HAL Id: jpa-00222244
https://hal.archives-ouvertes.fr/jpa-00222244
Submitted on 1 Jan 1982
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
INFLUENCE OF MBE GROWTH CONDITIONS ON THE PROPERTIES OF AlxGa1-xAs/GaAs
HETEROSTRUCTURES
H. Morkoç
To cite this version:
H. Morkoç. INFLUENCE OF MBE GROWTH CONDITIONS ON THE PROPERTIES OF AlxGa1- xAs/GaAs HETEROSTRUCTURES. Journal de Physique Colloques, 1982, 43 (C5), pp.C5-209-C5- 220. �10.1051/jphyscol:1982525�. �jpa-00222244�
JOURNAL DE PHYSIQUE
CoZZoque C5, suppZ6ment a u n012, Tome 43, dgcembre 1982 page C5-209
INFLUENCE OF MBE GROWTH CONDITIONS ON THE PROPERTIES OF A ~ ~ G ~ ~ - ~ A S / G ~ A ~ HETEROSTRUCTURES
Dept. o f E Z e c t r i c a Z E n g i n e e r i n g , Coordinated S c i e n c e Lab., U n i v e r s i t y o f I Z Z i n o i s , 1101 W. S p r i n g f i e l d , Urbana, IZ 61801, U.S.A.
R6sum6. Des structures hBterojonctions GaAs/Al Gal-,As
X
ont 6tB obtenues par Bpitaxie par jets molBculaires pour un grand Bventail de paramstres de croissance. Alors que des couches de haute qualit6 cristalline peuvent Gtre Blaborees .2 des temp6ratures de sub- strat d'approximativement 550'-650°C, la qualit6 de AlxGal-xAs s'amb- liore quand la temy6rature du substrat est portee a environ 750°C.
pour beaucoup de structures, cependant, il est necessaire dVelaborer des h6t6rojonctions simples ou multiples avec des dimensions suffisam- ment petites pour obtenir des effets quantiques. Dans de tels cas, un compromis doit Btre fait dans les conditions de croissance pour r6aliser la meilleure performance globale. Un effott doit Btre port6 a 11am61ioration des propriet6.s interfaciales de ces structures. A l'aide dVh6t6rostructures simples dopage modul&, les propri6tBs d'interface binaire/ternaire et ternaire/binaire ont &ti5 6tudi6es pour de faibles champs Blectriques et les conditions de croissance optimi- sBes. A forts champs, les proprietes ont 6te d6duites des performances FETs GaAs ayant des couches tampon AlxGal-xAs. Des interfaces relati- vement droits ont 6tB obtenus pour une grande gamme de conditions de croissance dans le cas des structures ternaire/binaire. Des conditions de croissance beaucoup plus strictes ont cependant St5 necessaires pour les structures inverses. I1 a Bt6 observe qu'une couche mince de GaAs amBliore la surface de la couche de AlGaAs et que son incorpora- tion facilite les conditions de croissance.
Abstract. G ~ A S / A ~ ~ G ~ ~ - ~ A S heterojunction structures have been qrown by molecular beam epitaxy under a wide range of qrowth parameters.
While hiqh quality GaAs layers can be grown in a substrate temperature range of approximately 550-650°C, the quality of AlxGal-xAs has been
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982525
C5-210 JOURNAL DE PHYSIQUE
found to improve as the substrate temperature is increased up to about 750°C. Y.any structures, however, require single or multiple interface heterojunctions with dimensions sufficiently small to give rise to quantum effects. In these cases a compromise in growth con- ditions must be made to obtain the best overall performance. Emphasis must be placed on improving interfacial properties of these struc- tures. Using single interface modulation doped structures, the inter- facial properties of binary on ternary and ternary on binary hetero- junctions were studied at low electric fields and the growth condi- tions were optimized. The high field properties were deduced using the performance of GaAs FETs with A1 Gal-xAs buffer layers. Rela- x tively smooth interfaces have been obtained in a wide range of growth conditions for the ternary on binary structures. Precisely controlled growth conditions were, however, reuuired for the inverted structures.
A thin layer of GaAs was observed to smooth the surface of the AlGaAs layer and its incorporation allowed growth conditions to be relaxed.
