGaAs behaves as an ideal solution of AlAs in GaAs [6], the mole fraction x of Al equals the activity aAJAs of AlAs in the solid. Similarly, a G aAs equals (1 - x). The concentrations of the species in eqs. (1)-(3) near the substrate surface are determined by their partial pressures in the bulk of the gas phase and by the mass transport occurring between the bulk and the substrate surface. The diffusion steps considered in the model are schematically presented in fig. 1. P; and p;D stand for the partial pressures of the species i in bulk of the gas phase and immediately above the substrate surface, respectively. .1/ describes the diffusion flux of gaseous species from the bulk of the gas phase towards the substrate, .1;- the diffusion flux from the substrate to the gas phase. As proposed by Korec and Heyen [7], at constant temperature in the gas phase the net flux of each species i can be described as follow:
(4) where k d . ; is the mass transfer coefficient for species i. Utilizing the principle of mass conservation for chlorine, gallium, aluminium and arsenic, we obtain expressions involving the rates of deposition of the binaries GaAs (rGaAs) and AlAs (rAlAs): k d •Ha (P Ha -
plla )
+kd.GaCI(PGaCI-
+ kd,AlCI (PAlCI -
PgCl )
plb)
+ 3k d,AIC13 ( pAla 3 kd,Gaa( PGaa -
p.BCI 3 ) = 0,
Pga ) = rG aAs ,
(5) (6)
kd,Ala( PAlC l- P.Ba)
= rAJAs,
(7)
p~J = rG aAs + rAJAs·
(8)
+kd,Ala 3(PAla 3- P.BaJ 4k d
,As.(PA s4 -
The growth rate 7' of the ternary AIGaAs may be expressed as the sum of the deposition rates of the
,.
bulk gas stream
Ji
I
P,
1 1J;
substrate
I
pD I
Fig. 1. Diffusion fluxes and partial pressures of component i in deposition system.
binary components:
(9) The ratio of the deposition rate of the AlAs component to the total growth rate determines the Al concentration of the film: (10)
It should be mentioned that the mass transfer coefficients kd" are free parameters in this approach; they are determined by fitting calculated curves against experimental data. Utilizing the method proposed by Riedl [8], the number of free parameters can be reduced to one by expressing the mass transfer in the gas phase in terms of fluxes normalized to one of them (here of HCl):
b; = kd,;/kd,Ha'
(n)
b, can be calculated from the ratio of the diffusion constants for the different species so that k d.HCI remains the only free parameter [9,10]. From eqs. (6) to (8) it follows that the value of k d.Ha , which depends on reactor geometry, gas velocity and total gas pressure, enters in the expression for the growth rate as a proportionality factor. However, the Al concentration x in the film, which is expressed as a ratio of deposition and growth rates (d. eq. (10», is independent of this parameter. Results of computer calculations based on this model for kd,Ha = 10 mm h- 1 bar "! will be compared with experimental results in section 4. 3. Reactor design As mentioned before, an important consideration in the design of the reactor is the corrosivity
M. Deschler et al. / Halogen VP E of A IGaAs for optoelectronic device applications
630
of the Al species which leads to high impurity incorporation in the grown film. Background doping levels lower than 1018 ern- 3 could not be reached in the growth apparatus proposed by Bachem and Heyen [5] where a quartz substrate holder was used and where the substrate zone was not shielded from the quartz walls of the reactor. In the newly designed .apparatus, the deposition is performed in a resistance heated reactor where all parts coming into contact with the reactants at high temperature are either made from graphite or are protected by a graphite liner. Fig. 2 shows a schematic diagram of the growth apparatus. The vertical arrangement permits bubbling of HCl/H 2 through the Ga-10%AI source, thus avoiding problems with the inevitable aluminium oxide crust over the source. A bubble path length of about 5
HCl .H 2 HC1by· H2
H2 I
-
11- 'l~
I
' rI
I~
~ I r;(
dip tube
.- cover
t-- crucible AI/Go alloy
./
! I
i
rr .I- :~
I~
~
quartz tube
-----
liner tube
nozzle substrate
:~
em was found to be sufficient for a complete source reaction. The source composition (10% A.I) was choosen to allow growth of AIGaAs with A.I concentrations over a large range at practical tem_ peratures. At this low Al concentration in the source, a solidification of the alloy is also avoided. Deposition on the reactor walls was minimized by introducing the arsine into the reaction zone immediately above the substrate through the sub_ strate holder. In the same way, H 2S used as n-tyPe dopant was injected into the mixing zone. Diethyj., zinc (DEZn), used for growing p-type films, was carried into the reactor at a high velocity (133 cmys) by means of a narrow capillary to aVoid thermal cracking of the metalorganic compound at the transition to the hot zone. After each epi_ taxial run, HO was introduced into the reactor through the substrate zone and via the bypass line (HO by ) to clean the apparatus by means of etch, ing. Injection of HCl by can also be used for shift_ ing the activity in the gas phase towards lOwer values during deposition. Typically, 150 mf/min of HCl/H 2 were bUb_ bled through the source, 150 mlyrnin of H 2 Were injected through the bypass and 450 mlyrnin of H 2-H 2S-DEZn-AsH 3 were introduced through the substrate holder. The flow rates of all the gases introduced into the reactor were controlled by mass flow meters and switched with electropneu_ matic valves. The GaAs substrates were oriented 2° off the (100) direction towards the (110) direction. The thickness of the epitaxially groWn films was measured optically upon staining. Car_ rier concentrations and mobilities were obtained from Hall measurements. The Al mole fraction was determined from photoluminescence and Xray double crystal diffractometry measurements.
substrate holder
I
quartz capillary
I
4. Results and discussion
I
I I
4.1. Characterization of the deposition system
I
~
~
L-
to exhaust to exhaust
AsH3• Hel • H2S• H2 DEZn· H2
Fig. 2. Schematic diagram of epitaxial reactor.
The Al concentration in the deposited layer was found to depend strongly on the process tempera_ ture. As shown in fig. 3a, by varying source and deposition temperature between 700 to 800° C AlxGa 1 _ x As films with Al concentrations be~
M. Deschler et 01. / Halogen VPE of AIGaAsfor optoelectronic deviceapplications XAl ,""%100
.. To=1O'I3K
To= '046 K
A
80
experimental
• To = me K p'
He'
60
la)
- - - calculated
-
=2 lCf'bar ,
P:....,.2 lCf bar
20
• To.me K
lb)
A" ='046 • To .tl73 K K
'0' 20 NK ~.'.... 10
W;;(, ......
P:.c,
=2 ld'bClf
p;....,=2 1Ir' bar
s
05'----"-----'---"-----'---------J 9 95 lOS ~ T.
