380

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Philips tech. Rev. 32, 380-384,1971, No. 9/10/11/12

Epitaxial growth of gallium arsenide

A. Boucher and B. C. Easton

Gallium arsenide has a special place among the semi-conductors used in solid-state microwave electronics.lts electron mobility and band gap are high comparedwith those of silicon or germanium. By introducingsuitable dopant impurities into the material a widerange of electrical conductivity values can be attained.These facts, together with the discovery of the Gunn ortransferred-electron effect [1] that occurs in galliumarsenide because of the particular band structure, havestimulated the interest in this material.

Like many .semiconductor devices and integratedcircuits, gallium-arsenide devices are constructed withtheir active regions fabricated within a layer grownepitaxially on a monocrystalline substrate of the samematerial. The electrical properties of the substrate canbe selected by suitable doping, so that it can act aseither 'a conducting or an insulating mechanical sup-port. The required properties in the epitaxial layer arealso obtained by doping with an appropriate impurityeither during growth or by subsequent diffusion. Epi-taxial layer thicknesses may range from less than onemicron to many tens of microns depending on theparticular device.

Gallium-arsenide microwave devices, in which epi-taxial material is used, include transferred-electronoscillators, varactor, mixer, tuning and avalanchediodes and field-effect transistors.

The majority of applications require thin N-typelayers on highly conducting N+ gallium-arsenide sub-strates. Sometimes an N+ layer is needed on top ofsuch an N layer to provide an ohmic semiconductorcontact to the N-type material. Other applications re-quire a number of successive layers with different car-rier concentrations, e.g. N+ substrate, N layer, N+layer or a series of alternate Nand N+ layers. This re-quirement and the need for localized epitaxial deposi-tion for discrete-device or integrated-circuit applica-tions demand a thorough control of all aspects of theepitaxial-growth process. Investigations with thisobjective have been made at the Laboratoires d'Elec-tronique et de Physique Appliquée (LEP) on some ofthe fundamental aspects of epitaxial growth (thermo-

Dr. A. Boucher, formerly with Laboratoires d'Electronique et dePhysique Appliquée, Limeil-Brevannes, Val-de-Marne, France isnolV with La Radiotechnique Compelec, S.A., Suresnes, France;B. C. Easton, M.Sc., is with. Mullard Research Laboratories,Redhill, Surrey, England.

dynamics, kinetics and anisotropic effects) [2] [3] andalso on new methods which can yield high-performancedevices [4]. At Mullard Research Laboratories (MRL)the incorporation and identification of impurities isbeing studied, and work is also being done on the auto-mation of the epitaxial-growth process. Impurities areidentified by electrical characterization and scanning ..mass-spectrographic techniques [5]. .

For some applications, e.g. field-effect transistors,semi-insulating gallium arsenide (resistivity higher than107 ncm) is needed as an insulating substrate. High-purity gallium arsenide, in which the impurity conduc-tion is due to shallow donor levels, may be given ahigher resistivity by creating deep traps by doping thematerial with chromium.

In recent years the major emphasis in the study ofthe epitaxial growth of gallium arsenide has been on thepreparation of material suitable for Gunn-oscillatordevices. Epitaxial layers for this purpose must be ofrelatively high purity with a free-donor concentrationin the region of 1015 cm-3, combined with a high elec-tron mobility (about 7500 cm2jVs at 293 K).

Two general methods of preparing high-purity epi-taxial gallium arsenide are available; the first is bychemical vapour deposition and the second by solutiongrowth from a gallium melt. We have investigated bothof these techniques; details will be given and the meth-ods used for characterization of the results by meansof electrical and optical techniques will be reviewed.

Epitaxial growth of gallium arsenide

Vapour growth

Most chemical vapour transport reactions for thepreparation of epitaxial gallium arsenide involvereactions between gallium, arsenic and hydrogenchloride. These reactants may be derived from theelements or their compounds; the particular combina-tion chosen has strong influence on the purity of thelayers produced. D. Effer [6] was the first to use thereactions between hydrogen, arsenic trichloride andgallium to produce layers of a higher purity than anypreviously prepared by other processes. This has beenthe method most commonly used for the growth ofepitaxial layers of suitable purity for Gunn-devicemanufacture.

