A METHOD FOR THE LOW TEMPERATURE CLEANING OF SUBSTRATES CONTAINING INDIUM OR ANTIMONY

Abstract: A method for low temperature cleaning of group III-V semiconductor substrates, in particular substrates containing indium or antimonide, whereby the substrate is heated in an oxygen-free environment and exposed to a chemical cleaning agent. The chemical is of the form (Me2N)3-X, where X is a group V element (e.g. arsenic (As), antimony (Sb), phosphorus (P)), also present in the substrate. Examples of the chemicals are tris(dimethylamino)arsine [TDMAAs] = (Me2N)3As, tris(dimethylamino)antimony [TDMASb] = (Me2N)3Sb, and tris(dimethylamino)phosphine [TDMAP] = (Me2N)3P. The cleaning process removes the native oxide layer on the surface of the substrate, leaving a smooth, atomically flat surface suitable for epitaxial growth. The process is of particular importance to thermally unstable substrates, for example indium antimonide (InSb), which are difficult to clean using conventional techniques.

Description: A method for the low temperature cleaning of substrates containing indium or antimony This invention relates to a method for the low temperature cleaning of group III-V semiconductor substrates, in particular substrates containing indium or antimony. The preparation of clean, atomically flat substrate surfaces, and the removal of native oxides prior to epitaxial growth, is of major importance in the manufacture of electronic and photonic devices based on group III-V semiconductors. The most important surface plane in device fabrication is the (001) plane and the technique most widely used to clean the substrate surface is thermal desorption ofthe native oxide, often using a group V overpressure to maintain the surface stoichiometry. However, this process is known to result in a roughening ofthe surface on an atomic scale [G.W. Smith et al. 1991, Appl. Phys. Lett. 59(25)3282] which has to be smoothed out by the growth of buffer layers before epitaxial growth is possible. Indium antimonide (InSb) is an important group III-V semiconductor and has electronic properties which are well known for their applications to mid-infrared emitters and detectors, and ultra high speed electronic devices. However, the conventional thermal desorption technique is not practical for cleaning InSb substrates. The oxide desorption temperature of InSb is very close to the bulk melting temperature of 525°C, and well above the non-congruent evaporation temperature of 325°C, and consequently the thermal desorption process takes several hours [J.F. Klem et al. 1991 J. Vac. Sci. Technol. A 9 (6) 2996] . The process also results in a very rough surface, characterized by large In droplets, which is not suitable for epitaxial growth. Another technique employed to remove the native oxide from InSb substrates is argon ion bombardment. This technique uses argon ions of several hundred eV to sputter the oxide from the surface, followed by low temperature annealing to reduce any structural damage induced by the ion beam. Like the thermal desorption technique, this process is time consuming and argon ion cleaned surfaces are usually indium rich, due to the preferential removal ofthe group V species. The cleaned surface also has an n-type layer as a consequence of damage by the ion beam. A more recently developed technique for cleaning InSb substrates makes use of an electron cyclotron resonance (ECR) hydrogen plasma, in combination with an antimony overpressure. Although this technique yields a surface which is free of indium islands and exhibits an antimony rich surface, the process requires expensive equipment. The present invention relates to a method for the low temperature preparation of oxide-free, atomically flat substrates using a chemical cleaning agent. The resulting surface is suitable for epitaxial growth. The method has important advantages for the preparation of thermally unstable substrates, particularly InSb, since it may be carried out at temperatures significantly below those required for thermal desorption techniques. The present method can be much less time- consuming than the thermal desorption process and also offers the advantage of a single gas source compared to a much more expensive plasma source. According to this invention, a method for removing oxide from a substrate containing indium or antimony comprises the steps of; placing said substrate inside an oxygen-free environment, heating said substrate to a suitably high temperature in excess of 