A METHOD OF PREPARING SEMICONDUCTOR SUBSTRATES

Abstract: A new method for GaAs substrate preparation which significantly reduces the formation of oval defects during MBE growth of selectively doped n-Al x Ga 1-x As/GaAs hetero­ structures. The method simply requires treatment in H₂SO₄ after mechano-chemical polishing in NaOCl solution and generation of a protective surface oxide during In soldering. Routinely a density of oval defects of less than 200 cm ² is achieved for 2-µm thick heterostructures. The efficiency of the new preparation procedure is demonstrated by 2DEG mobilities in excess of 10⁶ cm²/Vs at 6 K obtained with a spa­ cer width as narrow as 18 nm.

Description: The present invention relates to a method of preparing a GaAs semiconductor substrate to reduce the formation of defects, in particular of oval defects, during subsequent growth of semiconductor structures thereon, in particular growth of heterostructures by molecular beam epitaxy, wherein the substrate is first subjected to mechano-chemical etch polishing using an abrasive free fabric or paper polishing lap soaked with an oxidising etching solution and is provided with a thin oxide layer as protection against contamination during handling prior to growth of a said semiconductor structure thereon. A method of this kind is known from the document "Applied Physics Letters", vol. 39, No. 4, August 1981, pages 316-318. Here mechano-chemical etch polishing is carried out on InP and GaAs substrates using bromine in methanol on a paper polishing lap. The substrates required both a pre-treatment and also stripping in chemical solutions prior to further use. In general it is also known to chemically clean substrates and to provide them with a thin oxide layer as protection against contamination during handling prior to growth of a said semiconductor structure thereon. A method of this kind is described in United States patent No. 3 969 164 and also in the paper by Dr. K. Ploog in "Crystals: Growth, Properties and Applications" edited by H.C. Freyhardt, Springer Press, Berlin, Heidelberg 1980, vol. 3, page 73 and in a paper by A.Y. Cho entitled Growth of III-V Semiconductors by Molecular Beam Epitaxy and their Properties published in 1983 in "Thin Solid Films", vol. 100, page 291. A method of the above described kind is of particular interest in connection with the growth by molecular beam epitaxy of selectively doped n-Al x Ga l-x As/GaAs heterostructures, the method is however not restricted to these structures and preliminary investigations indicate that it can also be used with other semiconductor substrates. A contamination-free GaAs surface is crucial for the quality of the GaAs and Al x Ga l-x As layers subsequently grown by molecular beam epitaxy (MBE). The substrate wafers are normally cleaned with various chemical solutions, and then a thin oxide layer is produced to protect the GaAs surface from contamination during further handling. The most commonly used method to prepare GaAs substrates is chemical etching based on a H₂SO₄/H₂O/H₂O mixture, which was assumed to produce a passivating thin surface-oxide layer. This oxide layer, which is removable by heating prior to MBE growth, preserves the etched surface from carbon contamination during substrate handling after the etching process. Recently, however, it has been shown that the procedure of H₂SO₄/H₂O₂/H₂O etching and rinsing in water does in fact not produce any passivating oxide film on the (100) GaAs surface. Instead, the surface oxide is produced only by the sample handling in air after etching, in particular during heating for indium soldering on the molybdenum sample holder. described immediately above More specifically the prior art method/involves multiple degreasing by successively rinsing the substrate in trichloroethylene, acetone and then water. Thereafter the substrate is boiled in hydrochloric acid and free-etched in a stagnant solution of (3:1:1) H₂SO₄/H₂O₂/H₂O at 48°C for one minute. (The aforementioned concentration, temperature and time statements are typical values which may be varied in practice). To stop the etchant the substrate in wafer form is then flooded in water and blown-dry with filtered N₂ gas. It will be apparent from the foregoing that the known method involves a large number of separate steps and is particularly complicated by the need to use the boiling hydrochloric acid and an etching solution at an elevated temperature. The plurality of steps involved also means that substrate treatment takes a relatively long period of time. The object underlying the present invention is thus to provide a method of preparing semiconductor substrates of the initially named kind which is simple and can be carried out relatively rapidly and which reliably and drastically reduces the formation of