Philips tech. Rev. 32, No. 9/10/11/12 EPITAXIAL GROWTH OF GaAs 381

Fig. 1 shows a schematic diagram of an epitaxialreactor. High-purity hydrogen from a palladium dif-fuser unit is passed through an arsenic-trichloride sat-urator and into a silica reaction tube in a two-zonefurnace. Arsenic trichloride is initially reduced in thereaction tube to form arsenic vapour and hydrogenchloride; these reaction products then proceed downthe tube to the gallium source (99.9999% purity) con-tained in a silica boat at 800-850 °C. At this stage ar-senic dissolves in the gallium until it is saturated and asolid skin of gallium arsenide is formed on its surface.At the same time the hydrogen chloride reacts withgallium, mainly to form the mono chloride. When thesource is saturated the gaseous reaction products(principally gallium monochloride, arsenic and hydro-gen) pass into the second zone, kept at 750°C, wheredeposition occurs on the gallium-arsenide substrate,which has been polished chemically in a bromine-methanol solution. No growth occurs on the surround-ing silica ware at this temperature, but free arsenic,gallium arsenide and other by-products of the reaction

2~{ WM'~'\'\'\'\~

H2+A~CI3\ ~ III s~==frw"$/ffi'$'$'#~\..\\\\"-.~~I I I

: BDD-B5Dac: 75DaC :I A I B I

Fig. 1. Reactor for gas-phase deposition of epitaxial galliumarsenide. A stream of high-purity hydrogen and arsenic tri-chloride enters at inlet tube J and reacts with the gallium (Ga) inzone A of the furnace. The reactants proceed to zone B, wheregallium arsenide is epitaxially deposited on to the substrates Slib.The liner tube L permits collection and disposal of the reactionby-products that are deposited at the relatively cold downstreamend of the furnace. At port 2 gaseous dopant impurities can beintroduced into the reactor.

may be deposited on the wall at the end of the reactiontube. A liner is used there for ease of cleaning.The principal chemical reactions which occur are

summarized as follows:

l'4 AsCh + 6 H2-+As4 + 12 HCl

l' l'2 Ga + 2 HCl -+ 2 GaCI + H2 •

The overall equilibrium for the deposition reaction inthe temperature range 730-830 °C has been stated as [2]

2 GaCI + t AS4+ H2 ~ 2 GaAs + 2 HCI.

Layers for Gunn devices are generally grown on sub-strates cut on a surface 2-30 off a {lOO}crystal plane.

This orientation facilitates dicing and produces lowerelectrical impurity layers than on {110} or {lll} sur-faces. Chromium-doped semi-insulating or highly con-ducting N-type substrates, where the dopant may besilicon, tin or tellurium are used. Monitor layers grownon semi-insulating substrates are used for Hall meas-urements so that resistivity and carrier concentrationmay be rapidly assessed for each growth experiment.It has been found that these measurements relateclosely to the properties of layers grown at the sametime on highly doped substrates. Growth rates are inthe region of 20-30microns an hour and layers from 1to over 100 microns thick may be grown.The background impurities in an epitaxial system are

generally predominantly donors, and free-carrier con-centrations down to 1014 cm-3 may be obtained. Themost difficult impurity to eliminate or control duringgrowth is oxygen (usually introduced into the systemas water). Its influence on the electrical properties ofepitaxial layers is mainly indirect and complex; notonly can it affect the reactions controlling depositionbut it can react with other impurity atoms in thesystem. At high levels it has a harmful effect on thecrystal quality of the layers.Most devices require epitaxial material with free-

donor concentrations of 1015 cm-3 or more; the sim-plest method of doping during growth is to add tin(either as tin-doped gallium arsenide or free metal) tothe gallium source, but this approach has the disadvan-tage that it is inflexible. Doping systems which permitthe doping level to be varied, either between growthexperiments or during the deposition, rely on the in-troduction into the gas stream of a gaseous donorimpurity such as hydrogen sulphide or selenide be-tween the gallium source and the substrate (see fig. I).A more recent method developed at LEP, the back-dif-fusion method, is illustrated in fig. 2. It is used to pro-duce tin-doped epitaxial layers, and permits changesin dopant concentration over a wide range duringthe process.The main stream of gaseous reaction products is the

same as in the process given above. Here however a sidestream of hydrogen is introduced into the system, andthis hydrogen flows over a boat containing elementaltin. Hydrogen chloride from the main stream diffuses

