Liquid phase electroepitaxy

Surface

and Coatings

LIQUID

PHASE

L. PERALDO

Technology,

29 (1986)

1

- 12

ELECTROEPITAXY

BICELLI

Department of Applied Physical Chemistry of the Milan Polytechnic, Research Electrode Processes of the Consiglio Nazionale delle Ricerche, Piazza Leonardo 32 20133 Milan (Italy) (Received

Centre on da Vinci,

March 17, 1986)

Summary Liquid phase electroepitaxy is surveyed, comparing its potentialities with those of liquid phase epitaxy and considering the connected problems. Since the electric current circulating in the proper direction leads to Peltier cooling at the substrate-solution interface and may also cause solute electromigration to the growth interface, it is possible to control the overall epitaxial growth process through varying the current density, while the temperature of the growth system is kept at the equilibrium value. The principles of the method, the growth kinetics, particularly considering the interface stability, the dopant segregation and the electroepitaxy of multicomponent systems, more especially of ternary and quaternary compounds, are discussed and the more recent developments of the technique outlined.

1. Introduction Most types of electronic and optical devices are based on epitaxial layers grown from liquid solutions on bulk single-crystal substrates. In liquid phase epitaxy (LPE), the substrate (e.g. GaAs) is thermally equilibrated with the solution (e.g. a gallium-rich arsenic solution) and growth occurs upon supersaturation of the solution brought about by a controlled temperature decrease. Solidification takes place preferentially, although not always exclusively, on the substrate. The solute depletion at the growth interface creates a concentration gradient in the solution and thus solute transport for the growth process is provided by diffusion [ 11. External control of diffusion is hardly possible; consequently, the overall growth process control is limited to the control of the temperature changes as a function of time. Hence, the precise control of the microscopic growth rate of segregation and of the defect structure still presents a problem. It has been recognized that this control could be achieved through the passage of electric current across the 0257~8972/86/$3.50

@ Elsevier Sequoia/Printed

in The Netherlands

2

solution-substrate interface while the overall system temperature is kept constant. In fact, the current flowing in the proper direction induces Peltier cooling in the immediate proximity of the interface and causes solute electromigration to the growth interface. Although this approach has also been proposed for the growth of bulk single crystals from the melt [ 2 - 41, it has not been put into practice, owing to convective instabilities in the melt and to further complexities introduced by Joule heating [5]. Current-controlled growth was successfully carried out in the 1970’s in an LPE configuration [6] where Joule heating presents no significant difficulties because the dimensions of the substrate and the solution are relatively small. Further research showed that under high current densities high growth rates can be achieved without interference from interface kinetic phenomena [7]. Liquid phase electroepitaxy (LPEE) has therefore been employed to obtain films of various semiconductors (among them InSb [6], GaAs [l, 8 - 211, InP [22], GaAlAs [23 - 281, HgCdTe [29, 301 and garnet layers [ 311. Its advantages have been demonstrated in controlling the interface temperature and the growth rate [6, 12, 15, 281, the interface stability [7, 321, the composition [ 23 - 28, 311, the doping [6, 8, 9, 11, 12, 17, 331 and the recombination centres (in particular for GaAs [ 18]), as well as in improving the surface morphology, the defect structure [14] and the electronic properties of the layer [ 11, 13, 14, 251. Much of this research activity has been performed by H. C. Gatos and coworkers who developed a theoretical model for binary compounds [ 151, successively extended to multicomponent, more especially ternary and quaternary, compounds [28], which quantitatively related the growth velocity to the growth parameters. They also suggested a theoretical model of dopant segregation [ 171. All these models were found in agreement with experimental data on electroepitaxial growth [ 15, 17, 281. Various Soviet Union researchers have also studied LPEE, unfortunately publishing their results in Soviet journals that are practically inaccessible in the West. In this connection, it is worthwhile remembering that the introduction of an electric current in LPE was independently suggested by Golubyev et al in the Soviet Union [34] and by the Gatos group [6] in the United States. The Soviet scientists discussed many of the problems considered by Gates and coworkers. In fact, LPEE was examined as a new method of epitaxial layer growth [ 35 - 381, and its potentialities in assuring optimal growth conditions [39] and composition control 1401 were evidenced. Films of various binary and ternary compounds were studied, e.g. GaAs [ 41 - 441, Al,Ga, _,As [44 - 461, (Al, Ga, 1n)As [ 471, GaSb [ 35, 481, AlO,,Gao.sSb [35], GaAs,_,Sb, [48], Ali _,Ga,PandIn,_,Ga,P [49] and CdSnAs, [50], also with discussion of the related apparatus [ 511. The experimental results, particularly those for GaAs [ 42, 431 and Al,Ga, ,As [45, 461, were theoretically interpreted, more especially considering the Joule heating effect [ 52, 531. In the present review, the Gatos approach will mainly be followed.