1. Introduction
For its superior transport properties, GaAs has become a widely used semiconductor for high speed and low noise devices.' In addition the direct bandgap nature of GaAs allowed the development of semi- conductor lasers.2 However, room temperature CW operation of lasers was possible only by utilizin~ the larger bandaap and smaller refrac- tive index of lattice matched AlxGal-xAs in GaAs/Al Gal-xAs hetero-
X
junctions. With the advent of molecular beam epitaxy, it became uossible to develop many novel structures for fabricatinq devices as well as studying basic physical phenomena. Most of these structures reuuire extremely thin epitaxial layers and in many cases larae doping concentrations. These conditions can be met by the use of hetero- junction structures. The N33E technique possesses all the recuired features to meet the requirements imposed by these new classes of devices. Modulation d.oped structures, quantum well structures, and multilayered device ~tructures,~ such as those utilizing AlxGal-xAs as a buffer6 or large bandsa~ emitter in bipolar transi~tors,~ are a few among many reauiring heterojunctions.
While MBE is capable of producing the thin layers and abrupt heterojunctions, the growth conditions must be explored to obtain high performance interfaces in terms of transport parallel and perpen- dicular to the interfaces. Attention must also be paid to the effects of growth conditions on the optical properties of these structures,.
In this paper the effect of growth conditions on the properties of AlxGal-xAs/GaAs heterointerfaces will be described. Emphasis will be placed on the transport parallel to the interfaces at low and high electric fields. A brief correlation between the optical and electrical properties of heterojunctions will also be presented.
2. Experimental Procedure
Structures were grown by molecular beam epitaxy on (100)
oriented (or, in a few cases 2' off (100) toward 110) Cr doped semi- insulating substrates. The structures intended for optical measure- ments were grown on Si doped substrates instead. The substrates were polished using pellon cloth and optical paper saturated with Br- methanol until a mirror-like surface was obtained. Following a free chemical etch in H2S04:H 0 :H 0 (5:l:l) and rinsing in DI water, the
2 2 2
dried substrate was mounted on a molybdenum block with high grade indium. Following an initial pump down, the block was loaded into the system airlock and outaassed at 400°C to remove water vapor and other volatile contaminants. Once the airlock had been pum~ed down to the lo-' Torr range, the substrate block was transferred into the growth chamber.
Prior to the initiation of growth, the substrate was outgassed at a temperature of 630°C, as determined by an infrared detector.
The growth was started after the substrate temperature was lowered to between 580° and 590°C. Further changes in growth cond.i'.ions depended on the particular structure bein? grown. These will be described later.
Hall measurements were made using van der Pauw clover leaf patterns. Ohmic contacts were obtained by alloying Sn balls to the samples at 400°C for about 1.5 min in an H2 atmosphere. A cryostat was used to allow mobility measurements down to 10 K.
The high field properties were characterized by the use of field effect transistors (FETs). The device fabrication began with the
C5-212 JOURNAL DE PHYSIQUE
formation of mesas by etchinq in 3:l:l (H20:H202:HF). The etching process was continued until the current conduction between mesas was not noticeable at & 50 V. Source and drain contact regions were then defined using AZ1450J positive photoresist and a AuGe/Ni/Au metalliz- ation was evaporated and lifted off. Ohmic contacts were obtained by alloying at 500°C for 1 min in H2 atmosphere. The qate pattern was then defined, part of the channel layer was etched for a recessed gate structure, and A1 gate metallization was evaporated and lifted off.
The details for each type of FET will be described later.
3 . Result and Discussions
General: The two main parameters affecting the properties of GaAs and particularly AlxGal-xAs layers grown by MBE are the sub- strate temperature and the group V/III flux ratio.* Reducing the group V/III ratio used during growth improves the electrical and optical quality of the epitaxial layers. Care, however, must be exercised to avoid group I11 rich conditions which lead to a degraded surface morphology. This is not tolerable for devices requiring smooth interfaces. It should be noted though, that the bulk proper- ties improve unless the group I11 rich conditions are excessive. For the best overall performance slightly group V rich growth conditions must be employed.
The flux of constituents required for this condition depends on the substrate temperature very strongly. At higher temperatures, the surface lifetime of As4 is shorter, resulting in a shift toward the group I11 rich conditions. At extremely high temperatures, e.g.