300°C and exposing said substrate to a chemical ofthe form (Me 2 N) 3 -X, where X is a group V element, until the process of heating the substrate and exposing the substrate to said chemical removes said oxide on the surface ofthe substrate. -> In a preferred embodiment, the method includes the further steps of annealing said substrate for a period of time up to 30 minutes, and typically for 10 minutes, during which time the temperature ofthe substrate is maintained and the substrate remains exposed to the chemical. In a further preferred embodiment, the element X is one of arsenic (As), antimony (Sb) or phosphorus (P). In a further preferred embodiment, the chemical of the form (Me 2 N) 3 -X and the substrate are comprised of the same group V element. In one embodiment ofthe invention, the chemical is tris(dimethylamino)antimony, (Me 2 N) 3 Sb, and the substrate is indium antimonide (InSb). In another embodiment ofthe invention, the chemical is tris(dimethylamino)arsine, (Me 2 N) 3 As, and the substrate is indium arsenide (InAs). In another embodiment ofthe invention, the chemical is tris(dimethylamino)phosphine, (Me 2 N) 3 P, and the substrate is indium phosphide (InP). In another embodiment ofthe invention, the chemical is tris(dimethylamino)antimony, (Me 2 N) 3 Sb, and the substrate is gallium antimonide (GaSb). The above method may also be used for the low temperature cleaning of other group III-V semiconductor substrates, for example, gallium phosphide (GaP). The invention will now be described, by example only, with reference to the accompanying figures in which: - Figure 1 shows a schematic diagram of a system which may be used to remove the oxide from a group III-V semiconductor substrate, in particular a substrate containing indium or antimony, by exposure to a chemical acting as a cleaning agent and Figure 2 shows the structure of a chemical ofthe form (Me 2 N) 3 -X, where X is a group V element, e.g. arsenic (As), antimony (Sb) or phosphorus (P). Referring to Figure 1, the substrate 1 has an oxide layer 2 which must be removed, leaving a smooth group V terminated surface suitable for epitaxial growth. In this example, the reactor 4 is a standard CBE-style (Chemical Beam Epitaxy) reactor and the pressure inside the reactor 4 is maintained at a pressure of between 10 " and 10 " torr by means of a vacuum pump 5. The substrate 1 is heated by means of a heating element 6 and is exposed to a chemical 7 until the oxide layer 2 is seen to have been removed. Chemical 7 is ofthe form (Me 2 N) 3 -X. where X is a group V element, for example, As, Sb or P. Examples of these chemicals are tris(dimethylamino)arsine [TDMAAs] = (Me 2 N) 3 As, tris(dimethylamino)antimony [TDMASb] = (Me 2 N) 3 Sb and tris(dimethylamino)phosphine [TDMAP] = (Me 2 N) 3 P. The chemical 7 is introduced from a source 8 into the reactor 4. This is enabled by means of a main valve 9, which turns the chemical supply on and off, and a variable leak valve 10 which regulates the rate of flow ofthe chemical 7 into the reactor 4. The chemical 7 is passed into the reactor 4 along heated gas lines 11 to prevent condensation. Removal ofthe oxide layer is initiated by a chemical reaction between the compound (Me 2 N) 3 - X and the surface oxide species, whereas the presence ofthe arsenic, antimony or phosphorus in the chemical leaves a smooth surface morphology if used on a substrate which contains the same group V element. Therefore, preferably, the substrate 1 is exposed to a chemical 7 comprising of the group V element present in the substrate. For example, if the substrate 1 to be cleaned is InSb, preferably the chemical 7 contains antimony e.g. TDMASb, if the substrate 1 to be cleaned is InAs, preferably the chemical 7 contains arsenic, e.g. TDMAAs and if the substrate 1 to be cleaned is InP, preferably the chemical 7 contains phosphorus, e.g. TDMAP. However, oxide removal can still be initiated by using any other chemical ofthe form (Me 2 N) 3 -X, where X is a group V element. In order to initiate the chemical reaction that causes the oxide removal, a substrate temperature in excess of 300°C is required. As the reaction proceeds at a higher rate for higher substrate temperatures, it was found to be more convenient to heat the substrate to a temperature in excess ofthe minimum temperature required to cause the oxide removal. For example, the oxide was removed from InSb by holding the temperature ofthe substrate at 400°C and exposing to TDMASb. For InAs substrates, the