3

2. Principles

and methods

LPEE utilizes the standard LPE configuration modified to permit the passage of electric current. The substrate and solution, being different electrical conductors, have different thermoelectric coefficients. Thus, the current flow across their interface is accompanied by heat exchange. More precisely, heat absorption results when an n-type substrate has a positive polarity or a p-type substrate a negative polarity. This Peltier cooling is proportional to the difference in Peltier coefficients, and to the current density. For A”‘BV semiconductors and at the temperature range used in LPEE, the absorbed heat is around 1 W cme2 at a current density of about 10 A cm-2. In an equilibrated isothermal system, this heat absorption typically decreases the interfacial temperature by 0.1 - 1 “C, depending on the substrate thickness and thermal conductivity, and induces supersaturation of the liquid phase, thus leading to compound solidification as in standard LPE. Simultaneously, current flow (in the same direction) induces an electric field, in turn leading to the electromigration of the solute towards the substrate (arsenic in Ga-As solutions) at a velocity proportional to the solute mobility and to the electric field. This results in further supersaturation at the growth interface and in continued growth. It is thus apparent that electric current affects two key factors in epitaxial growth, i.e. the interface temperature and the solute transport. Figure 1 shows a schematic representation of a typical growth cell. n- or p-type doped GaAs substrates (1.2 cm X 1.2 cm) having a thickness varying from 200 to 300 pm were employed. Depending on the dimensions of the solution well, the growth area ranged from 0.2 to 1.4 cm2 and the height of the solution from 0.2 to 2 cm. In a typical growth experiment [ 131, the cell was brought to the desired temperature (800 - 1000 “C) and the solution was equilibrated on a dummy substrate for 2 - 5 h, depending on the melt height. The substrate was then brought into contact with the melt, and electric current at a current density of 0.3 - 60 A cmp2 was applied through the growth interface. The epitaxial

TEMPERATURE

ELECTRIC INSULATOR

-

ELECTRIC CONTACT

ELECTRIC CONTACT

-

+ Fig. 1. Schematic representation of the growth cell used for LPEE of GaAs from a Ga-As solution and of the temperature profile resulting from Peltier cooling at the substratesolution interface and from Peltier heating at the substrate-electric contact interface (taken from ref. 15).

4

layer thickness ranged from 1 to 150 pm and the growth time from a few minutes to a few hours. Two types of thickness fluctuations were observed: random fluctuations associated with non-uniform current flow through the substrate, mainly resulting from bad electric contacts, and systematic fluctuations associated with temperature gradients or convective flow in the solution [ 131. The solution of these problems will be discussed below.

3. Growth

kinetics

3.1. Model and hypotheses In the theoretical model of LPEE of the Gatos group [ 151 it is assumed, as in LPE [ 11, that the solute transported to the advancing interface is removed from the solution through epitaxial growth on the substrate only. Solute transport due to temperature gradients in the solution is neglected, but that due to convective flow is taken into account. Considering the binary phase diagram of Fig. 2, electroepitaxial growth is approximated as a transition from point A, (defined by the original equilibrium temperature T, and the solute concentration Co) to point A, on the BINARY

SYSTEM

I

I

,

I

,

I (

C’

I ,

co SOLUTE CONCENTRATION

Fig. 2. Schematic representation tem and the transition involved

of the liquidus in electroepitaxial

line in the phase diagram of a binary growth (taken from ref. 28).

sys-

liquidus line uniquely determined by the new interface temperature Ti = To + AT, (AT, is negative when T1 < To) and a new solute concentration C1 at the interface is defined. As the solid composition of a binary compound is known by definition, the growth velocity can be obtained by solving the solute transport equation. The basic assumption made here (whose validity has been experimentally verified [ 151) is that growth proceeds under thermal quasi-equilibrium; i.e. that the A0 -+ A, transition follows the liquidus line. The widely adopted isothermal diffusion treatment of LPE [l] is modified by including a current-dependent electromigration transport term [ 151. In solving the transport equation, the term u(X/&) is neglected (u is the growth rate and x the distance from the advancing growth interface) as it is very small in comparison with the diffusion and electromigration terms.

