METHOD OF FORMING QUANTUM DOTS FOR EXTENDED WAVELENGTH OPERATION
(19)AUSTRALIAN PATENT OFFICE(54)TitleMETHOD OF FORMING QUANTUM DOTS FOR EXTENDED WAVELENGTH OPERATION(51)6 International Patent Classification(s) H01L 021/00 (21) Application No: 2003239674 (22) Application Date: 2003.05.19(87) WIPONo: WO03/100833 (30) Priority Data (31) Number (32) Date (33) 0212055.8 2002.05.24 Country GB(43) Publication Date : 2003.12.12(43) Publication Journal Date : 2004.02.05(71) Applicant(s) IMPERIAL COLLEGE INNOVATIONS LIMITED(72)Inventor(s)JONES, Timothy, Simon; HOWE, Patrick; MURRAY, Ray; LE RU, Eric (H) Application NoAU2003239674 A1(19)AUSTRALIAN PATENT OFFICE(54)TitleMETHOD OF FORMING QUANTUM DOTS FOR EXTENDED WAVELENGTH OPERATION(51)6 International Patent Classification(s) H01L 021/00 (21) Application No: 2003239674 (22) Application Date: 2003.05.19(87) WIPONo: WO03/100833 (30) Priority Data (31) Number (32) Date (33) 0212055.8 2002.05.24 Country GB(43) Publication Date : 2003.12.12(43) Publication Journal Date : 2004.02.05(71) Applicant(s) IMPERIAL COLLEGE INNOVATIONS LIMITED(72)Inventor(s)JONES, Timothy, Simon; HOWE, Patrick; MURRAY, Ray; LE RU, Eric A method of forming the active region of an optoelectronic device incorporating semiconductor quantum dots whose ground state emission occurs at wavelengths beyond 1350 nm at a temperature of substantially 293 K is provided by forming a first layer of quantum dots covered by a spacer layer with strained areas extending there through. The spacer layer then forms a template upon which quantum dots of an active layer may be formed with a surface with a surface density and formation that is influenced by the underlying first layer of quantum dots. This allows a choice of growth parameters more favourable to the formation of quantum dots in the active layer emitting at long wavelengths with a narrow inhomogeneous broadening. As an example, the active layer of quantum dots may be formed at a lower temperature than the first layer of quantum dots. The quantum dots of the active layer are then subject to less intermixing with the surrounding spacer and capping layers, and can also preserve a more strain-relaxed state, which results in a longer wavelength emission with a narrower inhomogeneous broadening. This method is particularly well suited to the growth of the active region of an optoelectronic device on a GaAs substrate. A method of forming the active region of an optoelectronic device incorporating semiconductor quantum dots whose ground state emission occurs at wavelengths greater than 1350 nm at substantially 293 K, said method comprising the steps of: growing a first layer of quantum dots formed on one of a substrate layer or a buffer layer, said quantum dots of said first layer being subject to a strain due to a lattice mismatch between said substrate layer and said quantum dots of said first layer; growing a spacer layer over said first layer and said spacer layer being subject to a strain in strained areas overlying quantum dots of said first layer due to a lattice mismatch between said quantum dots of said first layer and said spacer layer; growing an active layer of quantum dots on said spacer layer, quantum dots of said active layer predominately forming upon strained areas of said spacer layer such that the surface density of quantum dots of said active layer is substantially determined by the surface density of quantum dots of said first layer, quantum dots of said active layer being in a strain-relaxed state in which said quantum dots of said active layer are subject to less strain than quantum dots grown on an unstrained surface, growth conditions for said active layer being different to those of the first layer and chosen appropriately, in particular substrate temperature being low enough, such as to substantially preserve said strain-relaxed state and to limit intermixing of said quantum dots of said active layer with said spacer layer; and growing a capping layer on said active layer, growth conditions for said capping layer chosen appropriately, in particular substrate temperature being low enough, such as to substantially preserve said strain-relaxed state and to limit intermixing of said quantum dots of said active layer with said spacer layer and with said capping layer. A method as claimed in claim 1, wherein said spacer layer is grown to a thickness of 3x10-9m to 3x10-8m. A method as claimed in any one of claims 1 and 2, wherein said first layer of quantum dots is grown at a growth rate of less than 0.06 monolayer per second. A method as claimed in any one of claims 1, 2 and 3, wherein said quantum dots in said first layer are grown to have a surface density of between 1013 and 1015 per square meter. A method as claimed in any one of claims 1, 2, 3 and 4, wherein said capping layer acts as a spacer layer for growth of one or more further active layers and capping layers. A method as claimed in any one of claims 1, 2, 3 and 4, comprising growing one or more further first layer, spacer layer, active layer and capping layer groups on said capping layer. A method as claimed in any one of the preceding claims, wherein said quantum dots are one of: (i) InAs quantum dots; (ii) InGaAs quantum dots; and (iii) GaInNAs quantum dots. A method as claimed in any one of the preceding claims, wherein at least part of said substrate layer or said buffer layer is one of: (i) GaAs; (ii) AlGaAs. A method as claimed in any one of the preceding claims, wherein at least part of said spacer layer is one of: (i) GaAs; (ii) AlGaAs; (iii) InGaAs; (iv) InAlGaAs; and (v) GaInNAs. A method as claimed in any one of the preceding claims, wherein at least part of said capping layer is one of: (i) GaAs; (ii) AlGaAs; (iii) InGaAs; (iv) InAlGaAs; and (v) GaInNAs. A method as claimed in any one of the preceding claims, wherein said active layer is operable to perform at least one of: (i) radiation emitting; (ii) radiation amplifying; (iii) radiation detecting; and (iv) radiation absorbing. A method as claimed in any one of the preceding claims, wherein the mean size of quantum dots in said active layer is different to the mean size of quantum dots in said first layer. A method as claimed in any one of the preceding claims, wherein said active layer is grown at a lower temperature than said first layer. A method as claimed in any one of the preceding claim, wherein said capping layer is grown at a lower temperature than said first layer. A method as claimed in any one of the preceding claims, wherein said spacer layer is annealed prior to growing said active layer on said spacer layer. A method as claimed in any one of the preceding claims, wherein growth is interrupted between said spacer layer and said active layer in order to change the growth parameters. A method as claimed in any one of the preceding claims, wherein growth is interrupted between said active layer and said capping layer in order to change the growth parameters. A method as claimed in any one of the preceding claims, wherein the quantum dots of said first layer are electronically coupled to the quantum dots of said active layer. A method as claimed in any one of the preceding claims, wherein said quantum dots of said active layer are operable to at least one of emit, absorb or amplify light in their ground states. A method as claimed in any one of the preceding claims, wherein said quantum dots of said active layer are operable to at least one of emit, absorb or amplify light in their excited states. An optoelectronic device containing an active region grown according to the method described in any one of claims 1 to 20.