Method of growing multilayer crystal films by metal organic vapor phase epitaxy

Abstract: Disclosed are methods of preparing multilayer structures with InGaAsP layers of different compositions by metal organic vapor phase epitaxy, which result in formation of sharp heterointerfaces. After an InGaAsP well layer has been grown, the process is kept on standby with a flow of AsH 3 and PH 3 , which are sources comprising elements of group V, at the well's composition ratios, and then with a flow of a source comprising an element of group V, including TBP (TBP/standby step), and an InGaAsP barrier layer is grown which has a smaller arsenic content than the well layer. TBP has a decomposition temperature.approximately 100°C lower than PH 3 , and thus provides a phosphorus pressure which is five times or more as high as that of PH 3 at identical growth temperatures and at identical V/III ratios. Therefore, during the process of growth of multilayer InGaAsP films, TBP may be used in a standby step with a flow of a source comprising an element of group V at the same composition ratio as is designed for an InGaAsP layer which has a smaller arsenic content than the previously grown layer and is subsequently grown, to prevent arsenic desorption from chamber walls and group V species desorption from the exposed interfaces, thus realizing sharp heterointerfaces.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of crystal growth by metal organic vapor phase epitaxy, and more particularly to a method of growing multilayer films of InGaAsP crystal layers having at least different group V element compositions by metal organic vapor phase epitaxy. Description of the Related Art Most lasers for use in subscriber system optical communications require small driving currents at high temperatures and have InGaAsP multiple quantum well (MQW) structures. In cases where such MQW structures have sharp heterointerfaces, injected carriers are satisfactorily caught in the well layers owing to low non-radiative recombination rates at the interfaces, thus realizing small driving currents at high temperatures. In metal organic vapor phase epitaxy (MOVPE) of an InGaAsP MQW structure according to the conventional art, after the well layer was grown, the growth sequence is kept on standby with a flow of arsine (AsH 3 ) and phosphine (PH 3 ), the sources for the group V elements, first with the same flow rate as for the well layer, and then with a flow of AsH 3 and PH 3 designed for the barrier layers, which is subsequently grown. The quality of the interface was optimized through optimization, of the standby times. Since the InGaAsP barrier layer has a greater bandgap than the InGaAsP well layer, the composition ratio of arsenic in the barrier layer must be smaller than that in the well layer. When the above mentioned method was used to optimize the interface, the following shortcomings are evident. That is, excess arsenic is taken into the barrier layer for standby times shorter than required, whereas for longer standby times, arsenic is desorbed from the growth surface due to insuffficient overpressure of arsenic. This causes defects and changes in the composition at the interface, and results in failure to form a narrow interface in InGaAs multilayer films having different compositions. With semiconductor lasers fabricated with such multilayer InGaAsP films, it is difficult to accomplish small threshold currents and high slope efficiencies. SUMMARY OF THE INVENTION The invention provides a method of metal organic vapour phase epitaxy as defined in the appendent independent claim. Preferred or advantageous features of the invention are defined in dependent subclaims. The present invention may thus provide a method for growing a multilayer crystal film of InGaAsP, where the layers have at least different group V element compositions. The invention is advantageously characterized in that tertiary-butyl phosphine (TBP) is used in a step of keeping the growth sequence on standby with a flow of a source gas used for growing an InGaAsP barrier layer which subsequently will be grown with a smaller arsenic content than the previously grown InGaAsP layer. This standby step is hereafter referred to as the TBP/standby step). In a preferred embodiment, the present invention may also provide a modification of the above method of growing multilayer crystal films by metal organic vapor phase epitaxy, characterized in that the TBP/standby step is performed with a flow of TBP alone in between a standby step with a flow of a source gas having a same composition and flow rate ratios as the previously grown InGaAsP layer, and a standby step with a flow of a source gas having a same composition and flow rate ratio for an InGaAsP layer which is subsequently grown and has a smaller arsenic content than the former InGaAsP layer. In a further preferred embodiment, the present invention may also provide another modification of the above-mentioned method of growing multilayer crystal films by metal organic vapor phase epitaxy, characterized in that phosphine is introduced in addition to TBP in the TBP/standby step. TBP, which has a decomposition temperature approximately 100°C lower than PH 3 , is believed to provide a phosphorus pressure which is five times or more as high as that of PH 3 at the same growth temperatures and at the same V/III gas supply ratios (molar ratio). This high phosphorus pressure prevents desorption of residual arsenic from the deposition chamber walls and of group V element atoms exposed at the growth interface. Therefore, during the process of growing multilayer InGaAsP films, TBP can be effectively used during a standby step with a flow of a source gas designed for an InGaAsP layer which has a smaller arsenic content than the previously grown InGaAsP layer and which is to be subsequently grown, so as to prevent arsenic