5

Moreover, it is assumed that the solute concentration for either x = 00 or x = 6 (where 6 is the thickness of the diffusion boundary layer) remains constant during the growth process. This occurs when feed material is present on top of the solution or when the thickness of the grown layer is very small. Obviously, in the absence of feed material, the maximum time of growth is limited owing to solution depletion and is of the order of h*/D, where h is the height of the solution and D is the solute diffusion coefficient 1541. The LPEE growth kinetics from solutions having a very small volume was examined in refs. 55 and 56. 3.2. Results The overall velocity in LPEE is given by the contribution sion transport (uT) and of the electromigration (uE): u = uTfk(E,8,t)

+

of the diffu-

uE

where fk accounts for solute transport other than diffusion (e.g. by convective flow) and E is the electric field intensity [ 151. In metallic-type solutions (as in those of A”‘BV compounds), after an initial transient fk = 1 in the absence of convection (h small) and fk = (rDt)“* in the presence of convection (h large). In the first case, the electroepitaxial growth is sustained through solute electromigration to the interface, while the contribution of Peltier cooling is relatively small. The growth velocity is proportional to the current density since both the electric field in the solution and the Peltier cooling are proportional to it. The Peltier-effect contribution to the growth velocity decreases with time as t-“2, which is behaviour typical of a diffusion-controlled process, while the contribution due to electromigration is time independent. In the second case, the presence of convection in the solution enhances the contribution of the Peltier effect which otherwise is important only in the initial growth stages, as already shown. The primary reason for this enhancement is the formation of a boundary layer with a pronounced concentration gradient which increases mass transport by diffusion. After a growth period t S 6*/4Ll the growth becomes independent of time. In summary, electromigration and the Peltier effect render electroepitaxial growth sensitive to the direction and density of the electrical current, to the solution electrical resistivity, to the carrier concentration and conductivity type (n or p) of the substrate and to the thickness of the latter. These parameters provide a high flexibility in controlling the growth process, which is unattainable in standard LPE. As to the influence of the substrate thickness, Peltier cooling at the substrate-solution interface is accompanied by Peltier heating at the substrate back-contact interface (Fig. 1). Thus, heat transport across the substrate affects the degree of cooling at the interface; indeed, AT, at the growth interface increases with increasing substrate thickness [ 571. Therefore, within thin substrates in the absence of convection, the electroepitaxial

6

growth is dominated by electromigration; in contrast, with thick substrates and in the presence of a significant convective flow, it is dominated by Peltier cooling. In the first case, LPEE takes place under essentially isothermal conditions, and as expected epitaxial layers are formed exhibiting improved surface morphology and decreased defect density [7, 141. The model was found to be in excellent agreement with extensive experimental data on the LPEE of GaAs from a Ga-As solution [ 151. As to LPEE from solutions having a limited volume [ 561, the limitation of the solution height causes a reduction of the influence of diffusion on the growth rate and results in better homogeneity of the grown layer. Moreover, the elimination of convective mixing in the solution volume allows more easy control of the growth process. 3.3. Interface stability A theoretical treatment has also been presented which defines the unusual features of LPEE with regard to interface stability [7]. In ref. 7 it was shown that the phenomena dominating electroepitaxial growth kinetics are also of fundamental importance to the interface stability. On the basis of the thermodynamic constitutional supercooling criterion as well as of the more rigorous dynamic criterion, electromigration was shown to play a significant role as an interface-stabilizing factor. Moreover, interface stability is sensitive to substrate characteristics such as carrier concentration, conductivity type and thickness, which all determine the Peltier effect at the interface. The treatment explains the experimentally attained stable electroepitaxial it defines the geomgrowth with rates as high as 25 pm min -‘, Furthermore, etry of the LPEE configuration and the growth parameters (current density and polarity, temperature and substrate characteristics) leading to the optimization of the surface morphology. 4. Dopant