T > 700°C, it is possible that As4 may be cracked to result in As2.
This process may be responsible for the hiqh quality bulk layers and interfaces obtained at these temperatures.
Effect of Substrate Temperature and Group V/III Ratio: For many years molecular beam epitaxy was touted to be a very low temperature growth process. While there are many advantages to a low temperature process, e.g., sharp doping profiles, recent advances indicate that the optical and electrical properties of the epitaxial layer degrade if the growth temperature is kept low indiscriminately. This is particularly true for semiconductors containing A1 which requires a relatively high substrate temperature to provide a reasonable surface mobility which is essential for high quality.
Epitaxial GaAs layers of electronic device quality can be grown by molecular beam epitaxy in an approximate substrate temperature range of 550-650°C. For optical devices the higher end of this
temperature range must be employed.' ~t the upper end of this temper- ature range the control of group V/III ratio may be relaxed. At
lower temperatures, however, care must be taken to avoid Ga vacancies, i.e., the group V/III ratio should be reduced. Nevertheless GaAs layers with qood properties can be prepared in a wide range of substrate temperatures.
Growth conditions for high quality A1 Gal-xAs require more X
stringent control, particularly of the substrate temperature. The importance of the substrate temperature was first reported in 1975. 10 Improved transport properties in bulk AlxGal-xAs layers grown at high temperatures were later reported.8 In all of these studies the substrate temperature was kept below 650°C. Since higher substrate temperatures lead to better optical and transport properties, a series of experiments was carried out where AlxGal-xAs growth took place under a variety of temperatures, up to about 750°C. We will first discuss the question of surface morphology.
In one set of experiments the group V/III ratios were kept constant at about 4 while the substrate temperatures were varied between 600°C and 750°C. In the second set the substrate temperatures were kept constant and the group V/III ratios were varied between 4 and 20. The surface morphology for mole fractions below 0.25 did not
Figure 1. Phase contrast photomicrograph of 1.5 pm thick AlxGal-xAs layers grown at constant group V/III ratio and
substrate temperatures of (a) 600°C, (b) 620°C, ( c ) 635OC, (dl 650°C, (e) 680aC, (f) 700°C.
C5-214 JOURNAL DE PHYSIQUE
show appreciable dependence on the substrate temperature. For increased mole fraction however, the morphology showed a profound dependence as depicted in Fiq. 1. Below about 630°C and above 700°C excellent surface morphologies can be obtained. Within these bound- aries, however, the surface morphology was observed to degrade. The improvement of the surface morphology at and above 700°C can be attributed to the increased surface mobility of Al. The degradation in the intermediate substrate temperature range is not yet fully understood.
In this intermediate range, increasing the group V/III ratio from 4 to 20 in succession steadily improved the surface morphology.
However, even at a ratio of 20, the surface morphology was inferior to that obtained above 700°C employing a ratio of 4.
Transport Parallel to the Heterointerfaces: Recently developed high speed devices with extremely short dimensions require high doping concentrations and very thin epitaxial layers. Heterojunction struc- ture may in fact be required to obtain these characteristics. A good example is the modulation doped AlxGal-xAs/GaAs heterostructure 11 where the electrons leave their parent donors in the doped
AlxGal-xAs and diffuse to the heterointerface where they are confined on the undoped GaAs side. Effective se~aration of electrons and donors leads to extremely high mobilities and slightly larger veloc- ities than obtainable in bulk doped GaAs. This is a direct result of the absence of impurity scattering. The high electron velocity is currently being studied for application to high speed logic devices by many laboratories. l2 The importance of the modulation doped structures from the point of view of crystal growth is that the electrons are confined to the heterointerface and their transport properties can be used to assess the heterointerface quality. The band diagram of such a single interface modulation doped heterostruc- ture is shown in Fig. 2.
By studying the transport parallel to the heterointerface in single period MD heterojunctions with either the GaAs on top of AlxGal-xAs ("inverted" structure) or the AlxGal-xAs on top of GaAs
("Normal" structure), the growth conditions can be optimized for each type of interface. Investigations so far have led to the discovery that the substrate temperature is the dominant parameter. The effect of substrate temperature was investigated using these single interface modulation doped structures. A schematic diagram of the inverted structure is shown in Fig. 3.