Fig. 3. Effect of source temperature on (a) AI concentration and (b) growth rate for various deposition temperatures
tween 0 and 80% could be grown, Plotted are mean values of the results obtained and the absolute spread around these values. The error bars indicate the typical spread (Ax = 0.05) as determined for a large number of experiments made at three different growth conditions. The scatter in the Al concentration for constant source and deposition temperature can be explained by incomplete control of the process temperatures which were measured, for technical reasons, outside the reactor. Also, precipitation of the Al compound at the wall in the source zone affects the reproducibility of the Al concentration of the grown film. The results obtained on the Al concentration as a function of the process temperatures qualitatively agree with those obtained by Bachem and Heyen [5]. However, the Al concentrations found in our study using a newly designed reactor are some-
631
what lower. This difference results from the changed geometry of the deposition apparatus, from the fact that the temperatures in source and deposition regions were measured outside of the growth system and from the different flow rates of the gas species used in the present study. The results of the model calculation, which are also plotted in fig. 3a, show the same trends as the experimental data, namely an increase of the aluminium concentration in the film with deposition temperature and with reciprocal source temperature. It may be seen, that experiment and calculation lead to similar values of x . Since the calculated values of x are independent of the diffusion coefficient kd,HCI' this indicates that the aluminium incorporation is essentialIy thermodynamically controlled and that the source reaction goes to completion. The small discrepancy between the slopes of the calculated and experimental curves is probably caused by the inaccurancy of the thermodynamic data used in the computations. The temperature dependence of the AlGaAs growth rates is represented in fig. 3b. The rates decrease strongly with increasing deposition temperature and decreasing source temperature (i.e. with increasing the Al concentration). The incomplete control of the reactor temperature is probably the cause of the pronounced scatter of the data points. A further reason are most likely the variations in the flow past the substrate surface (and thereby in kd,HCI) due to variation in the radial position of the substrate holder from run to run. The solid straight lines shown in fig. 3b were obtained by fitting to the mean values of the growth rates for constant deposition temperature using a least square method. In this case the relative scatter of the data points, which was approximately constant in all cases (38%), is indicated, The trend of high growth rates at high source temperatures, i.e. at low Al concentrations, was also predicted by the model calculations, as shown by the dashed lines in fig. 3b. The quantitative difference between the measured growth rates and those predicted from model calculations is probably caused by the inaccuracy of the thermodynamic data and by the inaccuracy of the parameter kd,HCI ' The fact that the largest discrepancy
M. Deschler et 01. /I/alogen VPE 01AIGaAslor optoelectronic deviceapplications
632
o
30
ts
(Q)
(a)
-- - calculated expenmental
30
6
2S
rc p~ • Zllfbar
20
TS .1015 K
U· 1046 K
T ~mlh
i
•••• calculated expenmental
lb)
--
10
p' .Z 1l-2 bar Htl
--
lb)
b.Il~6 K
,-
5
--- calC\Jlated - .xpermental
Ys .1015
K
0
2 .2 10- bar AsH3 TO' 1046 K
p'
2
Ts' 1015 K 5
10
20
50
ZO
---
10
5 100
)~ar --
Fig. 4. Effect of HCI pressure on: (a) AI- concentration; (b) growth rate.
Z
5
10
20
!>O. 100 PAsH3 3
lli
occurs at the highest deposition temperature may also point to the problem of material loss by wall deposition, which was most pronounced at these conditions. The dependence of the Al concentration and the growth rate of the film on the partial pressure of HCl is shown in fig. 4. In agreement with the model calculation, the composition of the film and the growth rates are only weakly affected by the HCl input pressure. Furthermore, the effect of the AsH) input pressure on the Al concentration and the growth rate of the film was investigated (see fig. 5). The continuous decrease of x observed for increasing PAsH , again agrees with results of the model calculation. Also, the experimentally found increase of the growth rates with PAsH) is predicted by eq. (8). With the addition of small amounts of HCl through the bypass, the wall deposition upstream of the surface is reduced leading to an increase of the growth rate (see fig. 6). For higher HCl bypass pressures, a distinct decrease of the growth rate is observed which is caused by the reduction of the
bat
ZOO
-
Fig. 5. Effect of AsH 3 pressure on: (a) Al concentration; (b) growth rate .
supersaturation of the gas phase for these cons], tions. The maximum of the curve lies around P H O •by = 2 X 10- 4 bar. An extrapolation of the calculated and experimental growth rate curves to zero growth appears to yield different PUC! pressures for this case. This is probably due to th~ above mentioned error in the growth rates. It should also be remarked that the thermodynamic data exhibit some uncertaincy. The Al concentra_ tion decreases only very slightly with increasing HCI bypass pressure, as predicted by the model,
4.2. Electrical characteristics The use of graphite tubes to protect the quartz reactor from reaction with corrosive compounds results in lowering of the background doping in comparison to the earlier study from our labora_' tory [4]. The measured electron concentrations in the first runs were in the lOU em -s region; after
633
M. Deschler et 01. I Halogen VPE of AIGaAsfor optoelectronic deviceapplications
~ .,.
j40
/'"
_
_ _
w
. . . . ..
.
...
...
_
20 • - - cok:ulated
p~'2~bc1r
_
'kl ,210-2 bc1
.xpemnentQl
lb l
To , 1046 K Is ,1015K
- •• calcubled
20 10
-... _- -
10
20
50.
100
2lXl
PHC1.by ~
lO"'bar
Fig. 6. Effect of HO bypass pressure on : (a) AI concentration; (b) growth rate.
accidental contact of the source with a quartz tube, this value increased to the high 1016 em - 3 range. It should be noted that a background doping level in this range could be reproducibly obtained although no in situ purification of AsH 3 was performed as has been found advantageous in MOCVD or MBE [2,11]. The electron mobilities are strongly dependent on the AI concentration in the film. In our study similar mobility values were obtained as reported for MOCVD, LPE or MBE grown layers [12,13]. Typically, in Alo.30Gao.7oAs films with carrier concentrations around 1 X lO17cm - 3 the mobilities range between 1000 and 1500 cm2IV . s. However, the carrier mobility in GaAs grown in our system was lower than typical values reported for pure GaAs films [14]. One may ask if a small amount of residual AI (x < 1%) could be responsible for this effect or if a different impurity would be involved. If the first explanation would hold, the growth of high quality GaAs along with GaAlAs would require the use of a two
source system, one of them being a pure Ga source for the growth of GaAs. Carbon, which was indeed found in these films, as will be discussed in the next section, could also be responsible of the mobility lowering. By injection of H 2S into the deposition zone, n-type AIGaAs was obtained. For a constant AI concentration in the film, the electron concentration exhibits a linear dependence on the dopant pressure PH2 S (fig. 7). Such a linear dependence was also found in the halide growth of GaAs [15} and in the MOCVD of GaAlAs [12,13]. Furthermore, like was reported for the growth of Si doped MBE films [16} and also for Si or Se doped MOCVD films [17], in our experiments the measured electron concentration decreased with increasing AI mole fraction at constant injection rate of the dopant gas H 2S. This behaviour appears to correlate with the increase of the ionisation energy of the donors observed at higher AI concentrations. Beside the Al concentration, also the AsH 3 partial pressure affects the level of dopant incorporation. An increase of the AsH 3 input pressure
~31
•
KCOO1
A
K=
•
Q3
18
10
•
•
..
... .. lOTI
.
1016' - ---'-_ _" - - - - ' - _........ _ Q2 01 05 2
........._
5
........ _ " ' - - - - _ - '
10
20
50
•
---~
p¥
l06bor
Fig. 7. Carrier concentration (300 K) versus H 2S input pressure (PH,S) for various AI concentrations (x) . The accuracy of the point indicated by the open triangle at n -1 x 10 17 cm - 3 is low because of the level of background doping.
634
M. Deschler et al. / Halogen VP Eo/ AIGaAs lor optoelectronic device applications
~1
1019,--
-,
---"--t'ooo
: 2 10 bar ' + PAI M ) : 2 10" bar
cri?FiS
OP A • H ,
P
';5
AI. Go,..As(xc Q01) • AI. Go....AsI02S...035 J 3000 + MOCVO latter Stnngfellowl1311 0
: 5 10" bar
"0 2000
o 1000
rf
ne
!if'
~
~
Fig. 9. Dependence or mobility on electron concentration (T - 300 K); numbers at solid square points indicate oX AI; curve: best empirical representation or data for n-doped GaAs
10
20
30
40
(14).