oval defects during subsequent growth of semiconductor structures on the substrate. It is a further object of the present invention to provide a method of preparing semiconductor substrates which allows them to be stored in air for a considerable period of time, of at least two to three weeks, prior to the growth of semiconductor structures thereon, without loosing their defect inhibiting properties. In order to satisfy these objects there is provided, in accordance with the present invention, a method of the initially named kind, which is characterised in that the substrate is treated after the mechano-chemical etch polishing by placing it in concentrated H₂SO₄ subjected to agitation and is then heated under dust free conditions to form said oxide layer. Applied Physics Letters, 47 (1985), Sept. No. 5, page 518 discloses the treatment of InP substrates by subjecting them to a Br-methanol polish, by boiling them in organic solvents and by swabbing them with sulphuric acid. Apart from the complexity of boiling the substrates in organic solvents no particulars are given of the concentration of the sulphuric acid. Patent Abstracts of Japan, vol. 6, No. 200 E-135 (1078), October 9th, 1982 discloses the removal of a very small amount of organic matter by a method wherein when a liquid-phase epitaxial growing is performed by providing a hole on an InP semiconductor substrate or an epitaxial crystal, the above is treated with concentrated sulphuric and as a pre-treatment. The oxidising etching solution used for the present invention is preferably NaOCl , preferably with a concentration in the range 0.5 to 6.5 % active chlorine, and in particular with a concentration of 2.0 %. The use of NaOCl as the oxidising etching solution during mechano-chemical etch polishing of III-V semiconductors is known per se from FR-A-2 168 036, which does not however give particulars of the concentration of active chlorine that is used. This French reference also discusses the use of bromine-methanol as the oxidising-etching solution. After polishing the substrates of the citation are again subjected to treatment with H₂SO₄, H₂O₂ and H₂O. The H₂SO₄ used for the presently proposed method preferably has a concentration of 98 %. With this method both the mechano-chemical etch polishing and the treatment in concentrated H₂SO₄ are carried out at room temperature. Accordingly no complicated and potentially dangerous thermal treatments are required. The treatment step in concentrated acid is preferably carried out by sequentially placing the substrate in two separate baths of the concentrated acid while the baths are subjected to ultrasonic agitation. It will be seen from the foregoing that the total number of steps involved are fewer than with the known method and that all treatment is carried out at room temperature and indeed using relatively simple chemicals and apparatus, all of which combine to yield a substantial time saving. Moreover, the efficiency of the new substrate preparation procedure is demonstrated by the excellent quality of selectively doped n-Al x Gal l-x As/GaAs heterostructures which have been produced and which exhibit a high mobility two-dimensional electron gas (2DEG). More specifically mobilities in access of 10⁶cm²/Vs have been obtained under illumination using a 18 nm undoped Al x Ga l-x As/GaAs spacer. These values are among the highest reported for this narrow spacer. Even more important is the fact that the density of overall defects is drastically reduced to routinely below 200 cm - ² by this substrate preparation method. In accordance with the invention the mechano-chemical polishing step is conveniently carried out using a polishing lap comprising lens paper of the type sold by the company Polytex under the trade designation Supreme 1241142. With a sawn substrate, the sawn surface is first polished with diamond paste to remove saw-cut damage prior to mechano-chemical etch polishing. Although NaOCl is the preferred oxidising etching solution for GaAs substrates, it is also possible to use other oxidising etching solutions such as bromine. methanol. The invention will now be more fully described with reference to a specific example and with reference to the drawings in which: Fig. 1 shows a micrograph of the semiconductor substrate surface after treatment in H₂SO₄, and Fig. 2 shows the Hall mobility as a function of temperature obtained from two heterostructures one deposited on a substrate prepared in accordance with the prior art method (sample 1) and the other deposited on a substrate prepared in accordance with the present invention (sample 2). In the example given below a gallium arsenide substrate is prepared for the subsequent growth of Si-doped n-Al x Ga l-x As/GaAs heterostructures. The selectively Si-doped n-Al x Ga l-x As heterostructures