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

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1 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 (12), pp < /jphys: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1964 HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published 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.

2 IIIV On Films LE JOURNAL DE PHYSIQUE TOME 25, JANVIERFEVRIER 1964, 212. 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. 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 IIIV. On a trouvé que la décroissance de l absorption 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. 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 IIIV 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. semiconductor compounds have many interesting and useful properties but in many cases they are difficult to prepare 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 stoichiometry 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 mixtures of IIIV compounds, a continuous variation of energy gap from GaP to InSb with the approximate 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 additional 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 polycrystalline growth reported for films grown on amorphous substrates at elevated temperatures, the microcrystal size and orientation increasing with substrate temperature [1, 2]. IIIV compound 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 fashion 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 " method of Günther has been used and modified to give greater control. The " three temperature " method, as applied to producing films of IIIV com as the conti pounds, can be summarised briefly 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 substrate. 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 reevaporating. If during this time they combine with a molecule of the nonvolatile 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

3 213 chiometric material may be expected under a range of conditions because of the reevaporation of the excess volatile component. The mass of material arriving unit time is given by : at unit area in 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 coefficient, a function of many variables. This equation provides a way of estimating source temperatures 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 temperature the temperature of the arsenic source was adjusted to give satisfactory films. Smooth hard coherent films were obtained which with Xray powder diffraction proved to be gallium arsenide. Films were successfully prepared on substrates higher temperatures and thick up to 700 OC. At 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 coefficient 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 although leading to the production of gallium arsenide 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 gallium over than that of arsenic vapour over the element. 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 figure 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 temperature 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 evaporation despite unequal losses. The mixture was contained in an alumina crucible at C. Another method of preparation of films of gallium arsenide used was by direct evaporation of the compound. On examination of vapour pressure curves available it would appear that dissociation 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 temperatures 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 suboxide, which is volatile, or by nondissociation of the vapour. Films were formed from compound source at a comparatively low temperature in open or semiclosed 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

4 Transmission 214 was found to be independent of oxygen pressure from about 105 torr to 1 torr, except that nonvolatile 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 reevaporation 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 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 interference 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 : 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 Assuming refractive index of the substrate only. a linear variation of refractive index with wavelength the order of fringe observed and the refractive index at other fringe positions can be found and hence also the film thickness. Where the film is absorbing to a reasonable extent the be obtained absorption coefficient can then easily at positions mid way between maximum and minimum 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 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 FIG. 2. 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 available [11, 12], indicated that the absorption edge extended to longer wavelengths in the film and was considerably less steep. Investigation ot the films by Xray powder diffraction revealed almost amorphous structure. Examination of films of

5 Transmission Refractive Refractive Comparison 215 FIG. 3. of a InP film grown on glass at ambient temperature. 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 wavelengths as shown in figure 8. An examination of FIG. 4. index versus wavelength for GaAs films. FIG. 7. of film and single crystal GaAs. FIG. 5. index versus wavelength for InP films. gallium arsenide grown on glass and quartz substrates at higher temperatures gave steeper absorption edges but there was still considerable excess of the films as a function of film thickness and of substrate temperature was under the properties 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 105 torr upwards. It can be seen that the steepness of the absorption., edge improves with increased substrate temperature but the maximum transmission falls after a substrate temperature of 550 C. The films were the same thickness which was all of approximately

6 Percentage Logarithmic Percentage 216 FIG. 8. transmission of GaAs film. transmission versus wavelength for GaAs films of varying thickness grown at a substrate temperature of 500 OC. FIG. 10. microcrystal 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. FIG. 9. transmission versus wavelength for GaAs films deposited at various temperatures. Investigation of the properties of one micron. 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. Influence of substrate temperature varies the film thickness at which this cloudy surface appears. Xray powder diffraction measurements indicated 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 temperatures 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. Britten, W. Morton and A. Porteous in the preparation and measurement of evaporated films and to the Ministry of Aviation who supported this work.

7 Pouvez 217 M. PAPARODITIS. Discussion 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 transport. The temperature of the quartz wool was measured. M. PERNY. 1) N y atil pas des niveaux discrets localises au voisinage de la bande d absorption fondamentale et qui pourraient etre dus a des excitons?. 2) comment avezvous trace les courbes de dispersion? Comment a ete determine l ordre d interférence 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 prepared 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 interference could be verified by the peak transmittance. Quantitative measurements were used for absorption coefficient. Interference order gave the variation 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 Symposium, 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 [4] PAPARODITIS (C.), As above, pp [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, 1962, 23, 567. [10] MELVILLE (H. N.) and GRAY (S. C.), Trans. Faraday Soc., 1936, 32, 271 and [11] NEWMAN (R.), Phys. Rev., 1958, 111, [12] STURGE (M. D.), Phys. Rev., 1962, 127, 768. [13] KUDMAN (I.) and SEIDEL (T.), J. Appl. Physics, 1962, 33, 771.