50 XAI
»:
Fig. 8. Dopant concentration (300 K) versus AI concentration x; parameter: PAsH , .
by a factor of 10 yields an increase of the electron concentration by roughly a factor 2 for constant Al concentration (fig. 8). This behaviour cannot be explained from thermodynamic considerations which indicate that
(12) and therefore p..\s~/4 - [VAs] - [S;sl = n ,
(13)
i.e. a decrease of the electron concentration with increasing AsH) pressure. In these equations VAs represents an As vacancy, S;s an ionized sulfur atom on an As site and n the electron concentration at room temperature. In a study of the sulfur doping of GaAs during growth in the hydride system it was found that the incorporation rate of S supplied to the surface is also affected by the rate of growth of the film [18]. However, as shown in fig. 5, an increase in PA sH ) leads to an increase in the growth rate. One would thus predict a decrease in the dopant concentration with increasing PAsH rather than the observed increase. At the prese~t time we cannot 'offer an explanation for this discrepancy. The electron mobilities of the doped layer as a function of the carrier concentration for different
AI concentration are plotted in fig. 9. Similar mobility values are obtained in layers prepared by other growth methods [12,13]. To deposit p-type AIGaAs films, DEZn Was used as a source material. Fig. 10 shows the dependence of the hole concentration on the DEZn partial pressure for two AI concentrations. Similar to the behaviour observed for the electron COncentration upon H 2S injection, an increase of the hole concentration with P O FZ o and a decrease of the doping for higher AI concentrations in the filtn is observed. However, the slope of the curves of doping level versus DEZn partial pressure is
~1101'.l em
"
-2
P>CI =110 bar p" =5 bar
10-)
AsH)
~" L .-"--'----"-----'---""'------.J 2
5
10
20
50
_ _---.._.
100
POEZ.!!..
10"bar Fig. 10. Hole concentration (300 K) versus DEZn input pres, sure; parameter: AI concentration x.
M. Deschler et al. / Halogen VPE of AIGaAsfor optoelectronic device applications
J\5t
tion is lower in the case of Zn doping. The relation between Hall mobili ty of the p-type layers and the carrier concentration for different AI concentrations is shown in fig. 11. The mobilities are comparable to those obtained by Mori and Watanabe [12].
Al x Ga,-lCAs .,,001 • Alx Ga",As IQ25'.' 035) + MOCVO (Morl el 01 1121) o
300
0 0
200
•
I2SJo
100 +
U S)
+
•
i1 >5l( J2l
'-2>f
Ii'
635
+
11>~
+00 0
' 35)
US)
l It)
(fJ l _
+
t ~,
, 261
4.3. Optical characteristics
+ +
IlS)
•
135l
lr!'
10"
10"
p
c;y;:i
F ig. 11. Hole mob ility versus carrier con centration (T - 300 K) ; numbers at solid square po int s indic ate xA\; curve : best empirical representation of data for p-type GaAs [21].
somewhat larger than 1. This behaviour is probably caused by a precipitation of Zn after thermal cracking of the metal organic compound at the transition to the hot reactor zone or by the formation of a stable compound ZnO due to small amounts of O2 in the reactor. The dependence of the ionization energy for acceptors on the Al concentration in GaAlAs is smaller than the dependence for donors [19,20]. Thus, the reduction in doping efficiency with increasing Al concentra-
Examples of low temperature photoluminescence spectra measured on AIGaAs layers with AI concentrations of 17% and lower than 1%, with background doping levels of 1 X 1017 em"? and 2 X 10 17 cm -3, respectively, are shown in fig. 12. The spectrum measured on the AlxGal_xAs layer with x < 1 % exhibits a sharp bound exciton peak (BE) at 1.528 eV and a free to bound peak (FB), 26 meV below the BE peak. The full width at half-maximum (FWHM) of the BE peak is 1.3 meV . The spectrum measured on the AIo.17Gao.83As film shows a BE peak at 1.746 eV with a FWHM of 4 meV and a FB peak at 1.707 eV. The BE peak energies measured on both layers agree with results obtained by other authors [22,23]. Furthermore, the excitonic linewidth of the BE peaks, which is an indication of the optical quality of the grown film, corresponds to the best
iooo
300 A;",GoCNAs
A I. Go" As I x<001)
BE t746 ltV
n ~l 'lO"cm)
~
I
i
"=2 lO"cm'
800
:;
:;200
s
S
l:-
l:-
BE 1528 tV ~
600
-iii c
-iii c QI
~
FWHM = 4 "",V
~ ~ 100
l:
400
...J
FB
FWHIo1=13mtN
Q..
1707 ""
200
0
170
172
174
176
Energy (eV) ~
1.48
FB
X"" ) 1.50
1.52
1.54
Energy (eV)
156
~
Fig. 12. Photoluminescence spec tra of AlGaAs films (T- 2 K ; P - 30 mW /cm2 ) at two different AI con centrations.
636
M. Deschler et al. / Halogen VPE of AIGaAs f or optoelectronic deri ce applications
value reported for LPE, MOCVD and MBE films [24-27]. The FB peak measured on the AIGaAs film with x < 1 % is probably related to a carbon acceptor transition whereas the FB peak measured on the Alo.17GaO.R3As layer can be assigned to a silicon acceptor transition [23,28,29]. These results suggest that some impurities from the carbon parts are incorporated into the deposited films. As mentioned above, the Si contamination of our samples appears to be of an accidental nature.
5
ios trcte
4.4. Morphology, uniformity and interface sharpness The surfaces of AlGaAs films with Al concentrations lower than 50% are mirror like . At higher AI content, the surface gradually roughens. Layers grown from the gas phase at atmospheric pressure tend to have a somewhat smaller thickness at the downstream part of the substrate. Such a behaviour was also observed in the present reactor. On a 20 X 20 mnr' substrate this inhomogeneity in the flow direction was less than 15%; perpendicular to the flow an inhomogeneity below 5% was observed. The variation of the AI concentration over the thickness of films was less than
1%. As discussed above, the film composition is strongly dependent on the growth temperature whereas the other parameters have only a small effect. Therefore, in order to prepare heterostructures, the growth process has to be interrupted by switching off the HCI source flow. Also, the growth temperature has to be changed before depositing the next layer. It is helpful for keeping the film composition constant over its full depth that this composition is nearly independent of PH C1 (see fig. 4a). The interface sharpness of the AI concentration was within the resolution of our secondary ion mass spectrometry (SIMS) measurements, i.e. within 8 nm. The scanning electron micrograph of a multilayer structure in fig. 13 demonstrates the absence of any defects. Also shown in this figure is the X-ray rocking curve of the structure; it is indicative of the excellent depth homogeneity of the layers.
b
c.
.::=============-:--
Go As Su bst rote
----7 int ensi ty ( A. u.) Fig. 13. (a) Scanning electr on micrograph of cross-sec tio n through hetero structure and (b) corresponding X-ray rOCking curve .
S. Device application Using the technique described in the foregoing sections light emitting diodes were fabricated. Multilayer structures consisting of four dOPed layers were deposited on an n-type substrate. The e1ectroluminescence spectrum of such a device is shown in fig. 14 together with the J- V characteris_ tic of the LED. It may be seen that the diode exhibits a very low leakage current, which proves the high quality of the p-n junction. The etectns, luminescence signal of the light emitting diode,
637
M. Deschler et al. / Halog en VPE of AIGaA s for optoelectronic device applications
tion of the layer thicknesses and doping levels. Thi s optimization was not carri ed out for our VPE grown LEDs.
AI O ~ Go O Il" A s - LE O
1 , 31 mA
p e GaAs p
e
Al os Gao ~ As
0 :' iJm 2 ~ ~m
\ n e Alo ~ Goor As n e Galu
9S 30
:~
.
~m
6. Conclusion
lJm
II
,
b .--
i
IV
I
I
..
