The preparation and study of the optical absorption edge of thin films of gallium arsenide

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The preparation and study of the optical absorption edge of thin films of gallium arsenide

R.P. Howson

To cite this version:

R.P. Howson. The preparation and study of the optical absorption edge of thin films of gallium arsenide. Journal de Physique, 1964, 25 (1-2), pp.212-217. �10.1051/jphys:01964002501-2021200�.

�jpa-00205740�

THE PREPARATION AND STUDY OF THE OPTICAL ABSORPTION EDGE OF THIN FILMS OF GALLIUM ARSENIDE

Par R. P. HOWSON,

The Plessey ^Co. (U. K.) ^Limited.

Résumé.

On a préparé des films d’arséniure de gallium par évaporation ^{sur des} supports amorphes. ^{On décrit une méthode} simple et efficace pour évaporer l’arséniure de gallium, ^méthode qui pourrait ^être applicable ^{à d’autres} composes ^{III-V. On a} trouvé que la décroissance ^de l’absorp-

tion au voisinage ^de la bande était moins rapide pour les couches minces que ^{celle à} laquelle ^on pourrait s’attendre à partir des données concernant un seul cristal, l’absorption s’étendant dans

l’infrarouge ; ^elle dépend ^{de la} température ^du support.

Abstract.

Films of gallium arsenide have been prepared by evaporation ^{on to} amorphous

substrates. A simple ^and effective method of evaporating gallium ^arsenide, which would also be

applicable ^to other III-V compounds, is described. The optical absorption edge ^has proved ^to

be less steep ^{in the} films than expected ^from single crystal data, ^the absorption extending ^into

the infrared, ^{and is a} function of substrate temperature.

Introduction.

III-V semiconductor com-

pounds have many interesting ^{and useful} pro-

perties ^{but in} many ^cases they ^{are difficult to} pre- pare ⁱⁿ single ^or polycrystalline ^{form. The main} difficulty ^{is the} high vapour pressure of the volatile

component ^{over the molten} compound ^which

makes high pressure apparatus necessary if stoi-

chiometry ^{is to} be maintained when material is grown from a melt. These difficulties become

greater ^if advantage ^{is to be taken of the} properties

of mixed crystals ^{which offer,} with various mix- tures of III-V compounds, ^a continuous variation of energy gap ^from GaP to InSb with the approxi-

mate range of 2 to 0.15 eV, optical absorption edges

of from 0.6 to 8 microns. Evaporation ^{offers a}

means of producing ^a large range of compounds ^and

mixed compounds ^{without undue} complication

where the advantages ^{of films are} required, ^i.e.

large ^{areas of thin} material and the disadvantages

of uncertain structure can be tolerated.

The optical properties ^exhibited by metal films

especially ^{in the} infra red have been known for a

long ^{time to} be different from those of the material in bulk ; ^{this is} associated with the difference in carrier transport processes. Semiconductors whose electrical properties ^{are dominated} by ^either

structural or impurity imperfections ^{would be} expected ^to show very marked differences when

evaporated. ^These properties may be expected ^to

indicate the progress ⁱⁿ technique ^for producing

semiconductor films with good ^carrier transport properties. ^The optical absorption edge ^{and addi-}

tional optical absorption would indicate deviations from perfection ^{in the} lattice and the progress made in producing structurally good ^films.

Films of the element semiconductors Ge and Si

have been produced by evaporation ^and poly- crystalline growth reported ^for films grown ^on

amorphous substrates at elevated temperatures,

the micro-crystal size and orientation increasing

with substrate temperature [1, 2]. ^{III-V com-} pound semiconductors have been evaporated ^from

element [3] ^and compound [4] ^{sources with some} difficulty ^and complication ^and optical properties

of films of InSb [5] ^{and GaAs} [6] grown ^{in this} fas- hion have been measured. Electrical measurements have shown that properties ^{close to those} given by single crystal ^{material can} be achieved with such films [3]. ^GaAs films have also been produced by sputtering [7].

Preparation.

^{The " three} temperature " ^me-

thod of Günther has been used and modified to give greater ^{control. The " three} temperature " ^me- thod, ^as applied ^to producing films of III-V com-

pounds, ^can be summarised briefly ^{as the conti-}

nuous evaporation of each element separately ^and

their reaction on the substrate surface. The

precise control that would soem to be indicated is lessened by ^{the fact that one} of the constituent elements is more volatile and ^at the substrate

temperature ^{used is not} condensed in element form upon ^the substrate and hence an excess of this vapour ^can be allowed to impinge upon the sub- strate. Molecules of this more volatile element

are incident upon the substrate and ^are mobile on

it for a certain time, depending upon the substrate material and its temperature, ^before re-evaporating.