were grown in an MBE system of the quasi-horizontal evaporation type which comprises a continuously rotating substrate holder and a liquid nitrogen (LN₂) cryoshroud encircling the entire growth region. The heterostructures were deposited on (100) oriented undoped semi-insulating GaAs substrate (etch-pitch density ca. 5 x 10⁴cm - ²) using a GaAs growth rate of 1 µm/hr and a growth temperature as low as 600°C. Two substrate slices cleaved from the same 2-inch diameter liquid encapsulation Czochralsky (LEC) grown GaAs wafer were soldered after treatment as described below to the Mo sample holder for simultaneous deposition. The uncleaved wafer was first polished with diamond paste to remove saw-cut damage followed by etch-polishing (lapping) on an abrasive-free lens paper soaked with NaOCl solution leaving a mirror-like finish. Half of the wafer was then etched in the following conventional manner. The slice was first rinsed successively in trichloroethylene, acetone and water for a final degrease. After boiling in hydrochloric acid, the substrate was free-etched in a stagnant solution of (3:1:1) H₂SO₄/H₂O₂/H₂O at 48°C for one minute. To stop the etchant the wafer was flooded in water and blown-dry with filtered N₂ gas. The second half of the GaAs wafer was simply placed twice in concentrated H₂SO₄ kept at 300 K and stirred ultrasonically. The slice was then carefully rinsed in water and blown-dry with N₂. Both parts of the original wafer were then soldered with liquid indium to the preheated (250°C) molybdenum sample holder under dust-free conditions, on a clean bench in a clean room, and immediately transferred to the sample load-lock system. For sample 2 the substrate is first heated to 250° to 300°C for three minutes under dust-free conditions prior to soldering. This preheating is an important step of the invention. The duration of the two treatments in H₂SO₄ is uncritical and normally amounts to 5 to 10 minutes (per treatment). A substantially longer treatment time is not disadvantageous provided precautions are taken to ensure that the temperature of the sulphuric acid bath remains at room temperature. An increase in temperature is to be feared because the bath is ultrasonically stirred. The treatment in two subsequent acid baths is preferred so that contamination remains in the first bath. Massies and Contour have shown that during this soldering process, which takes several minutes, the oxide on the (100) GaAs substrate is generated to protect the surface from carbon contamination. Their work is described in the Journal of Applied Physics, vol. 58, page 806 (1985). After transfer to the MBE growth chamber the surface oxide is thermally absorbed by heating the substrate wafers to 530°C to 560°C in a flux of As₄. When the desorption process is finished a clear (2 x 4) surface reconstruction is observed in the reflection high energy electron diffraction (RHEED) pattern. The layer sequence of the heterostructures includes an undoped 2-µm thick p - -GaAs buffer layer with a residual doping concentration of 10¹⁴cm - ³, an undoped Al₀.₃₃Ga₀.₆₇As spacer of 18 nm thickness, a Si-doped Al₀.₃₃Ga₀.₆₇As layer of 53 nm thickness, and an undoped GaAs cap layer of 8 nm thickness. After growth the epilayers were examined by a phase-contrast microscope. Hall effect measurements in the temperature range 4K < T < 300 K were performed on cleaved square-shaped specimens using the van der Pauw technique. The samples were mounted in a continuous He flow cryostat, and the measurements were controlled by a computer. Fig. 1 shows the micrograph of a typical area of the wafer treated in H₂SO₄ only, which clearly indicates the absence of any oval defects, i.e. defects of the type described in the papers by C.E.C. Wood, L. Rathbun, H. Ohno, D. De Simone in the Journal of Crystal Growth, vol. 51, pages 299 (1981) and by Y. Suzuki, M. Seki, Y. Horikoshi and H. Okamoto in the Japanese Journal of Applied Physics, vol. 23, page 164 (1984). The total density of oval defects, originating mostly at the epilayer-substrate interface, is less than 200 cm - ² for the 2 µm thick heterostructure grown on a substrate prepared in accordance with the invention. This result demonstrates that simple but careful preparation procedure of the GaAs substrate surface proposed herein, is crucial in obtaining nearly defect-free epilayers. It is important to note that a similar low density of oval defects for the specimen chemically etched in the H₂SO₄/H₂O₄/H₂O mixture can be achieved only if the generation of the surface oxide during In soldering is performed under extremely dust-free conditions. Treatment of the GaAs substrate in accordance with the invention with H₂SO₄ makes the surface less sensitive to particulate contamination. The advantages of the simple H₂SO₄ treatment of GaAs substrates can be demonstrated by means of Hall effect measurements on selectively doped n-Al₀.