, III
,..
r"I
II
T
i
Fig. 14. (a) Electroluminescence spectrum of LED ; (b) 1- V characteristic.
measured at room temperature, exhibits a maximum at a wavelength of 751 nm, as expected for an Al concentration in the active layer of 19%. On similar LPE grown structures up to 10 times larger power efficiencies were observed. However, it should be noted that this result required optirnizaTable 1 Constants in eq. (15) for
~G o
We demonstrate in this paper that the halid e transport system is a suitable method for growing device grade AIGaA s material. Thereby. the carbon shielding of all parts coming in contact with the corrosive Al species proves to be essential to reduce the background doping level and to improve the electrical and optical quality of the grown layers. The process temperature is one of the major parameters governing the composition of the deposited films. The growth behaviour may be reproduced by a model which indicates that the growth reactions are essentially thermodynami cally controlled. In the halide system, sulfur and zinc are found to be well-behaved n- and p-type dopants respect ively for obtaining carrier concentrations up to 10 18 cm- 3 • As also found in MBE. MOCVD and LPE grown films, an increase of the Al mole fraction leads to a reduction of the concentrations of the ionized donor and acceptor atoms. The material obtained by this technique appears to be suitable for the fabrication of devices.
Appendix
The temperature dependent equilibrium constants of the reactions described in section 2 can
for different species
Compounds
A
B
C
H2 HC I G a(I) AI(1) GaCI AICI AICI ) AS4 GaAs(s) AIAs(s)
- 1938.4 - 23915 -682.7 297.04 -20290 - 15215 -144750 31017 - 24143 - 32497
12.623 -2.005 30.339 41.323 2.6233 5.8599 54.798 54.957 55.552 56.372
-6.52 -6.34 -6.65 - 7.59 - 8.93 -8.94 -19.59 -19.82 - 10.46 -10.5
E( xI0 5)
D
-600
-13000 0 0 19000 36500 -118500 61500 0 0
-39 -55 0 0 0 5.5 - 7.5
-1.5 - 140 -75
Rd.
(30) (30) (31) (30] (31) (30) (30) (32) [31,33) [32]
638
M. Deschler et 01. / Halogen VPE of AIGaAsfor optoelectronic deviceapplications
be expressed by means of the standard Gibbs free energies .1Gjo: K, = exp( -.1G,o/RT),
(14)
with .1Go =A +BT+ CT In(T)+D/T+ET 2 •
(15)
R represents the gas constant, T the temperature and A to E are constants specific to each component. These constants are summarized in table 1 for the main species appearing in the deposition reactor. Acknowledgements
The authors would like to thank J. Korec for fruitful discussions on the thermodynamics of the growth process. Thanks are also due to K.H. Goetz and W. Richter for photoluminescence measurements and discussions. We are grateful to M. Maier for performing SIMS measurements and to H. Monchs for scanning electron micrographs. References [1] H. Kressel, J. Electron. Mater. 3 (1974) 747. [2] T. Nakanisi, I. Crystal Growth 68 (1984) 282. [3] M. Heiblum, E.E. Mendez and L. Osterling, I. App!. Pbys. 54 (1983) 6982. [4J K.H. Bacbem and M. Heyen, in: Proc. 8tb Intern. Symp. on GaAs and Related Compounds, Vienna, 1980, Inst, Phys. Conf. Ser. 56, Ed. H.W. Thim (Inst. Phys., London-Bristol, 1981) p. 65. [5] K.H. Bachem and M. Heyen, I. Crystal Growth 55 (1981) 330. [6] M.B. Panisb and M. Illegems, in: Progress in Solid State Chemistry, Vol. 7, Eds. H. Reiss and 1.0. McCaldin (Pergamon, Oxford, 1972) p. 39. ,7] J. Korec and M. Heyen, I. Crystal Growth 60 (1982) 297. [8] W.J. Riedl, in: Advances in Epitaxy and Endotaxy, Eds. H.G. Schneider, V. Ruth and T. Kormany (Elsevier, Amsterdam, 1976) p. 97. [9] I. Bloem and L.J. Giling, in: Current Topics in Materials Science, Vo!. 1, Ed. M. Kaldis (North-Holland, Amsterdam, 1978) p, 147. [10J P. van der Putte, L.I. Giling and I. Bloem, I. Crystal Growth 31 (1975) 299.
[11] L.M. Fraas, JA Cape, P.S. McLeod and L.D. Partain, J. Vacuum Sci. Techno!. B3 (1985) 921. [12) Y. Mori and N. Watanabe, J. AppI. Phys. 52 (1981) 2792. (13) G.B. Stringfellow, I. Crystal Growth 53 (1981) 42. [14] Landolt-Bornstein, Numerical Data and Functional Rela, tionships in Science and Technique, Vol. 17, Eds. O. Madelung, M. Schulz and H. Weiss (Springer, Berlin, 1982) p. 532. [IS) E. Veuhoff, M. Maier, K.-H. Bachem and P. Balk. J. Crystal Growth 53 (1981) 598. [16] N. Chand, T. Henderson, I. Klem, W.T. Masselink, R. Fisher, Y.c. Chang and H. Morkoe, Phys. Rev. B30 (1984) 4481. [17J I. Hallais, I.P. Andre, A. Mircea-Roussel, M. Mahieu, J. Varon, M.C. Boissy and A.C. Vink, I. Electron. Mater. 10 (1981) 665. [18J I. Korec, D. Grundmann and M. Heyen, I. Electrochelll. Soc. 131 (1984) 1433. (19) 1.1. Yang, W.I. Simpson and L.A. Moudy, I. Appl. Phys. 53 (1982) 771. [20J K. Masu, M. Konagai and K. Takahashi, I. Appl. Phys, S 1 (1980) 1060. [21J I.D. Wiley, in: Semiconductors and Semimetals, Vol. 10 Eds. R.K. Willardson and A.C. Beer (Academic Press' New York, 1975) p. 154. ' (22) R. Dingle, R.A. Logan and I.R. Arthur, Ir., in: Proc. 6th Intern. Symp. on GaAs and Related Compounds, Edin. burgh, 1976, Inst. Phys. Conf. Ser, 33a, Ed. C. Hilsum (Inst. Phys., London-Bristol, 1977) p. 210. (23) M. Mihara, Y. Nomura, M. Mannoh, N. Yamanaka, S. Naritsuka, K. Shinozaki, T. Yuasa and M. Ishii, J. Appl. Phys. 55 (1984) 3760. [24] E.F. Schubert, E.O. Gobel, Y. Horikoshi, K. Ploog and H.J. Queisser, Phys. Rev. B30 (1984) 813. [25] I.M. Ballingall and D.M. Collins, J. Appl. Phys. 54 (1983) 341. [26] J.R. Shealy, V.G. Kreismanis, D.K. Wagner and J.M. Woodall, Appl. Phys. Letters 42 (1983) 83. [27J D.C. Reynolds, K.K. Bajaj, C.W. Litton, P.W. Yu, J. Klem, C.K. Peng, H. Morkoc and J. Singh, Appl. Phys. Letters 48 (1986) 727. [28J G. Wicks, W.I. Wang, C.E.C. Wood, L.F. Eastman and L. Rathbun, J. Appl. Phys. 52 (1981) 5792. [29] V. Swaminathan, J.L. Zilko, W.T. Tsang and W.R. Wagner, I. Appl. Pbys. 53 (1982) 5163. [30] JANAF Thermochemical Tables (National Bureau of Standards, Washington, DC, 1971, Suppl. 1974/1975). (31) D.W. Shaw, I. Phys. Chern. Solids 36 (1975)'111. (32) I. Barin, O. Knacke and O. Kubaschewski, Thermochem_ ical Properties of Inorganic Substances, Supplement (Springer, Berlin, 1977). [33] J.R. Arthur, I. Phys. Chern. Solids 28 (1967) 2257.