If during this time they combine with a mole- cule of the non-volatile constituent to form the

compound they ^{remain on the} substrate, ^{as the} compound ^{is stable at that} temperature. ^Stoi-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01964002501-2021200

213

chiometric material may ^be expected ^{under a}

range of conditions because of the re-evaporation

of the excess volatile component.

The mass of material arriving ^{at unit area in}

unit time is given by :

where p is the partial pressure of the material of molecular weight ^{M at a} temperature ^{T and of}

area A. No is Avogadro’s ^{number and R is the}

gas ^{constant. r} is the distance of the substrate from the source inclined at an angle 0. This is a

maximum rate of evaporation, ^the actual material condensed depends upon the condensation coeffi-

cient, ^a function of many variables. This equa- tion provides ^a way of estimating ^source tempera-

tures required ^{for a} given evaporation ^rate using partial pressure ^curves given by Honig (1962) [9].

Gallium was evaporated ^{from a} graphite ^boat

at 1200 oC and arsenic through quartz ^{wool in a}

fused silica crucible held at a temperature ^between

320 OC and 350 oC. These sources were close

together ^to give good ^{mixture to} the vapour streams. The gallium ^source temperature ^was

held constant and for any given ^substrate tempe-

rature the temperature of the arsenic source was

adjusted ^to give satisfactory films. Smooth hard coherent films were obtained which with X-ray powder diffraction proved ^{to be} gallium ^arsenide.

Films were successfully prepared ^{on substrates}

up ^to 700 OC. At higher temperatures ^{and thick-}

nesses an uneven loose surface to the film became

apparent. ^Indium phosphide ^{films were also} prepared using ^{an indium source at} 850 °C and red

phosphorus ^at 330 0C. An excessive amount of

phosphorus vapour had ^to be produced ^{in this case}

due to the exceptionally low condensation coeffi- cient for this material and hence molecules would be only ^a short time mobile on the substrate [10].

Successful films were only ^{obtained on substrates}

at ambient temperature.

The three temperature method described al-

though leading ^{to the} production ^of gallium ^arse-

nide films required ^careful control, ^the evaporation

rate of the element sources being very sensitive ^to

temperature, ^{and was} cumbersome to operate.

Study ^{of the} principles involved has led to a

much simpler ^{and more} controllable system. ^The

vapour pressure ^of arsenic over gallium ^arsenide

as As4 ^{is much closer to that of} gallium ^over gallium ^{than that} of arsenic vapour ^{over the} ele- ment. The vapour pressures ^{as a} function of

temperature ^are given ⁱⁿ figure ^{1. It is} possible

to envisage operating ^{both sources at the same} temperature but of different effective areas and in this case one source of ^a mixture of the source

materials may be used. It ^{can be seen} from fi- gure 1 that ^the ratio of vapour pressures ^is subs-

FIG. 1. - Vapour pressure of arsenic ^over gallium ^arsenide

and gallium ^over gallium.

tantially independent ^of temperature ^{over the}

range ^{that is} likely ^{to be used.’ The} proportion ^of

the mixture will then determine the correct ratio of element molecules in the vapour and the tempe-

rature will determine the rate of evaporation, giving ^a system ^as controllable as a single ^element evaporation. The mixture ^to give satisfactory

films was found to be about 15 % gallium ^arsenide by weight ⁱⁿ gallium ^{but was not} critical and could,

with practice, ^be judged by eye. The volume of

.source material was such as to maintain the mixture approximately ^constant during ^the evapo- ration despite unequal losses. The mixture was

contained in an alumina crucible at 1 000 °C.

Another method of preparation ^{of films of} gal-

lium arsenide used ^was by ^direct evaporation ^of

the compound. On examination of vapour pres-

sure curves available it would appear ^that disso- ciation of the compound ^takes place liberating predominantly arsenic vapour, leaving ^residual gallium. Experimental evidence however was

obtained in an open and ^{in a} closed system ^that compound material transfer could occur at tempe-

ratures as low as 300°C. This may be due ^{to a} reversible reaction of gallium ^or gallium ^arsenide

with the residual oxygen ^{in the} system ^to give ^the gallium sub-oxide, ^{which is} volatile, ^or by ^non-

dissociation of the vapour. Films ^were formed from compound ^{source at a} comparatively ^low temperature ⁱⁿ open ^or semi-closed system ^but evaporation ^{rates were found to be} low, ^{of the}

order of 3 A _per second, and arsenic was lost at too

great ^a rate, giving large usage of gallium ^arsenide

source material. The rate of transfer of material

was found to be independent ^of oxygen pressure from about 10-5 torr to 1 torr, except ^{that non-}

volatile Ga2o3 ^{formed on the source at the} highest

pressure obscuring ^it.