desorption from the chamber walls as well as from the growth interfaces and thus realizing sharp heterointerfaces. In addition, due to accelerated decomposition of the PH 3 in the presence of the TBP, the combined use of TBP and PH 3 results in a higher phosphorus pressure than in the cases where TBP alone is used. Thus, the desorption of arsenic is prevented more efficiently to realize sharp heterointerfaces between the InGaAsP layers. It should be noted that, the Japanese Unexamined Patent Application Disclosure HEI 5-102039 describes, with reference to FIG. 1 appended thereto, a growth suspension step in the process of growth of InGaAsP/InP by MOVPE, wherein the process is kept on standby with a flow of either TBA and TBP combined, or TBP alone after the growth of the InGaAsP, but before the growth of the InP. That invention disclosed in the official gazette is a solution to a problem which is encountered in the growth of the InP, different from the problem of the growth of multilayer InGaAsP films to be solved by the present invention; the operations and the effects are also different. BRIEF EXPLANATION OF THE DRAWINGS The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which: FIG. 1 is a view illustrative of a gas changeover sequence for transition from a well layer to a barrier layer during the process of growth of an InGaAsP MQW structure according to the present invention; FIG. 2 is a view illustrative of a gas changeover sequence for transition from a well layer to a barrier layer during the process of growth of an InGaAsP MQW structure according to the present invention; and FIG. 3 is a view illustrative of a gas changeover sequence for transition from a well layer to a barrier layer during the process of growth of an InGaAsP MQW structure according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a view illustrative of a gas changeover sequence for growth of an InGaAsP MQW structure according to a method by metal organic vapor phase epitaxy which is an embodiment of the invention. The present embodiment will be explained regarding the case of a MQW structure comprising an InGaAsP well layer of composition for a wavelength of 1.4 µm, and an InGaAsP barrier layer of composition for a wavelength of 1.13 µm. In addition, the standby time for the interface from a barrier layer to a well layer is set to zero seconds. A horizontal low-pressure MOVPE system equipped with a susceptor-rotating mechanism was used for growth. The growth temperature was set to 650°C, and the growth pressure to 2x10 4 Pa (50 Torr). For the growth of a well layer, there were used trimethyl indium (TMI) and triethyl gallium (TEG) as sources for group III elements, and AsH 3 and PH 3 as sources for group V elements with a V/III ratio (molar ratio of group V element sources to group III element sources) of 140. For the growth of a barrier layer, TMI and TEG were used as group III sources, and TBA and TBP as group V sources with a V/III ratio of 60. After the well layer was grown, flow of group III sources for the well layer was suspended, and the process was kept on standby for 10 seconds with only a flow of the group V (PH 3 and AsH 3 ) sources with the same composition ratios as in the well layer (standby with group V sources at the well's composition ratios). Then, but prior to the growth of a barrier layer, the process was kept on standby for 45 seconds with only a flow of group V (TBP and TBA) sources with the same composition ratios as were predetermined for the barrier layer (standby with group V sources at the barrier's composition ratios which also functions as the TBP/standby). Thereafter, a flow of group III sources for the barrier layer was introduced to grow the barrier layer, thus realizing an InGaAsP MQW structure. Although the present embodiment is an example of the case where the step of standby with group V sources at the barrier's composition ratios functions as the TBP/standby step, the TBP/standby step with a flow of TBP alone may first be carried out, and the step of standby with group V sources at the barrier's composition ratios which uses TBP and TBA may then be carried out to grow the barrier layer. separately, in order to optimize an InGaAsP MQW structures as described above, AsH 3 and PH 3 were used as sources for group V elements for growing a barrier layer at a V/III ratio of 250 to grow the structure while varying, from 0 to 100 seconds, the standby time during which the group V sources flow prior to the growth of the barrier layer. As a result, the photoluminescent line width at half maximum, which is an indicator of the sharpness and quality of an heterointerface, was found to be minimal when the standby time was 20 seconds. These two types of MQWs were buried in a DCPBH structure to prepare a semiconductor laser which was then tested with the following results: for cavity length = 300 µm, 30% front coating and 70% back coating, the threshold current was 20 mA, and the slope efficient was 0.3 W/A at 85°C, for the case where TBA and TBP were used as the group V sources for the barrier layer, whereas the threshold current was 25 mA, and the slope efficient was 0.25 W/A for the case where AsH 3 and PH 3 were used. Since the TBP/standby step contributed to formation of a sharp heterointerface between the InGaAsP well and barrier layers, the characteristics of the semiconductor laser were improved. FIG. 2 illustrates a gas changeover sequence for growth of an InGaAsP MQW structure by a method of growth which is according to an embodiment of the invention. The deposition chamber and the growth conditions used were the same as those used in the first embodiment. For the growth of a well layer and a barrier layer there were used TMI and TEG as group III sources, and AsH 3 and PH 3 as group V sources with V/III ratios of 140 and 250, respectively. After the well layer was grown, the