628
HALOGEN VPE OF AIGaAs FOR OPTOELECfRONIC DEVICE APPLICAnONS M. DESCHLER, M. COPPERS, A. BRAUERS, M. HEYEN and P. BALK Institute c/ Semiconductor Electronics, Technical Uniuersity Aachen, Sommerfeldstrasse, D·5JOO Aachen, Fed Rep.
0/ Germany
Received 6 August 1986; manuscript received in final form 3 November 1986
We report on the growth of the ternary Al x Gaj ; x As in a specially designed halide transport reactor. High quality layers with background doping levels in the 1016 cm - 3 range have been achieved. Furthermore. the small FWHM (for example. 4 meV measured on an Ala l,GaO.83As film) obtained from low temperature photoluminescence demonstrates the excellent optical quality of the rilms. By varying the growth temperature between 700 and 800°C, the AI concentration of the layers can be changed between < 1 and 70% whereas the AsH], HO and HCI-bypass partial pressures have hardly any effect on the composition of the films. These results are i~ accordance with a thermodynamic model which takes into account a diffusion step from the bulk gas phase to the substrate surface. The doping behaviour with DEZn and H 2S as sources for n and p-type doping respectively leads to similar results as have been reported for MOCVD, MBE or LPE layers. The fabrication of a light emitting d iode with satisfactory efficiency demonstrates the suitability of this method for growing device quality films.
1. Introduction The ternary semiconductor AIGaAs is one of the most important materials for the fabrication of microwave or optoelectronic devices. This material is commonly grown by liquid phase epitaxy (LPE) , metalorganic vapour phase epitaxy (MOCVD) or molecular beam epitaxy (MBE) [1-3]. It is also possible to deposit epitaxial films of this ternary in a chlorine transport system, as was reported in earlier publications from our laboratory by Bachem and Heyen [4,5]. Using a Ga-10% AI source, these authors were able to grow AIGaAs films with Al concentrations up to 40% at temperatures between 700 and 800°C. However, the deposition of high quality material was not achieved in their work since the highly corrosive nature of the reactants, especially the Al chlorides, at the typical growth temperatures lead to high unintentional doping of the films. In this paper we will describe the deposition of device quality AIGaAs films in a reactor specifically designed to avoid this contamination problem. Data will be presented on the effect of the deposition parameters on growth rate and film composition. We will also present an expanded
modelling treatment of these data including the diffusion of the reactants towards the substrate surface. We will show that the electrical and opu, cal characteristics of the material obtained in this study are comparable to those obtained by other growth techniques. The fabrication of a light emi tting diode will be used as an example of the viability of the method for growing device grade films of controlled doping.
2. Model considerations The deposition of AlxGa1_xAs on a GaAs substrate is governed by the following equilibriulll equations: I
GaCi + AIel +
l I
4
1
K1
AS4 +! H 2 ~ GaAs + HCI, 1
K2
AS4 + ! H 2 ~ AlAs + HCI,
(1)
(2)
The chlorides are provided by reaction of Hel with the Ga-AI source.
0022-0248/87/$03.50 e Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
629
M. Deschler et al. / Halogen VPE of AIGaAsfor optoelectronic device applications
The growth kinetics will be described for the case of a small deviation from equilibrium. At this condition the activities of GaAs and AlAs in the gas phase near the substrate surface are close to their values in the solid phase. Assuming that AlGaAs behaves as an ideal solution of AlAs in GaAs [6], the mole fraction x of Al equals the activity aAJAs of AlAs in the solid. Similarly, a G aAs equals (1 - x). The concentrations of the species in eqs. (1)-(3) near the substrate surface are determined by their partial pressures in the bulk of the gas phase and by the mass transport occurring between the bulk and the substrate surface. The diffusion steps considered in the model are schematically presented in fig. 1. P; and p;D stand for the partial pressures of the species i in bulk of the gas phase and immediately above the substrate surface, respectively. .1/ describes the diffusion flux of gaseous species from the bulk of the gas phase towards the substrate, .1;- the diffusion flux from the substrate to the gas phase. As proposed by Korec and Heyen [7], at constant temperature in the gas phase the net flux of each species i can be described as follow:
(4) where k d . ; is the mass transfer coefficient for species i. Utilizing the principle of mass conservation for chlorine, gallium, aluminium and arsenic, we obtain expressions involving the rates of deposition of the binaries GaAs (rGaAs) and AlAs (rAlAs): k d •Ha (P Ha -
plla )
+kd.GaCI(PGaCI-
+ kd,AlCI (PAlCI -
PgCl )
plb)
+ 3k d,AIC13 ( pAla 3 kd,Gaa( PGaa -
p.BCI 3 ) = 0,
Pga ) = rG aAs ,
(5) (6)
kd,Ala( PAlC l- P.Ba)
= rAJAs,
(7)
p~J = rG aAs + rAJAs·
(8)
+kd,Ala 3(PAla 3- P.BaJ 4k d
,As.(PA s4 -
The growth rate 7' of the ternary AIGaAs may be expressed as the sum of the deposition rates of the
,.
bulk gas stream
Ji
I
P,
1 1J;
substrate
I
pD I
Fig. 1. Diffusion fluxes and partial pressures of component i in deposition system.
binary components:
(9) The ratio of the deposition rate of the AlAs component to the total growth rate determines the Al concentration of the film: (10)
It should be mentioned that the mass transfer coefficients kd" are free parameters in this approach; they are determined by fitting calculated curves against experimental data. Utilizing the method proposed by Riedl [8], the number of free parameters can be reduced to one by expressing the mass transfer in the gas phase in terms of fluxes normalized to one of them (here of HCl):
b; = kd,;/kd,Ha'
(n)
b, can be calculated from the ratio of the diffusion constants for the different species so that k d.HCI remains the only free parameter [9,10]. From eqs. (6) to (8) it follows that the value of k d.Ha , which depends on reactor geometry, gas velocity and total gas pressure, enters in the expression for the growth rate as a proportionality factor. However, the Al concentration x in the film, which is expressed as a ratio of deposition and growth rates (d. eq. (10», is independent of this parameter. Results of computer calculations based on this model for kd,Ha = 10 mm h- 1 bar "! will be compared with experimental results in section 4. 3. Reactor design As mentioned before, an important consideration in the design of the reactor is the corrosivity
M. Deschler et al. / Halogen VP E of A IGaAs for optoelectronic device applications
630
of the Al species which leads to high impurity incorporation in the grown film. Background doping levels lower than 1018 ern- 3 could not be reached in the growth apparatus proposed by Bachem and Heyen [5] where a quartz substrate holder was used and where the substrate zone was not shielded from the quartz walls of the reactor. In the newly designed .apparatus, the deposition is performed in a resistance heated reactor where all parts coming into contact with the reactants at high temperature are either made from graphite or are protected by a graphite liner. Fig. 2 shows a schematic diagram of the growth apparatus. The vertical arrangement permits bubbling of HCl/H 2 through the Ga-10%AI source, thus avoiding problems with the inevitable aluminium oxide crust over the source. A bubble path length of about 5
HCl .H 2 HC1by· H2
H2 I
-
11- 'l~
I
' rI
I~
~ I r;(
dip tube
.- cover
t-- crucible AI/Go alloy
./
! I
i
rr .I- :~
I~
~
quartz tube
-----
liner tube
nozzle substrate
:~
em was found to be sufficient for a complete source reaction. The source composition (10% A.I) was choosen to allow growth of AIGaAs with A.I concentrations over a large range at practical tem_ peratures. At this low Al concentration in the source, a solidification of the alloy is also avoided. Deposition on the reactor walls was minimized by introducing the arsine into the reaction zone immediately above the substrate through the sub_ strate holder. In the same way, H 2S used as n-tyPe dopant was injected into the mixing zone. Diethyj., zinc (DEZn), used for growing p-type films, was carried into the reactor at a high velocity (133 cmys) by means of a narrow capillary to aVoid thermal cracking of the metalorganic compound at the transition to the hot zone. After each epi_ taxial run, HO was introduced into the reactor through the substrate zone and via the bypass line (HO by ) to clean the apparatus by means of etch, ing. Injection of HCl by can also be used for shift_ ing the activity in the gas phase towards lOwer values during deposition. Typically, 150 mf/min of HCl/H 2 were bUb_ bled through the source, 150 mlyrnin of H 2 Were injected through the bypass and 450 mlyrnin of H 2-H 2S-DEZn-AsH 3 were introduced through the substrate holder. The flow rates of all the gases introduced into the reactor were controlled by mass flow meters and switched with electropneu_ matic valves. The GaAs substrates were oriented 2° off the (100) direction towards the (110) direction. The thickness of the epitaxially groWn films was measured optically upon staining. Car_ rier concentrations and mobilities were obtained from Hall measurements. The Al mole fraction was determined from photoluminescence and Xray double crystal diffractometry measurements.