₃₃Ga₀.₆₇As/GaAs heterostructures. In Fig. 2 the Hall mobility is shown as a function of temperature obtained from two heterostructures deposited simultaneously on H₂SO₄/H₂O₂/H₂O-etched (sample 1) and on H₂SO₄ -treated (sample 2) substrate slices. For the heterostructure grown on the H₂SO₄ -treated surface (2) a Hall mobility is achieved of µ = 7.3 x 10⁴ cm²/Vs with a 2DEG channel density of n s = 4.4 x 10¹¹ cm - ² at 300 K. With reduced sample temperature the mobility strongly increases and at 6 K reaches a value of 0.59 x 10 6 cm²/Vs with n s = 4.3 x 10¹¹ cm - ² measured after illumination both at a magnetic field of 0.1 T. These mobility values are among the highest reported so far for selectively doped n-Al x Ga l-x As/GaAs heterostructures having a spacer width as narrow as 18 nm. Similar mobility values and carrier densities were obtained for sample 1, which was grown on a conventionally H₂SO₄/H₂O₂/H₂O -etched substrate, to within 10% of the sample 2 data (Fig. 2). This small difference is due to the uncertainty of the applied measuring method. In conclusion, a new time-saving method for preparation of GaAs substrates for MBE growth has been provided which simply requires a treatment in H₂SO₄ after mechano-chemical polishing in NaOCl solution. This treatment significantly facilitates the generation of protective surface oxides during In soldering at elevated temperature and thus avoids particulate contamination. A density of oval defects of less than 200 cm - ² is routinely achieved for 2 µm thick heterostructures grown on H₂SO₄ -treated substrates. The advantage of the new substrate preparation method is demonstrated by a high-mobility 2DEG in selectively doped n-Al x Ga x- ₁As/GaAs heterostructures. Using a spacer as narrow as 18 nm, mobilities in excess of 10⁶ cm²/Vs are achieved at 6 K under illumination.

Claims: A method of preparing a GaAs semiconductor substrate to reduce the formation of defects, in particular of oval defects, during subsequent growth of semiconductor structures thereon, in particular growth of heterostructures by molecular beam epitaxy, wherein the substrate is first subjected to mechano-chemical etch polishing using an abrasive free fabric or paper polishing lap soaked with an oxidising etching solution and is provided with a thin oxide layer as protection against contamination during handling prior to growth of a said semiconductor structure thereon, characterised in that the substrate is treated after the mechano-chemical etch polishing by placing it in concentrated H₂SO₄ subjected to agitation and is then heated under dust free conditions to form said oxide layer. A method in accordance with claim 1, characterised in that said oxidising etching solution is NaOCl, preferably with a concentration in the range 0.5 to 6.5 % active chlorine, and in particular with a concentration of 2.0 %. A method in accordance with claim 1 or claim 2, characterised in that the H₂SO₄ preferably has a concentration of 98 %. A method in accordance with any one of the preceding claims, wherein both said mechano-chemical etch polishing and said treatment in concentrated H₂SO₄ are carried out at room temperature. A method in accordance with any one of the preceding I claims, characterised in that the substrate is sequentially placed in two separate baths of said concentrated H₂SO₄ and said baths are preferably subjected to ultrasonic agitation, with the duration of the treatment in the concentrated acid preferably lying between 5 and 10 minutes in each bath. A method in accordance with any one of the preceding claims, characterised in that the polishing lap comprises lens paper of the type sold by the company Polytex under the trade designation Supreme 1241142. A method in accordance with any one of the preceding claims, characterised in that the substrate is preheated under dust-free conditions in air prior to mounting to the substrate holder. A method in accordance with claim 7, wherein the substrate is preheated in air at 250 - 300°C for three minutes. A method in accordance with any one of the preceding claims, characterised in that, with a sawn substrate the sawn surface is first polished with diamond paste to remove saw-cut damage prior to mechano-chemical etch polishing. A method in accordance with claim 1, characterised in that the treatment in the concentrated acid is effected for approximately 5 minutes or longer. A method in accordance with claim 1, characterised in that said oxidising-etching solution is bromine methanol.

Inventors: FISCHER, ALBRECHT, FRONIUS, HANS, PLOOG, KLAUS
Applicants: MAX-PLANCK-GESELLSCHAFT ZUR FOERDERUNG DER WISSENSCHAFTEN E.V.
Application year: 1986
Main classification: H01L21
IPC codes: H01L21