Conditions could presumably be balanced such that with temperature control and evaporation

rates satisfactory ^films could be formed with

optimum usage ^{of source material} but this would lead to cumbersome conditions. This method

seems essentially ^{that of the " three} temperature

method ", ^{arsenic rich films} being prevented by re-evaporation ^{from the hot} substrate. This will of course provide ^{a method of} forming ^{films in}

closed systems.

Measurement techniques.

^The measurements

’

made on the films have been of the transmission in the region ^{of the} optical absorption edge ^at

0.9 microns. The transmission of the film on the substrate was measured with a single pass grating

monochromator with a grating ^of 7 500 lines per inch using ^{either a S1} photomultiplier ^or Golay

cell as a detector. A resolution of between 4 and 8 millimicrons was achieved.

On favourable films exhibiting ^{ideal inter-}

ference fringes ^{it was} possible ^to obtain tabulated data of thickness, refractive index and absorption

coefficient. The expression for the transmission of a film giving interference attached to a large

substrate can be written :

where R4 ^{is the} total energy reflection coefficient at the substrate film interface and is

r and t are the amplitude coefficients and the

subscripts ^{refer to} the surfaces met in the order of

transversing ^the system in the direction of film to substrate. R3 ^{is the} energy reflection coefficient at the substrate air interface. _{no, nl} and n2 ^are

the refraction indices of air, gallium arsenide and substrate material respectively. 8 and p represent

the real and imaginary parts ^{of the} phase change

in the beam passing through ^the gallium ^arsenide

film.

and

where k is the extinction coefficient and x the

absorption coefficient.

Minimum transmission occurs when and

and maximum transmission when cos 28 = + 1,

i.e. mx

2n, ^d

and

It will be ssen that the maximum transmission

’is substantially independent ^of the refractive index of the gallium arsenide film and the minimum is

a function of the square of ^it.

The refractive index of a film of gallium ^arsenide showing theoretical interference fringes ^{can be}

estimated by the transmission of the system ^at

minimum in a region where there is no absorption.

Theoretical interference fringes ^can be verified by

the peak transmission which is a function of the refractive index of the substrate only. Assuming

a linear variation of refractive index with wave-

length ^{the order of} fringe observed and the refrac- tive index at other fringe positions ^{can be found}

and hence also the film thickness. Where the film is absorbing ^{to a} reasonable extent the

absorption coefficient can then easily be obtained at positions ^mid way between maximum and mi- nimum fringes.

Results.

The transmission of GaAs and InP films grown ^{on to} glass substrates at ambient

temperature ^{are shown for the} region ^{of the} optical absorption edge ⁱⁿ figures 2 and 3. These and

FIG. 2.

Transmission of a GaAs film grown ^on glass

at ambient temperature.

similar films could be analysed and the results for

absorption coefficient and refractive index are

presented ⁱⁿ figures 4, 5, ^6. Comparison ^{of these}

results with single crystal data which was avai- lable [11, 12], indicated that the absorption edge

extended to longer wavelengths ^{in the film and was} considerably ^less steep. Investigation ^{ot the}

films by X-ray powder diffraction revealed almost

amorphous structure. Examination of films of

215

FIG. 3.

Transmission of a InP film grown ^on glass

at ambient temperature.

FIG. 4.

Refractive index versus wavelength

for GaAs films.

FIG. 5.

Refractive index versus wavelength

for InP films.

gallium arsenide grown ^on glass ^and quartz ^sub-

strates at higher temperatures gave steeper absorp-

tion edges ^{but there was still} considerable excess

FIG. 6.- Absorption coefficient versus wavelength

for GaAs and InP films.

absorption ^at higher wavelengths than shown by single crystals. ^{This is} illustrated in figure ^7.

Absorption is however strong ^{at lower wave-} lengths ^{as shown in} figure ^8. An examination of

FIG. 7.

Comparison ^{of film and} single crystal ^GaAs.

the properties ^{of the films as a} function of film thickness and of substrate temperature ^{was under-}

taken and the results are displayed ⁱⁿ figures ⁹

and 10. The properties ^{of films were found to be} independent ^of evaporation ^rates between 30 and

2Å/s ^{and vacuum} pressure from 10-5 torr upwards.

It can be seen that the steepness ^{of the} absorption., edge improves with increased substrate tempera-

ture but the maximum transmission falls after a

substrate temperature of 550 °C. ^The films were

all of approximately ^{the same} thickness which was

FIG. 8.

Logarithmic transmission of GaAs film.

FIG. 9.

Percentage transmission versus wavelength

for GaAs films deposited ^{at various} temperatures.

one micron. Investigation ^{of the} properties ^of

the system with film thickness for films grown ^on

glass substrates at 500°C indicated the peak

transmission fell rapidly ^{above a} thickness of about 2 microns, ^{this was} associated with the appearance ^{of a} cloudy ^{surface on the film. In-}

fluence of substrate temperature varies the film thickness at which this cloudy ^surface appears.