substrate holder
I
quartz capillary
I
4. Results and discussion
I
I I
4.1. Characterization of the deposition system
I
~
~
L-
to exhaust to exhaust
AsH3• Hel • H2S• H2 DEZn· H2
Fig. 2. Schematic diagram of epitaxial reactor.
The Al concentration in the deposited layer was found to depend strongly on the process tempera_ ture. As shown in fig. 3a, by varying source and deposition temperature between 700 to 800° C AlxGa 1 _ x As films with Al concentrations be~
M. Deschler et 01. / Halogen VPE of AIGaAsfor optoelectronic deviceapplications XAl ,""%100
.. To=1O'I3K
To= '046 K
A
80
experimental
• To = me K p'
He'
60
la)
- - - calculated
-
=2 lCf'bar ,
P:....,.2 lCf bar
20
• To.me K
lb)
A" ='046 • To .tl73 K K
'0' 20 NK ~.'.... 10
W;;(, ......
P:.c,
=2 ld'bClf
p;....,=2 1Ir' bar
s
05'----"-----'---"-----'---------J 9 95 lOS ~ T.
Fig. 3. Effect of source temperature on (a) AI concentration and (b) growth rate for various deposition temperatures
tween 0 and 80% could be grown, Plotted are mean values of the results obtained and the absolute spread around these values. The error bars indicate the typical spread (Ax = 0.05) as determined for a large number of experiments made at three different growth conditions. The scatter in the Al concentration for constant source and deposition temperature can be explained by incomplete control of the process temperatures which were measured, for technical reasons, outside the reactor. Also, precipitation of the Al compound at the wall in the source zone affects the reproducibility of the Al concentration of the grown film. The results obtained on the Al concentration as a function of the process temperatures qualitatively agree with those obtained by Bachem and Heyen [5]. However, the Al concentrations found in our study using a newly designed reactor are some-
631
what lower. This difference results from the changed geometry of the deposition apparatus, from the fact that the temperatures in source and deposition regions were measured outside of the growth system and from the different flow rates of the gas species used in the present study. The results of the model calculation, which are also plotted in fig. 3a, show the same trends as the experimental data, namely an increase of the aluminium concentration in the film with deposition temperature and with reciprocal source temperature. It may be seen, that experiment and calculation lead to similar values of x . Since the calculated values of x are independent of the diffusion coefficient kd,HCI' this indicates that the aluminium incorporation is essentialIy thermodynamically controlled and that the source reaction goes to completion. The small discrepancy between the slopes of the calculated and experimental curves is probably caused by the inaccurancy of the thermodynamic data used in the computations. The temperature dependence of the AlGaAs growth rates is represented in fig. 3b. The rates decrease strongly with increasing deposition temperature and decreasing source temperature (i.e. with increasing the Al concentration). The incomplete control of the reactor temperature is probably the cause of the pronounced scatter of the data points. A further reason are most likely the variations in the flow past the substrate surface (and thereby in kd,HCI) due to variation in the radial position of the substrate holder from run to run. The solid straight lines shown in fig. 3b were obtained by fitting to the mean values of the growth rates for constant deposition temperature using a least square method. In this case the relative scatter of the data points, which was approximately constant in all cases (38%), is indicated, The trend of high growth rates at high source temperatures, i.e. at low Al concentrations, was also predicted by the model calculations, as shown by the dashed lines in fig. 3b. The quantitative difference between the measured growth rates and those predicted from model calculations is probably caused by the inaccuracy of the thermodynamic data and by the inaccuracy of the parameter kd,HCI ' The fact that the largest discrepancy
M. Deschler et 01. /I/alogen VPE 01AIGaAslor optoelectronic deviceapplications
632
o
30
ts
(Q)
(a)
-- - calculated expenmental
30
6
2S
rc p~ • Zllfbar
20
TS .1015 K
U· 1046 K
T ~mlh
i
•••• calculated expenmental
lb)
--
10
p' .Z 1l-2 bar Htl
--
lb)
b.Il~6 K
,-
5
--- calC\Jlated - .xpermental
Ys .1015
K
0
2 .2 10- bar AsH3 TO' 1046 K
p'
2
Ts' 1015 K 5
10
20
50
ZO
---
10
5 100
)~ar --
Fig. 4. Effect of HCI pressure on: (a) AI- concentration; (b) growth rate.
Z
5
10
20
!>O. 100 PAsH3 3
lli
occurs at the highest deposition temperature may also point to the problem of material loss by wall deposition, which was most pronounced at these conditions. The dependence of the Al concentration and the growth rate of the film on the partial pressure of HCl is shown in fig. 4. In agreement with the model calculation, the composition of the film and the growth rates are only weakly affected by the HCl input pressure. Furthermore, the effect of the AsH) input pressure on the Al concentration and the growth rate of the film was investigated (see fig. 5). The continuous decrease of x observed for increasing PAsH , again agrees with results of the model calculation. Also, the experimentally found increase of the growth rates with PAsH) is predicted by eq. (8). With the addition of small amounts of HCl through the bypass, the wall deposition upstream of the surface is reduced leading to an increase of the growth rate (see fig. 6). For higher HCl bypass pressures, a distinct decrease of the growth rate is observed which is caused by the reduction of the
bat
ZOO
-
Fig. 5. Effect of AsH 3 pressure on: (a) Al concentration; (b) growth rate .
supersaturation of the gas phase for these cons], tions. The maximum of the curve lies around P H O •by = 2 X 10- 4 bar. An extrapolation of the calculated and experimental growth rate curves to zero growth appears to yield different PUC! pressures for this case. This is probably due to th~ above mentioned error in the growth rates. It should also be remarked that the thermodynamic data exhibit some uncertaincy. The Al concentra_ tion decreases only very slightly with increasing HCI bypass pressure, as predicted by the model,
4.2. Electrical characteristics The use of graphite tubes to protect the quartz reactor from reaction with corrosive compounds results in lowering of the background doping in comparison to the earlier study from our labora_' tory [4]. The measured electron concentrations in the first runs were in the lOU em -s region; after
633
M. Deschler et 01. I Halogen VPE of AIGaAsfor optoelectronic deviceapplications
~ .,.
j40
/'"
_
_ _
w
. . . . ..
.
...
...
_
20 • - - cok:ulated
p~'2~bc1r
_
'kl ,210-2 bc1
.xpemnentQl
lb l
To , 1046 K Is ,1015K
- •• calcubled
20 10
-... _- -
10
20
50.