flow of the group III sources for the well layer was suspended, and the process was kept on standby for 10 seconds with only a flow of the group V sources with the same composition ratios as in the well layer (standby with group V sources at the well's composition ratios). The process was then kept on standby for 20 seconds while only bubbled TBP was introduced at a flow rate of 150 cc/min at 20°C (TBP/standby), and then for 20 seconds with a flow of group V sources with the same composition ratios as were designed for the barrier layer (standby with group V sources at the barrier' composition ratios), and after this, sources comprising elements of group III for the barrier layer were introduced to grow a barrier layer, thus realising an InGaAsP MQW structure. This MQW was buried in a DCPBH structure to prepare a semiconductor layer which was then tested with the following results: for cavity length = 300 µm, 30% front coating and 70% back coating, the threshold current was 18 mA, and the slope efficient was 0.32 W/A at 85°C. This is due to improved sharpness of the interface between the well layer and the barrier layer according to the present embodiment. FIG. 3 illustrates a gas changeover sequence for growth of an InGaAsP MQW structure by a method for growth which is according to another embodiment of the invention. The InGaAsP MQW structure was prepared in the same manner as in the second embodiment except that PH 3 was added at a flow rate of 100 cc/min during the TBP/standby between the duration of the standby with the group V sources at the well's composition ratios and the duration of the standby with the group V sources at the barrier's composition ratios. This MQW was buried in a DCPBH structure to prepare a semiconductor layer which was then tested with the following results: for cavity length = 300 µm, 30% front coating and 70% back coating the threshold current was 15 mA, and the slope efficient was 0.35 W/A at 85°C. Since a higher phosphorus pressure was realized according to the present embodiment, a sharper heterointerface was accomplished between the InGaAsP well and barrier layers, and contributed to the more excellent characteristics of the semiconductor laser. As described above, the methods for growth by metal organic vapor phase epitaxy according to the present invention allow formation of sharp heterointerfaces in multilayer InGaAsP films of InGaAsP layers of different compositions in terms of elements of group V. The present invention may be applied, with particular advantages, to MQW structures, superlattice structures, etc., the physical properties of which are greatly influenced by the degree of sharpness of the interfaces. For example, application of the present invention to preparation of MQW structures of semiconductor lasers provides noticeable improvements in the characteristics of the lasers. In addition, although not demonstrated with embodiments, application of the present invention to preparation of superlattice structures of resonance tunnel diodes, and superlattice structures of avalanche photodiodes results in improved device characteristics.

Claims: A method of metal organic vapour phase epitaxy for growing multiply laminated layers of InGaAsP having different group V element compositions, characterised in that in at least a portion of a standby step, between steps of growing first and second laminated layers, a gas comprising tertiary-butyl phosphine is passed over the first layer. A method according to claim 1, in which the gas comprises two group V element source gases, one of which is tertiary-butyl phosphine, the flow of the two group V element source gases being continued after the standby step as source gases for the growth of the second layer, and the flow rate ratio of the two group V element source gases during the standby step being the same ratio as for the growth of the second layer. A method according to claim 1, in which the gas comprises only tertiary-butyl phosphine. A method according to claim 1, in which the gas comprises tertiary-butyl phosphine and PH 3 . A method according to any preceding claim in which; a flow of group III element source gases used during growth of the first layer is turned off at the end of the step of growing the first layer; in a first portion of the following standby step a flow of group V element source gases used during the step of growing the first layer is continued, the continued flow of the group V element source gases having the same group V element composition ratio as during the growth of the first layer; the continued flow of the group V element source gases is turned off at the end of the first portion of the standby step; and the gas comprising tertiary-butyl phosphine flows during a second portion of the standby step following the first portion. A method according to any of claims 1, 3, 4 or 5, in which the standby step comprises a third portion, following the portion in which the gas comprising tertiary-butyl phosphine is passed over the first layer, and in the third portion the flow of the gas comprising tertiary-butyl phosphine is turned off and a flow of the group V element source gases to be used during the following step of growing the second layer is turned on, the flow during the third portion of the standby step having the same group V element composition ratio as during the growth of the second layer, and in which a flow of the group III element source gases to be used during the step of growing the second layer is turned on at the beginning of that step. A method according to any preceding claim, in which the second layer has a smaller arsenic concentration than the first layer.

Inventors: AE SATOSHI, NAKAMURA TAKAHIRO, AE, SATOSHI, NAKAMURA, TAKAHIRO
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1996
Main classification: C30B25
IPC codes: H01L21, C23C16, C30B25, H01S5