X-ray powder diffraction measurements indicated

FIG. 10.

Percentage transmission versus wavelength ^for

GaAs films of varying thickness grown at ^a substrate

temperature ^{of 500 OC.}

micro-crystal sizes of 100 to 1000 A for films grown above 300 °C. The cloudy surface would ^seem to be due to scattering by larger crystallites growing

out of the films.

Conclusions.

Films of gallium arsenide have been grown using evaporation ^{trom a} single ^source

to give reproducible easily produced films. These films are very adherent to glass substrates and hard and difficult to scratch. They have exhibited

optical absorption edges ^less steep ^than expected

from single crystal ^{data and} extending ^to longer wavelengths. Films grown ^at ambient substrate

temperatures ^{are almost} amorphous and difference in optical properties ^can be attributed to this, ^but

films grown ^at higher temperatures have sufficient

crystal ^{size to make this structure} unlikely ^to

affect the optical absorption edge. The differences between films grown ^at higher ^substrate tempe-

ratures and single crystal material is thought ^at

the moment to be due to strain. There is some

evidence that strain affects the optical absorption

of thin single crystals [12]. It is known that

impurity effects would push ^the absorption edge

towards low wavelengths [13], ^{the reverse of what}

is observed. Investigations ^of the effect of strain

are now in progress.

I would like to acknowledge ^the help of J. Brit- ten, W. Morton and A. Porteous in the preparation

and measurement of evaporated ^{films and to the}

Ministry ^{of Aviation who} supported ^{this work.}

217

Discussion

M. PAPARODITIS.

Pouvez vous donner plus

de details sur les conditions d’évaporation ^de

l’arsenic ?

Riponse : ^{Arsenic was} evaporated through quartz wool, ^the temperature ^of the element being higher ^{than that of the wool to ensure} good ^trans- port. ^The temperature ^{of the} quartz ^{wool was}

measured.

M. PERNY. -1) N’y ^a-t-il pas ^{des niveaux dis-}

crets localises au voisinage ^{de la bande} d’absorp-

tion fondamentale ^et qui pourraient ^{etre dus a des}

excitons? .

2) ^{comment avez-vous trace les} courbes de

dispersion ? ^{Comment a ete} determine l’ordre d’in- terférence dans la relation 2ni ^e

mi Ài.

Reponse : No evidence of absorption ^{due to}

localised states was observed but this could be confused with imperfect interference fringes.

Optical ^{constants were obtained for} films pre-

pared ^on substrates at ambient temperatures by measuring ^the transmittance at a minimum where the value was a function of approximately ^the

square of ^the film refractive index. Good inter- ference could be verified by ^the peak transmittance.

Quantitative measurements were used for absorp-

tion coefficient. Interference order gave the varia- tion of refractive index.

En réponse à ^une question posee : ^Published

evidence indicates that the optical absorption edge ^is pushed ^{to lower} wavelengths ^{when the} impurity concentration is increased in GaAs.

This is the reverse of what is observed.

REFERENCES [1] ^COLLINS (F. M.), ^Trans. Eight National Vacuum Sym-

posium, Pergamon, 1961, 2, ^899.

[2] ^DAVEY (J. E.), ^J. Applied Physics, 1961, ^{32, 877.}

[3] GÜNTHER (K. G.), Compound Semiconductors Part 1, Edited by ^{R. K.} Willardson and H. L. Goering (Reinhold ^Pub. Corp.), 1962, pp. 313-325.

[4] PAPARODITIS (C.), ^As above, pp. 326-336.

[5] ^POTTER (R. F.) ^and KRETSCHMAR (G. C.), ^{J. O. S.} A., 1961, 51, ^693.

[6] MARTINUZZI (S.), ^{C. R. Acad.} Sc., 1961, 253, 1157 ^and 1962, 255, ^110.

[7] ^MOULTON (C.), Nature, 1962, 195, ^793.

[8] ^GÜNTHER (K. G.), Naturwiss., 1958, 45, 415.

[9] ^HÖNIG (R. E.), ^{R. C.} A., Review, 196-2, 23, 567.

[10] ^MELVILLE (H. N.) ^{and GRAY} (S. C.), ^Trans. Faraday Soc., 1936, 32, 271 ^{and 1026.}

[11] ^NEWMAN (R.), Phys. Rev., 1958, 111, 1518.

[12] ^STURGE (M. D.), Phys. Rev., 1962, 127, 768.

[13] ^KUDMAN (I.) ^{and SEIDEL} (T.), ^J. Appl. Physics, 1962,

33, ^771.