oxide was removed by holding the temperature ofthe substrate at 380°C and exposing to TDMAAs. These temperatures are considerably lower than those required for conventional thermal desorption processes. During the process, the system shown in Figure 1 was monitored by using RHEED (Reflection High Energy Electron Diffraction) which requires a RHEED gun 12 on one side ofthe reactor and a RHEED screen 13 on the opposite side ofthe reactor. The appearance of diffraction spots in the RHEED pattern, observed by means ofthe RHEED screen 13, indicates the onset ofthe oxide removal. The use of a RHEED technique in this way would be conventional to one skilled in the art. When the oxide layer 2 is observed to have been removed, the substrate 1 is annealed for a period of time up to 30 minutes, and typically for 10 minutes. During this time, the substrate 1 continues to be exposed to the chemical 7 and the temperature ofthe substrate 1 is maintained throughout this time. For example, where an InSb substrate is to be cleaned, the substrate is maintained at a temperature of approximately 400°C, and where an InAs substrate is to be cleaned, the substrate temperature is maintained at approximately 380°C. The temperature ofthe substrate was monitored by means of a thermocouple calibrated using a technique conventional to one skilled in the art. Following the annealing process, the substrate is cooled at a rate of approximately 30°C per minute. When the temperature decreases to approximately 300°C the exposure to the chemical 6 is ceased and the substrate is ready for subsequent processing steps, such as epitaxial growth of the device layers. Alternatively, at this stage, the substrate 1 may be removed from the reactor 4 and Atomic Force Microscopy (AFM) images may be used to indicate the state ofthe surface morphology. The process of AFM would be conventional to one skilled in the art. As an alternative example to the system shown in Figure 1 , the process may also be carried out in a standard MOVPE-style (Metal-Organic Vapour Phase Epitaxy) reactor. Referring to Figure 2, fhe diagram shows the structure of a chemical ofthe form (Me 2 N) 3 -X, where 14 is a group V element. For example, if the element 14 is As, the chemical is tris(dimethylamino)arsine [TDMAAs], if the element 14 is Sb, the chemical is tris(dimethylamino)antimony [TDMASb] and the if element 14 is P, the chemical is tris(dimethylamino)phosphine [TDMAP].

Claims: Claims 1. A method for removing oxide from the surface of a substrate containing indium or antimony comprising fhe steps of:- placing said substrate inside an oxygen-free environment, heating said substrate to a suitably high temperature in excess of 300°C and exposing said substrate to a chemical ofthe form (Me 2 N) 3 -X, where X is a group V element, until the process of heating the substrate and exposing the substrate to said chemical removes said oxide on the surface ofthe substrate. 2. The method of claim 1, including the further steps of annealing said substrate for a period of time up to 30 minutes, and typically for 10 minutes, during which time the temperature ofthe substrate is maintained and the substrate remains exposed to the chemical. 3. The method ofthe preceding claims wherein the element X is one of arsenic (As), antimony (Sb) or phosphorus (P). 4. The method of claim 3 wherein the chemical of the form (Me 2 N) 3 -X and the substrate are comprised ofthe same group V element. 5. The method of claim 3 wherein the chemical is tris(dimethylamino)antimony, (Me 2 N) 3 Sb, and the substrate is indium antimonide (InSb). 6. The method of claim 3 wherein the chemical is tris(dimethylamino)arsine, (Me 2 N) 3 As, and the substrate is indium arsenide (InAs). 7. The method of claim 3 wherein the chemical is tris(dimethylamino)phosphine, (Me 2 N) 3 P, and the substrate is indium phosphide (InP). 8. The method of claim 3 wherein the chemical is tris(dimethylamino)antimony, (Me 2 N) 3 Sb, and the substrate is gallium antimonide (GaSb).

Inventors: FREER RICHARD WILLIAM, JOHNSON ANDREW DAVID, MARTIN TREVOR, WHITAKER TIMOTHY JOHN, FREER, RICHARD, WILLIAM, JOHNSON, ANDREW, DAVID, MARTIN, TREVOR, WHITAKER, TIMOTHY, JOHN
Applicants: FREER RICHARD WILLIAM, JOHNSON ANDREW DAVID, MARTIN TREVOR, SECR DEFENCE, WHITAKER TIMOTHY JOHN, FREER, RICHARD, WILLIAM, JOHNSON, ANDREW, DAVID, MARTIN, TREVOR, THE SECRETARY OF STATE FOR DEFENCE, WHITAKER, TIMOTHY, JOHN
Application year: 1996
Main classification: H01L21
IPC codes: H01L21