segregation

In considering dopant segregation in LPEE, it is assumed that the dopant concentration in the solution is sufficiently small for it to have no appreciable effect on the crystal growth velocity. Moreover, diffusion of impurities within the solid as well as interface phenomena, such as adsorbed layers on the substrate surface, are not taken into account [ 171. In developing the model, a similar approach is used to that employed in the theoretical treatment of growth kinetics. Two representative cases have been analysed. In the case of virtual absence of convection, the growth kinetics is controlled by electromigration and the growth velocity is time independent. When the dopant electromigration proceeds towards the growth interface, the effective distribution coefficient heff (the ratio between the dopant concentration in the solid at the interface and in the solution at time zero) increases with increasing current density and with increasing time for a constant current density.

7

When convection is present in the solution, the role of the Peltier effect becomes significant, as usual, and after a transient period growth proceeds under essentially steady state conditions. keff decreases with the magnitude of the increasing convective flow for a constant current density even below the value of the equilibrium distribution coefficient. Since Peltier cooling increases with substrate thickness, heff depends on the thickness for a constant current density and solution height. The experimental verification of the proposed model has been carried out for the electroepitaxial growth of tin-doped GaAs [ 171. Further improvements of the theoretical model were performed by Mazuruk and Bryskiewicz [ 581, and, for better agreement with some experimental results [ 8, 9, 591, by Bryskiewicz [ 601. 5. Multicomponent

systems

In multicomponent systems, such as ternary and quaternary compounds, mass transport is dominated by electromigration in the absence of convection and by diffusion in the presence of convection. The growth rate is proportional to the current density [ 281. The solid composition is accounted for in terms of mass transport in the liquid and of phase diagram relationships. It is controlled by temperature changes due to the Peltier effect at the growth interface, by the mobilities and the diffusion coefficients of the solute components and by the growth velocity, e.g. the current density. A unique feature of LPEE is that the solid composition remains constant even for prolonged growth, provided the current density is kept constant. However, for a given solution composition, the solid composition can be varied by varying the current density, The experimental results on the growth rate and solid composition of the Ga, _,Al,As system were analysed on the basis of the model and were found to be quantitatively consistent with all its aspects [ 281. The feasibility of electroepitaxial growth of Hg, ._,Cd,Te, ideally suited for advanced IR detection and imaging, was demonstrated both from mercury-rich solutions at temperatures as low as 290 “C [29] and from telluriumrich solutions at 501 “C [30]. In the second case, for Hg, _.Cd,Te in the region of low x values (e.g. x = 0.2) which is of greatest interest for practical applications, the average growth velocity was about 0.8 pm mini’ at a current density of 10 A cm-*, and electromigration was found to be the dominant growth mechanism. The results could be accounted for quantitatively by the multicomponent theory of LPEE already considered [ 281. Conversely, such theory allowed calculation of the growth velocity for other solid compositions and growth temperatures. 6. Formation

of recombination

centres

Ad hoc experiments [18] demonstrated the importance of growth dynamics and in particular of growth acceleration in the formation of recombi-

8

nation centres during LPEE. It is likely that these centres originate from point defects generated at the growth interface in response to the abrupt deviation from steady state conditions. In fact, surface nucleation effects commonly considered to be of no significance in the LPEE process with slowly varying growth velocity can be of primary importance when there are abrupt increases in growth velocity. The experiments were carried out on thin GaAs substrates so that growth was controlled by electromigration, since the electromigration flux adjusts instantaneously (within a few milliseconds) to current density changes, whereas the thermal relaxation time of the interface temperature is typically of the order of 0.1 s. It should be pointed out that the growth acceleration rather than the absolute value is responsible for the formation of recombination centres. For example, by increasing the current density from 8 to 20 pm min ’ not instantaneously but in 10 s an undetectable quantity of defects appeared [18].

7. A new approach In the LPEE systems initially employed (Fig. 3), a thin metal foil or multilayer was used for the electrical contact at the back side of the substrate [13]. This type of contact showed a high electrical resistance, comparable with the overall resistance of the system at growth temperatures. Thus, undesirable localized Joule heating, more especially at a high current density, was observed, as well as the variations already mentioned in the electroepitaxial layer thickness produced by local changes in the contact

Fig. with

3. Schematic representation (taken from ref. 13) of the electroepitaxy apparatus electrical contact at the substrate back side (SS, stainless steel; TC, thermocouple).