[1] This effect is treated in detail in the paper by G. A. Acket,R. Tijburg and P. J. de Waard in this issue, page 370.

[2] A. Boucher and L. Hollan, J. Electrochem. Soc. 117, 932,1970.

[3] L. Hollan and C. Schiller, J. Crystal Growth 13/14, 319, 1972.[4] L. Hollan, J. Hallais and C. Schiller, J. Crystal Growth 9,

165, 1971, and A. Boucher, J. P. Chané and E. Fabre, Rev.Physique appl. 6, 5, 1971.

[5] J., B. Clegg, E. J. Millett and J. A. Roberts, Anal. Chem. 42,713, 1970.

[6] D. Effer, J. Electrochem. Soc. 112, 1020, 1965.

382 A. BOUCHER and B. C. EASTON Philips tech. Rev. 32, No. 9/ 10/11/12

back into the hydrogen flow and reacts with the tin,which is kept at a temperature of 750°C. In this waytin is introduced into the system as a chloride. Thetin-chloride content is controlled through the flow rateof the hydrogen. Using such techniques layers may bedoped in the range 1015-1018 free donors cm=', andepitaxial structures built up of high- and low-dopedregions can also be obtained (jig. 3).

At MRL an improved epitaxial system has been de-veloped in which a more complex gas-handling systemhas been constructed, enabling the different growthparameters to be varied and their effect on the layerproperties to be studied. It incorporates electricallyoperated solenoid valves. A programme-timer unitenables the growth process to proceed automatically.The new system provides better control and consistencyin the growth process with reduced reliance on opera-tor skill.

Development of the vapour growth has resulted ingreater versatility, permitting doping control and auto-mation of the various process operations. It can beargued that the arsenic-trichloride process is inherentlyunsuitable for precise control since true equilibrium isnever attained at the gallium source. To overcome thisdifficulty alternative procedures for introducing ar-senic into the system have been used, e.g. use ofelemental arsenic or arsine. ln general, however, thealternative processes do not produce layers of suffi-ciently high purity, largely because sufficiently purestarting materials are not available.

Liquid growth

Two methods have been used for epitaxial growth ofGaAs from the liquid phase. Essentially they bothdepend upon the deposition of GaAs from a saturatedsolution in gallium. The first, described by H. Nel-son [7J, is the horizontal method and the second, re-ported by H. Rupprecht [8J, is the vertical method.

E. André and J. M. Le Duc from RTC (La Radio-technique-Compelec) at Caen have developed a specialversion of the horizontal method (fig. 4). Liquid gal-lium saturated with gallium arsenide in the range650-900 °C is brought into contact with the substrateby pulling back a shutter at the bottom of the galliumcontainer. Growth occurs on the substrate as thesolution is allowed to cool down. Material has beenproduced with free-donor concentrations down to1014 cm-3 and Hall mobilities in excess of 105 cm2fYsat 77 K [9J.

A schematic diagram of the vertical system used atMRL is shown in fig. 5. The saturated gallium is con-tained in a silica crucible; the substrate is introducedinto it from above, suspended from a silica rod, and thegrowth system is maintained under a pure hydrogen

Fig.2. Back-diffusion reactor for gas-phase deposition of tin-doped epitaxial gallium arsenide. High-purity hydrogen andarsenic trichloride, introduced through inlet 1, react with thegallium (Ga). A proportion of the reactants diffuses upstream inthe hydrogen flow, entering at inlet 2, and reacts with the tin(Sn). From the reactant flow containing tin, gallium and arsenic,tin-doped gallium arsenide is deposited on substrates downstreamin the furnace. Regulation of the hydrogen flow rate over the tinpermits accurate control of the dopant content in the reactantmixture.

atmosphere. Again, under optimum conditions, layerswith free-donor concentrations down to 1014 cm-3 havebeen obtained.In both the horizontal and vertical systems the layer

thickness is controlled by the temperature range overwhich deposition is allowed to occur; in this respect thevertical method permits better control. With coolingrates of 0.1 to 0.2 °C rnin! and well defined growthtimes, thinner layers can be grown by the verticalmethod than by the horizontal technique.

