VCSELs by MBE
Introduction
SHARP is a world leader in the production of optoelectronic components such as red (650nm) and infra-red (780nm) semiconductor laser diodes (LD). The principal commercial usage of these devices is in the current DVD and CD reader/writer products. Sharp Laboratories of Europe (SLE) has contributed to the development of these technologies by providing long term and short term research for SHARP Corporation. SLE has gained a large expertise in LD growth by Molecular Beam Epitaxy (MBE) as well as device processing. SLE has been working on edge-emitting LDs for the last ten years and started to work on Vertical Cavity Surface Emitting Lasers (VCSEL) about five years ago.
In the last decade, a large interest has been demonstrated worldwide on the development of VCSEL devices. Up to now, VCSELs have been mainly used as transmitters in optical fibre telecoms and data transmission (850nm-980nm). Recently new applications (optical storage, white lighting, biophotonics, displays etc…) have directed research towards shorter wavelengths. With the recent advance in nitride LDs that SLE has made, the laboratory is in a good position to develop nitride VCSELs. SLE is now working on novel and challenging VCSEL devices that will be part of future technology and new products.
Semiconductor laser diode
Radiative recombination in a semiconductor LD occurs when an electron in the conduction band recombines with an empty state (hole) in the valence band and the excess energy is emitted in a form of a photon. The optical processes associated with radiative recombination in semiconductors are i) spontaneous emission, ii) absorption and iii) stimulated emission. Stimulated emission (lasing) occurs when an incoming photon stimulates an electron in the conduction band to fall down to the valence band, thereby emitting a new photon that has the same energy and momentum as the incident photon.
VCSEL device properties
A VCSEL is a type of semiconductor LD with the laser beam emission perpendicular to the chip surface, in contrast to conventional edge-emitting semiconductor lasers where the laser light is emitted from the edge (usually formed by cleaving).
Figure 1
The VCSEL consists of a light-emitting medium (active region or also called optical cavity), usually composed of quantum wells sandwiched between two highly reflective Distributed Bragg Reflector (DBR) mirrors (Figure 1). The mirrors consist of layers with alternating high and low refractive indices (Figure 2). Each layer has a thicknessequal to a quarter of the laser wavelength in the material. These mirrors have the high reflectivites (>99%) required to obtain lasing in VCSELs. Current is injected into the active region where stimulated emission provides gain for an optical mode, and lasing light is emitted vertically from one of the mirrors.
Figure 2:
Cross-section scanning electron microscope image of a VCSEL structure. The alternated contrast regions correspond to the top and bottom DBRs which are placed on each side of the optical cavity.
VCSELs have some intrinsic advantages over edge emitting laser diodes. The emitted beam is circular which makes it easier to couple into a fibre. Due to the shorter cavity, the lasing threshold is lower and devices can be driven at higher modulation speed. The devices are smaller, and because the light is emitted from the surface testing can be performed on wafer, which reduces the cost of manufacturing.
Development of nitride based VCSEL by molecular beam epitaxy
In 2003 researchers at SLE achieved a world first by demonstrating a blue edge-emitting LD grown by MBE (Electron. Lett., 2004, 40, (1) pp.33-34) (For further information on MBE click here ).
SLE has developed a considerable knowledge of nitrides growth and processing of light-emitting devices, and is now beginning research work on nitride based VCSELs. This project is seen as very challenging as there have not yet been any reports in the scientific community about electrically pumped lasing nitride VCSELs. Researchers at SLE will be concentrating on developing the growth of high reflectivity (Al,Ga,In,N)-based DBRs, high efficiency active regions and p-type doping of high aluminium content material, with the aim of demonstrating an electrically pumped lasing nitride VCSEL.
Figure 3:
MBE system where VCSEL structure will be grown