SOLUTION

Fig. 4. Top view of apparatus for from ref. 19). The holder segments steel; TC, thermocouple).

electroepitaxial are of graphite

growth without back contact (taken and the slider is of BN. (SS, stainless

resistance. Therefore, a new LPEE configuration was developed (Fig. 4), positioning the substrate between two identically large segments of saturated solution [ 191. Thus, the preparation of a back-side contact to the substrate is eliminated. Furthermore, the reduction in the cell resistance makes it possible to monitor in situ the epitaxial growth process through measurements of the resistance changes associated with the growing layer. Such measurements are precise because all other electrical resistance components in the system are constant at a constant temperature [21]. Chromium-doped (100) GaAs (500 pm thick) and silicon-doped (100) GaAs (340 (urn thick) were used as substrates. Their growth area was 0.4 cm2 and each solution segment contained 9 g gallium saturated with GaAs at 850 “C. After the electric current was sent through the two solid-solution interfaces (at a current density ranging from 3 to 18 A cm-2), growth took place on the side of the substrate with a positive polarity with respect to the solution, whereas dissolution took place on the opposite side. During the runs of the process, which lasted 100 min, the temperature variations were within 0.5 “C [19]. Utilizing this new configuration, the simultaneous study of growth and dissolution processes became possible for the first time. The configuration can easily be extended to simultaneous growth on a number of parallel substrates separated by relatively thin solution layers to limit Peltier heating. In agreement with theoretical predictions [ 151, the growth and dissolution processes exhibited the same rate when the substrate was highly doped. In contrast, a pronounced Joule heating which promoted dissolution over growth was observed for high resistance substrates [ 191.

8. Conclusions The features and advantages of LPEE which were the main cause of its rapid development and widespread utilization have been discussed above. Nevertheless, it is worth recalling the most important ones: (1) the possibility to obtain epitaxial layers presenting morphological, structural and physical, and in particular electronic, characteristics far superior to those of layers grown by standard thermal epitaxy; (2) the achievement of high growth rates

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under a high current density, i.e. velocities as high as 25 pm min ‘ have been attained [ 71; (3) the modification of the dopant concentration in semiconductor compounds by varying the current density; (4) the controlled growth of multicomponent systems with a remarkable stabilization of the composition along the growth direction, and improved properties. These unique advantages and the high flexibility of the LPEE method allowed various types of applications, in particular the epitaxial growth of films for optical and electro-optical device structures, eventually in multilayer configuration, with the desired composition, thickness and electronic characteristics. For example, on increasing the current density from 1 to 80 A cm-‘, homogeneous layers of Ga, _,Al,As having x ranging from 0.11 to 0.20 and a band gap from 1.63 to 1.61 eV were obtained [28]. LPEE was also used to produce integrated circuit structures, e.g. on semi-insulating SiO,-masked GaAs [ 201. Selective dissolution and electroepitaxial growth were carried out under identical conditions (except under opposite current polarities) by controlling the current density across the solution-substrate interface. To evaluate the growth interface morphology and the microscopic growth rate, demarcation lines [61] were introduced by high current density (200 A cm-*) pulses lasting 5 s, superimposed on the base current (not higher than 80 A cm-*) at 5 min intervals. This so-called interface-demarcation [ 611 or rate-striation [ 61 technique is another interesting opportunity allowed by LPEE. It is evident, however, that these advantages are partly counterbalanced by difficulties essentially related to the growth apparatus which presently permit preparation of films of low dimensions only. Moreover, the possibility of changing the dopant concentration in semiconductor compounds by varying the current density has to be more deeply analysed. In fact, as the current density changes from 0 to 40 A cm-*, the doping level typically increases by about 40%, this value being insufficient to lead to possible wider applications [ 601. Therefore, according to Bryskiewicz [ 601, LPEE exhibits its most important feature in the growth of homogeneous layers at a fixed current density [ 24,281. Despite the wide range of research performed, the potentialities of electroepitaxy do not yet seem to have been fully exploited, although the importance of the method has been fully recognized. Hence, research is still proceeding highly actively [40, 43, 44, 46, 48, 53, 601.

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