100
2lXl
PHC1.by ~
lO"'bar
Fig. 6. Effect of HO bypass pressure on : (a) AI concentration; (b) growth rate.
accidental contact of the source with a quartz tube, this value increased to the high 1016 em - 3 range. It should be noted that a background doping level in this range could be reproducibly obtained although no in situ purification of AsH 3 was performed as has been found advantageous in MOCVD or MBE [2,11]. The electron mobilities are strongly dependent on the AI concentration in the film. In our study similar mobility values were obtained as reported for MOCVD, LPE or MBE grown layers [12,13]. Typically, in Alo.30Gao.7oAs films with carrier concentrations around 1 X lO17cm - 3 the mobilities range between 1000 and 1500 cm2IV . s. However, the carrier mobility in GaAs grown in our system was lower than typical values reported for pure GaAs films [14]. One may ask if a small amount of residual AI (x < 1%) could be responsible for this effect or if a different impurity would be involved. If the first explanation would hold, the growth of high quality GaAs along with GaAlAs would require the use of a two
source system, one of them being a pure Ga source for the growth of GaAs. Carbon, which was indeed found in these films, as will be discussed in the next section, could also be responsible of the mobility lowering. By injection of H 2S into the deposition zone, n-type AIGaAs was obtained. For a constant AI concentration in the film, the electron concentration exhibits a linear dependence on the dopant pressure PH2 S (fig. 7). Such a linear dependence was also found in the halide growth of GaAs [15} and in the MOCVD of GaAlAs [12,13]. Furthermore, like was reported for the growth of Si doped MBE films [16} and also for Si or Se doped MOCVD films [17], in our experiments the measured electron concentration decreased with increasing AI mole fraction at constant injection rate of the dopant gas H 2S. This behaviour appears to correlate with the increase of the ionisation energy of the donors observed at higher AI concentrations. Beside the Al concentration, also the AsH 3 partial pressure affects the level of dopant incorporation. An increase of the AsH 3 input pressure
~31
•
KCOO1
A
K=
•
Q3
18
10
•
•
..
... .. lOTI
.
1016' - ---'-_ _" - - - - ' - _........ _ Q2 01 05 2
........._
5
........ _ " ' - - - - _ - '
10
20
50
•
---~
p¥
l06bor
Fig. 7. Carrier concentration (300 K) versus H 2S input pressure (PH,S) for various AI concentrations (x) . The accuracy of the point indicated by the open triangle at n -1 x 10 17 cm - 3 is low because of the level of background doping.
634
M. Deschler et al. / Halogen VP Eo/ AIGaAs lor optoelectronic device applications
~1
1019,--
-,
---"--t'ooo
: 2 10 bar ' + PAI M ) : 2 10" bar
cri?FiS
OP A • H ,
P
';5
AI. Go,..As(xc Q01) • AI. Go....AsI02S...035 J 3000 + MOCVO latter Stnngfellowl1311 0
: 5 10" bar
"0 2000
o 1000
rf
ne
!if'
~
~
Fig. 9. Dependence or mobility on electron concentration (T - 300 K); numbers at solid square points indicate oX AI; curve: best empirical representation or data for n-doped GaAs
10
20
30
40
(14).
50 XAI
»:
Fig. 8. Dopant concentration (300 K) versus AI concentration x; parameter: PAsH , .
by a factor of 10 yields an increase of the electron concentration by roughly a factor 2 for constant Al concentration (fig. 8). This behaviour cannot be explained from thermodynamic considerations which indicate that
(12) and therefore p..\s~/4 - [VAs] - [S;sl = n ,
(13)
i.e. a decrease of the electron concentration with increasing AsH) pressure. In these equations VAs represents an As vacancy, S;s an ionized sulfur atom on an As site and n the electron concentration at room temperature. In a study of the sulfur doping of GaAs during growth in the hydride system it was found that the incorporation rate of S supplied to the surface is also affected by the rate of growth of the film [18]. However, as shown in fig. 5, an increase in PA sH ) leads to an increase in the growth rate. One would thus predict a decrease in the dopant concentration with increasing PAsH rather than the observed increase. At the prese~t time we cannot 'offer an explanation for this discrepancy. The electron mobilities of the doped layer as a function of the carrier concentration for different
AI concentration are plotted in fig. 9. Similar mobility values are obtained in layers prepared by other growth methods [12,13]. To deposit p-type AIGaAs films, DEZn Was used as a source material. Fig. 10 shows the dependence of the hole concentration on the DEZn partial pressure for two AI concentrations. Similar to the behaviour observed for the electron COncentration upon H 2S injection, an increase of the hole concentration with P O FZ o and a decrease of the doping for higher AI concentrations in the filtn is observed. However, the slope of the curves of doping level versus DEZn partial pressure is
~1101'.l em
"
-2
P>CI =110 bar p" =5 bar
10-)
AsH)
~" L .-"--'----"-----'---""'------.J 2
5
10
20
50
_ _---.._.
100
POEZ.!!..
10"bar Fig. 10. Hole concentration (300 K) versus DEZn input pres, sure; parameter: AI concentration x.
M. Deschler et al. / Halogen VPE of AIGaAsfor optoelectronic device applications
J\5t
tion is lower in the case of Zn doping. The relation between Hall mobili ty of the p-type layers and the carrier concentration for different AI concentrations is shown in fig. 11. The mobilities are comparable to those obtained by Mori and Watanabe [12].
Al x Ga,-lCAs .,,001 • Alx Ga",As IQ25'.' 035) + MOCVO (Morl el 01 1121) o
300
0 0
200
•
I2SJo
100 +
U S)
+
•
i1 >5l( J2l
'-2>f
Ii'
635
+
11>~
+00 0
' 35)
US)
l It)
(fJ l _
+
t ~,
, 261
4.3. Optical characteristics
+ +
IlS)
•
135l
lr!'
10"
10"
p
c;y;:i
F ig. 11. Hole mob ility versus carrier con centration (T - 300 K) ; numbers at solid square po int s indic ate xA\; curve : best empirical representation of data for p-type GaAs [21].
somewhat larger than 1. This behaviour is probably caused by a precipitation of Zn after thermal cracking of the metal organic compound at the transition to the hot reactor zone or by the formation of a stable compound ZnO due to small amounts of O2 in the reactor. The dependence of the ionization energy for acceptors on the Al concentration in GaAlAs is smaller than the dependence for donors [19,20]. Thus, the reduction in doping efficiency with increasing Al concentra-
Examples of low temperature photoluminescence spectra measured on AIGaAs layers with AI concentrations of 17% and lower than 1%, with background doping levels of 1 X 1017 em"? and 2 X 10 17 cm -3, respectively, are shown in fig. 12. The spectrum measured on the AlxGal_xAs layer with x < 1 % exhibits a sharp bound exciton peak (BE) at 1.528 eV and a free to bound peak (FB), 26 meV below the BE peak. The full width at half-maximum (FWHM) of the BE peak is 1.3 meV . The spectrum measured on the AIo.17Gao.83As film shows a BE peak at 1.746 eV with a FWHM of 4 meV and a FB peak at 1.707 eV. The BE peak energies measured on both layers agree with results obtained by other authors [22,23]. Furthermore, the excitonic linewidth of the BE peaks, which is an indication of the optical quality of the grown film, corresponds to the best
iooo
300 A;",GoCNAs
A I. Go" As I x<001)
BE t746 ltV
n ~l 'lO"cm)
~
I
i
"=2 lO"cm'
800
:;
:;200
s
S
l:-
l:-
BE 1528 tV ~
600
-iii c
-iii c QI
~
FWHM = 4 "",V
~ ~ 100
l:
400
...J
FB
FWHIo1=13mtN
Q..