Tin can be added to the melt to provide additionaldoping. An advantage of the liquid-growth techniquesis that more uniformly doped layers can be grown.G urm-device assessment of these layers shows thatstandard X-band (about 10 GHz) oscillators preparedfrom liquid epitaxial material have d.c. to r.f. con-version efficiencies that compare favourably withthe best results obtained when vapour-grown ma-terial is used.

3~~~-~~~~~~~~~~~~--

5 10_X

Fig.3. Typical result of the back-diffusion method. The free-carrier concentration n is plotted as a function of the depth xbelow the surface of a multiple-layer structure. Sub substrate.

Philips tech. Rev. 32, Nq. 9/10/11/12 EPITAXIAL GROWTH OF GaAs 38~

Characterization

Before an epitaxial layer is used in a microwavedevice it is desirable to determine its suitability by rou-tine measurements. In practice close correlation of thematerial properties with the device behaviour may belargely masked by subsequent device-fabrication tech-nologies, in particular when making ohmic contacts.The layer-assessment techniques employed at our lab-oratories are indicated in the following sections.

Ga+GaAs

Fig. 4. Apparatus used in a modified version of the horizontalliquid-phase deposition method. The substrate Slib is located atthe bottom of the sample holder. In the top compartment acharge of gallium is heated under pure hydrogen with an ac-curately determined quantity of gallium arsenide until the latteris dissolved. The shutter B at the bottom of the gallium compart-ment is then withdrawn and the solution spreads over the sub-strate. On slowly cooling the solution over a well defined tempera-ture range, an epitaxial layer of accurately controlled thickness isdeposited.

t

Ga+GaAs

Fig. 5. The deposition of gallium arsenide using the verticalmethod. A crucible contains gallium arsenide dissolved in gal-lium. At the temperature at which the solution is saturated, thesubstrate Slib is immersed in the solution. When the solutioncools down gallium arsenide is deposited on the substrate. Layerthickness is determined by the temperature range through whichthe melt is cooled. The system is flushed with pure hydrogenduring the process.

Crystallographic and optical methods

Microscopie examination of a surface reveals surfacedefects, and metallographic staining of a polished orcleaved cross-section reveals the interface between layerand substrate. The perfection and uniformity of thesubstrate can be studied and the thickness of the layermeasured directly. Infra-red interference techniques [10]

can be used for measuring non-destructively thethickness of N-type layers on N+ substrates.X-ray-reflection topographic examination [11] of lay-

ers and substrates is used to show dislocation density,strain and damage arising from cutting or polishingoperations; these factors are particularly important fordetermining the substrate quality required to producelayers with a low density of surface defects. C. Schil:Ier [12] has recently extended the technique to examina-tion of the layer-substrate interface region and hascorrelated cyrstallographic disturbances at the inter-face with substrate quality, polishing and pre-treatmentbefore epitaxial growth.

Electrical properties

The Van der Pauw technique [13] for measurement ofHall constant and resistivity is used for layers grown onsemi-insulating substrates, and gives values for resis-tivity, free-carrier concentration and mobility.Variation of dopant concentration with depth in the

epitaxial layer can be found from experiments on ametal-semiconductor diode (Schottky-barrier diode)obtained by evaporating a metal film on to the sample,or more conveniently by applying a mercury contactwith a defined area.The relation between reverse-bias voltage and diode

capacity yields the impurity concentration at a depthdetermined by the bias [14]; the largest depth on whichdata can be obtained is the depletion-layer thicknessat the breakdown voltage of the diode. Fig. 6 showsresults from these measurements. On applying thisprinciple in conjunction with suitable measuring equip-ment the doping level can be plotted automatically andits uniformity through an epitaxial layer can be in-vestigated. Hall and Schottky-barrier measurementsare used as routine assessment procedures. They can be