The 77 K electron mobilities in normal and inverted modulation doped structures grown in a substrate temperature range of 600-780°C
Figure 2. Band diagram of an A10,35Ga0.65A~/GaAs modulation doped heterostructure.
0.2k GaAs r, ncr_
0 . 2 ~ GaAs Buffer
GaAsZCr Substrate
Figure 3. Schematic diagram of an inverted modulation doped heterostructure.
JOURNAL DE PHYSIQUE
Figure 4 , Electron mobility at 77 K of normal and inverted modulation doped heterostructures grown in a substrate temperature range of 600°C - 780°C.
are shown in Fig. 4. The conclusions that can be drawn are as follows:
(i) The interface properties of normal AlxGal-xAs/GaAs hetero- junctions are fairly insensitive to the growth temperature below 700°C but degrade sharply above 700°C. This degrada- tion is a result of As loss during the growth of GaAs at these extremely high temperatures.
(ii) The quality of the inverted G ~ A S / A ~ ~ G ~ ~ - ~ A ~ interface improves steadily up to 700°C and degrades above 700°C again as a result of As loss during the GaAs qrowth. This indicates that at 700°C, a smooth A1 Gal-xAs surface can be obtained. High
X
quality AlxGal-xAs was grown up to 750°C as discussed earlier.
(iii) The best overall heterointerface in multiple period modula- tion doped, double heterojunction laser, and multiquantum well laser structures can be obtained at a growth temperature of 700°C. This assertion has been verified by photoluminescence studies13 and demonstration of 28 GaAs quantumwell lasers. 14 Applications to Heterojunction GaAs FETs: GaAs FETs with larger bandgap A1 Gal-xAs buffer layers and other device structures using
X
GaAs growth on A1 Gal-xAs require a high quality interface. As
X
discussed in the previous section, high guality heterointerfaces can
Gate
(AI,Ga)As Buffer
GaAs
S.
I.
Substrate YP-213Figure 5. Schematic cross-section of a GaAs FET with A1 Gal-,As
buffer layer. Undulations at the heterointerrace represent interfacial roughness caused by non-optimum srowth condi- tions.
be obtained when the structure is grown at 700°C. If grown at lower temperatures, interface roughness can lead to specular scattering of electrons at the interface as shown in Fiq. 5. For illustrative purposes the interface roughness is exaggerated; it is expected to be on the atomic scale only. This roughness was observed to be very sensitive to the growth conditions, particularly the substrate temper- ature. Deviations of about 5'C in substrate temperature could be detected with photoluminescence measurements made on multi-quantum well heterostructures.
In order to relax the dependence on the substrate temperature, a thin undoped GaAs smoothing layer can be grown on the A1 Ga As
x 1-x buffer layer prior to the active channel layer. This GaAs layer serves to smooth interfaces when the structure is grown under non- ideal conditions. The schematic diagram of this structure is shown in Fig. 6. The minimum required thickness of the smoothing layer is difficult to determine accurately from the electrical measurements.
For this set of experiments, a 2 0 0 f j GaAs smoothing layer was used successfully; Heterostructures grown under conditions deviating slightly from ideal should require only a thin smoothing layer.
Photoluminescence measurements on single quantum well structures should be useful for determining this thickness.
Electron velocity vs. depth profiles deduced from the dc charac- teristics of FETs with and without smoothing layers are plotted in Fig. 7 for various growth temperatures. The effect of the smoothing
JOURNAL DE PHYSIQUE
Gate
As Active
s
\-
Ga As Smoothing (AI,Ga)As Buffer
GaAs Buffer
S.
I.
Substrate YP-Z12Figure 6. FET structure of Fig. 5 with the addition of a 200 fi thick GaAs smoothing layer at the heterointerface.
No Smoothing Layer 580°C 640°C 700°C
2008 GaAs Smoothing Layer
I I I I I I I I I I 1 1 1 1
Distance (1008 /div) YP-191
Figure 7. Electron velocity vs. depth profiles, with and without a smoothing layer, in the GaAs channel of a GaAs FET with A1,Gal-,As buffer layers grown between 580 and 700°C.