1707 ""
200
0
170
172
174
176
Energy (eV) ~
1.48
FB
X"" ) 1.50
1.52
1.54
Energy (eV)
156
~
Fig. 12. Photoluminescence spec tra of AlGaAs films (T- 2 K ; P - 30 mW /cm2 ) at two different AI con centrations.
636
M. Deschler et al. / Halogen VPE of AIGaAs f or optoelectronic deri ce applications
value reported for LPE, MOCVD and MBE films [24-27]. The FB peak measured on the AIGaAs film with x < 1 % is probably related to a carbon acceptor transition whereas the FB peak measured on the Alo.17GaO.R3As layer can be assigned to a silicon acceptor transition [23,28,29]. These results suggest that some impurities from the carbon parts are incorporated into the deposited films. As mentioned above, the Si contamination of our samples appears to be of an accidental nature.
5
ios trcte
4.4. Morphology, uniformity and interface sharpness The surfaces of AlGaAs films with Al concentrations lower than 50% are mirror like . At higher AI content, the surface gradually roughens. Layers grown from the gas phase at atmospheric pressure tend to have a somewhat smaller thickness at the downstream part of the substrate. Such a behaviour was also observed in the present reactor. On a 20 X 20 mnr' substrate this inhomogeneity in the flow direction was less than 15%; perpendicular to the flow an inhomogeneity below 5% was observed. The variation of the AI concentration over the thickness of films was less than
1%. As discussed above, the film composition is strongly dependent on the growth temperature whereas the other parameters have only a small effect. Therefore, in order to prepare heterostructures, the growth process has to be interrupted by switching off the HCI source flow. Also, the growth temperature has to be changed before depositing the next layer. It is helpful for keeping the film composition constant over its full depth that this composition is nearly independent of PH C1 (see fig. 4a). The interface sharpness of the AI concentration was within the resolution of our secondary ion mass spectrometry (SIMS) measurements, i.e. within 8 nm. The scanning electron micrograph of a multilayer structure in fig. 13 demonstrates the absence of any defects. Also shown in this figure is the X-ray rocking curve of the structure; it is indicative of the excellent depth homogeneity of the layers.
b
c.
.::=============-:--
Go As Su bst rote
----7 int ensi ty ( A. u.) Fig. 13. (a) Scanning electr on micrograph of cross-sec tio n through hetero structure and (b) corresponding X-ray rOCking curve .
S. Device application Using the technique described in the foregoing sections light emitting diodes were fabricated. Multilayer structures consisting of four dOPed layers were deposited on an n-type substrate. The e1ectroluminescence spectrum of such a device is shown in fig. 14 together with the J- V characteris_ tic of the LED. It may be seen that the diode exhibits a very low leakage current, which proves the high quality of the p-n junction. The etectns, luminescence signal of the light emitting diode,
637
M. Deschler et al. / Halog en VPE of AIGaA s for optoelectronic device applications
tion of the layer thicknesses and doping levels. Thi s optimization was not carri ed out for our VPE grown LEDs.
AI O ~ Go O Il" A s - LE O
1 , 31 mA
p e GaAs p
e
Al os Gao ~ As
0 :' iJm 2 ~ ~m
\ n e Alo ~ Goor As n e Galu
9S 30
:~
.
~m
6. Conclusion
lJm
II
,
b .--
i
IV
I
I
..
, III
,..
r"I
II
T
i
Fig. 14. (a) Electroluminescence spectrum of LED ; (b) 1- V characteristic.
measured at room temperature, exhibits a maximum at a wavelength of 751 nm, as expected for an Al concentration in the active layer of 19%. On similar LPE grown structures up to 10 times larger power efficiencies were observed. However, it should be noted that this result required optirnizaTable 1 Constants in eq. (15) for
~G o
We demonstrate in this paper that the halid e transport system is a suitable method for growing device grade AIGaA s material. Thereby. the carbon shielding of all parts coming in contact with the corrosive Al species proves to be essential to reduce the background doping level and to improve the electrical and optical quality of the grown layers. The process temperature is one of the major parameters governing the composition of the deposited films. The growth behaviour may be reproduced by a model which indicates that the growth reactions are essentially thermodynami cally controlled. In the halide system, sulfur and zinc are found to be well-behaved n- and p-type dopants respect ively for obtaining carrier concentrations up to 10 18 cm- 3 • As also found in MBE. MOCVD and LPE grown films, an increase of the Al mole fraction leads to a reduction of the concentrations of the ionized donor and acceptor atoms. The material obtained by this technique appears to be suitable for the fabrication of devices.
Appendix
The temperature dependent equilibrium constants of the reactions described in section 2 can
for different species
Compounds
A
B
C
H2 HC I G a(I) AI(1) GaCI AICI AICI ) AS4 GaAs(s) AIAs(s)
- 1938.4 - 23915 -682.7 297.04 -20290 - 15215 -144750 31017 - 24143 - 32497
12.623 -2.005 30.339 41.323 2.6233 5.8599 54.798 54.957 55.552 56.372
-6.52 -6.34 -6.65 - 7.59 - 8.93 -8.94 -19.59 -19.82 - 10.46 -10.5
E( xI0 5)
D
-600
-13000 0 0 19000 36500 -118500 61500 0 0
-39 -55 0 0 0 5.5 - 7.5
-1.5 - 140 -75
Rd.
(30) (30) (31) (30] (31) (30) (30) (32) [31,33) [32]
638
M. Deschler et 01. / Halogen VPE of AIGaAsfor optoelectronic deviceapplications
be expressed by means of the standard Gibbs free energies .1Gjo: K, = exp( -.1G,o/RT),
(14)
with .1Go =A +BT+ CT In(T)+D/T+ET 2 •
(15)
R represents the gas constant, T the temperature and A to E are constants specific to each component. These constants are summarized in table 1 for the main species appearing in the deposition reactor. Acknowledgements
The authors would like to thank J. Korec for fruitful discussions on the thermodynamics of the growth process. Thanks are also due to K.H. Goetz and W. Richter for photoluminescence measurements and discussions. We are grateful to M. Maier for performing SIMS measurements and to H. Monchs for scanning electron micrographs. References [1] H. Kressel, J. Electron. Mater. 3 (1974) 747. [2] T. Nakanisi, I. Crystal Growth 68 (1984) 282. [3] M. Heiblum, E.E. Mendez and L. Osterling, I. App!. Pbys. 54 (1983) 6982. [4J K.H. Bacbem and M. Heyen, in: Proc. 8tb Intern. Symp. on GaAs and Related Compounds, Vienna, 1980, Inst, Phys. Conf. Ser. 56, Ed. H.W. Thim (Inst. Phys., London-Bristol, 1981) p. 65. [5] K.H. Bachem and M. Heyen, I. Crystal Growth 55 (1981) 330. [6] M.B. Panisb and M. Illegems, in: Progress in Solid State Chemistry, Vol. 7, Eds. H. Reiss and 1.0. McCaldin (Pergamon, Oxford, 1972) p. 39. ,7] J. Korec and M. Heyen, I. Crystal Growth 60 (1982) 297. [8] W.J. Riedl, in: Advances in Epitaxy and Endotaxy, Eds. H.G. Schneider, V. Ruth and T. Kormany (Elsevier, Amsterdam, 1976) p. 97. [9] I. Bloem and L.J. Giling, in: Current Topics in Materials Science, Vo!. 1, Ed. M. Kaldis (North-Holland, Amsterdam, 1978) p, 147. [10J P. van der Putte, L.I. Giling and I. Bloem, I. Crystal Growth 31 (1975) 299.
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