The (Al,Ga,In)N material system includes materials having the general formula AlxGayIn1-x-yN where 0≦x≦1 and 0≦y≦1. In this application, a member of the (Al,Ga,In)N material system that has non-zero mole fractions of aluminium, gallium and indium will be referred to as AlGaInN, a member that has a zero aluminium mole fraction but that has non-zero mole fractions of gallium and indium will be referred to as InGaN, a member that has a zero indium mole fraction but that has non-zero mole fractions of gallium and aluminium will be referred to as AlGaN, and so on. There is currently considerable interest in fabricating semiconductor light-emitting devices in the (Al,Ga,In)N material system since devices fabricated in this system can emit light in the blue-violet wavelength range of the spectrum (corresponding to wavelengths in the range of approximately 380-450 nm).
FIG. 1 is a schematic sectional view of a semiconductor laser device. The laser device consists of a laser structure 2 that is grown over a substrate 1. In this example, the laser device is a separate confinement (SCH) laser device and the laser structure 2 comprises, in sequence, a first cladding region 4, a first optical guiding region 5, an active region for laser oscillation 6, a second optical guiding region 7, and a second cladding region 8. A buffer layer 3 is provided between the substrate 1 and the laser structure 2. The first cladding region 4 and the first optical guiding region 5 are doped n-type, and the second cladding region 8 and the first optical guiding region 7 are doped p-type. A p-type contact layer 9 is disposed on the p-type cladding region 8. The laser further has a second, n-type contact, and this may be provided on the same surface of the substrate 1 as the laser structure (as indicated at 10a) or it may be provided on the opposite surface of the substrate 1 to the laser structure (as indicated at 10b).
The first cladding region 4, first optical guiding region 5, active region 6, second optical guiding region 7, second cladding region 8 and the contact layer 9 have been etched to a desired width w, to form a mesa. The mesa may be a ridge mesa and extend into the plane of the paper, or the mesa may have circular symmetry about the vertical axis. The width w of the mesa will be small, typically a few μm, so that the surface area of the mesa is small (particularly in the case of a circularly symmetric mesa). In order to ensure good electrical contact to the laser it is therefore conventional to deposit an electrically conductive “pad” 11 over the laser. The conductive pad 11 has a much greater surface area than the mesa, and so it is easier to make an electrical contact to the pad 11 than it is to make an electrical contact direct to the contact layer 9.
Providing the electrically conductive pad 11 directly over the laser structure, as shown in FIG. 1, has the disadvantage that the conductive pad 11 is disposed directly on the portions of the buffer layer 3 that are exposed in the step of etching to form the mesa, and so makes electrical contact with the buffer layer 3. As a result, low resistance current paths 12,13 exist that go from the electrically conductive pad 11 to the n-type contact 10a or 10b without passing through the active region 6. The current paths 12, 13 that do not pass through the active region 6 are in parallel to the desired current path 14 through the active region 6, and degrade the characteristics of the laser-current that flows along these current paths 12,13 does not contribute to laser oscillation. It is therefore usual to provide a current blocking layer 15 having a high electrical resistance between the buffer layer 3 and the conductive pad 11, as shown in FIG. 2. The laser of FIG. 2 is generally similar to the laser of FIG. 1, except for the provision of the high resistance layer 15. The high resistance layer 15 suppresses the current paths 12,13 of FIG. 1 that do not flow through the active region 6, and so improves the characteristics of the laser. The high resistance layer 15 may be, for example, a layer of silica (silicon dioxide).
Although the high resistance layer 15 improves the characteristics of the laser, providing the high resistance layer 15 does make the process of fabricating the laser significantly more complicated. The high resistance layer must not be deposited over the contact layer 9, since this would increase the resistance of the desired current path 14 through the active region 6. Depositing the high resistance layer 15 therefore requires additional etching and masking steps—in general, the insulating layer 15 is deposited over the entire buffer layer 3 and the mesa, the portions of the insulating layer that are not over the mesa are masked, and the portion of the insulating layer that overlies the mesa is then removed via an etching step.
It is known to provide the laser structure with an additional layer that can be oxidised during the growth process to form the high resistance layer 15. However, if the additional layer extends through the mesa, it will affect the bandgap profile and refractive index profile of the laser, and this is undesirable. If the additional layer is to be deposited only outside the mesa, then additional masking and etching steps are required.