[7) H. Nelson, RCA Rev. 24, 603, 1963.[8) H. Rupprecht, Proc. Int. Symp. on Gallium Arsenide, Read-

ing 1966, page 57.(0) E. André and J. M. Le Duc, Mat. Res. Bull. 3, 1, 1968.[10) P. J. Severin, Appl. Optics 9, 2381, 1970 and Appl. Optics 11,

691, 1972 (No. 3).[11) J. B. Newkirk, J. appl. Phys, 29, 995, 1958.[12) C. Schiller, Solid-State Electronics 13, 1163, 1970.[13) L. J. van der Pauw, Philips Res. Repts. 13, 1, 1958; see also

Philips tech. Rev. 20, 220, 1958/59.[14) General theory has been given -in: W. Schottky, Z. Physik

118, 539, 1941/42. The particular application referred to hasbeen described in: C. O. Thomas, D. Kahng and R. C. Manz,J. Electrochem. Soc. 109, 1055, 1962.

384 EPITAXIAL GROWTH OF GaAs Philips tech. Rev. 32, No. 9/10/11/12

n

Ild6f----+---+--+----tI-tcm-3

2 4o 6 8 IOfJm-x

Fig. 6. Concentration 11 of dopant impurities (number of freedonors cm-3) as a function of the distance x beneath the surfaceof an epitaxial layer. Sub substrate. The curves have been ob-tained from Schottky-barrier diode measurements. Curve a istypical of a layer into which dopant impurity from the substratehas been incorporated during growth. Curve b is frequently ob-tained when silicon- or tin-doped substrates are used and thegrowth conditions are insufficiently well controlled.

extended by investigating the influence of temperature,and in the case of capacitance measurements, the effectof infra-red radiation. Such studies provide further in-formation [15] about the nature and concentration ofthe electrically active centres, i.e. donors, acceptors,and traps in the epitaxial layer. Other techniques such

(15J H. T. Ralph and F. D. Hughes, Solid State Comm. 9, 1477,1971 (No. 17), and G. A. Acket, Philips Res. Repts, 26, 261,1971 (No. 4).

06J C. M. Wolfe and G. E. Stillman, Proc. 3rd Int. Syrnp. onGallium Arsenide and Related Compounds, Aachen 1970,page 3.

as those employing photo- or cathodoluminescence [16]

and magnetoresistance are used to complement theHall and Schottky measurements.As far as possible it is the aim of the electrical char-

acterization to correlate the results obtained, on onehand with the concentration and nature of impurityatoms, divergence from stoichiometry and crystallo-graphic defects in the epitaxial layer, and on the otherwith the microwave performance of devices made fromparticular samples of material.

Finally, although gallium arsenide is the principalcompound semiconductor for microwave applications,other lIljV cornpounds with theoretical advantages arebeing studied at MRL and LEP. Particular examplesare indium phosphide and various compositions of theindium-phosphidejgallium-arsenide alloy system. Suchmaterials require techniques for their preparationsimilar to those established for gallium arsenide, andprelirninary studies of the various growth systems havebeen commenced for these new materials.

Summary. Processes for the epitaxial deposition of gallium-arsenide layers for use in microwave electronics, are under in-vestigation at M ullard Research Laborarories (Redhili), at theLaboratoires d'Electronique et de Physique Appliquée (Limeil-Brévannes) as well as in some other laboratories within the Ph ilipsgroup of companies. Epitaxial layers are deposited both fromgas-phase reactions between gallium and arsenic cornpounds andfrom a saturated solution of Ga As in liquid gallium. The layersobtained are studied by means of X-ray diffraction, infra-redinterference cathodo- or photoluminescence and electrical meas-urements (conductivity, Hall effect, Schottky-barrier diodemeasurements and magnero resistance).