Figure 8. Drain I/V characteristic of a GaAs FET with an A1,Gal- As buffer layer grown at 700°C. FETs having a GaAs smootging layer and grown between 640 and 700°C showed similar characteristics.
layer is most profound at 580°C substrate temperature as the graded interfacial region is reduced from 260 A to 150 by its incorporation.
Drain I-V characteristics of an FET grown under ideal conditions are shown in Fig. 8. The characteristics are free of looping and imply a transconductance of about 160 mS per mm of gate width.
4. Conclusions
The properties of GaAs, AlxGal-xAs epitaxial layers and G ~ A S / A ~ ~ G ~ ~ - ~ A S heterojunction structures grown by molecular beam epitaxy depend on the growth conditions. In particular, the inter- facial properties of the heterojunction structures exhibit a veqy pronounced dependence on the substrate temperature and to some extent on the group V/III ratio. Bulk A1 Gal-xAs layers were observed to
X
grow with extremely good surface morphology below 630°C and above 700°C. Using single interface modulation doped structures, it has been determined that heterointerfacial properties of AlxGal-xAs grown on GaAs are not sensitive to growth conditions, in contrast to
inverted structures which show a strong dependence on substrate temperature. The properties of the inverted structures are best when
C5-220 JOURNAL DE PHYSIQUE
grown at 700°C. Optical properties of multiquantum well structures are dominated by the binary on ternary interface and thus a 700°C growth temperature is optimum.
Field effect transistor structures with AlxGal-xAs buffer layers are also optimized at 700°C substrate temperature. This was deter- mined primarily by the profile of the electron velocity near the heterointerface. At 700°C an interfacial sharpness of 40 was obtained. By using a GaAs smoothing layer between the AlGaAs buffer layer and the active layer, the dependence on the substrate temper- ature was observed to decline. This smoothing layer covers up the roughness present on the surface of the AlxGal-xAs buffer when grown under non-optimum conditions.
Acknowledqements
This project is funded by the U. S. Air Force Office of Scien- tific Research and the Joint Services Electronics Program. The author would like to express his sincere thanks to Drs, A. Y, Cho, E. Mendez and E. Brody for many stimulating discussions and to T. J.
Drummond, W. Kopp, J. Klem, R. Fischer, W. G. Lyons, S. L. Su and R. E. Thorne for technical assistance.
References
1. See special issue of IEEE Trans. on Electron Dev. ED-25 (1978).
2. CASEY, H.C. Jr. andM.B.PARISH, Heterostructure Lasers, Part A, Academic Press, N.Y. (1978).
3. DINGLE, R., H. L. STORMER, A.C. GOSSARD and W. WIEGMANN, Appl.
~ h y s . Lett. 37 (1978) 805.
4. DINGLE,R., Festkorperproblem g (1975) 21.
5. THORNE, R.E., S.L. SU, W. KOPP, R. FISCHER, T.J.DRUMMOND and H. MORKOC, J. Appl. Phys. 53 (1982) 5951.
6. MORKOC, H., W. KOPP, T.J. DRUMMOND, S.L. SU and R.E. THORNE, IEEE Trans. on Electron Dev. ED-29 (1982) 1013.
7. KROEMER, H., Proc. of IEEE
2
(1982) 13.8. MORKOC, H., A.Y. CHO and C RADICE, J. Appl. Phys. 2 (1980) 4882.
9. CHO, A.Y., H.C. CASEY, Jr., C. RADICE and P.W. FOY, Electron.
Lett. 16 (1980) 59.
10. CASEY, H.C. Jr., A.Y. CHO and P.A. BARNES, IEEE J. Quant. Elect.
QE-11 (1975) 467.
11. DRUMMOND, T.J., H. MORKOC and A.Y. CHO, J. Appl. Phys. 52 (1981) 1380.
12. MORKOC, H. in The Technology and Physics of Molecular Beam
Epitaxy, Ed. E.H.C. Parker and B.M.G. Dowsett, Plenum, N.Y.(1983).
13. MENDEZ, E. and H. MORKOC, unpublished.
14. MORKOC, H., T.J. DRUMMOND, M.D. CAMRAS and N. HOLONYAK, Jr., Appl. Phys. Lett. &(1982) 18. I
