Vertical-cavity surface-emitting lasers

2.2.1 Fundamental of VCSELs

Diode lasers, like the other types of lasers, incorporate the following three ingredients. (1) Gain medium: The gain medium consisting of a material which normally absorbs incident radiation over some wavelength range of interest. But, if it is pumped by inputting either electrical or optical energy, the electrons within the material can be excited to the higher, nonequilibrium energy level, so that the incident radiation can be amplified rather than absorbed by stimulating the de-excitation of these electrons along with the generation of additional radiation. If the resulting gain is sufficient to overcome the losses of some resonant optical mode of the cavity, this mode is said to have reached threshold, and relatively coherent light will be emitted.

(2) Pumping source: Pumping source provides the energy that can excite the electrons within gain medium at lower energy level to higher energy level. It could be either optical or electrical energy. (3) Resonant cavity: The resonant cavity provides the necessary positive feedback for the radiation being amplified, so that a lasing oscillation can be established and sustained above threshold pumping level.

Diode lasers are distinguished from the other two types lasers (gas lasers and solid-state lasers) mainly by their ability to be pumped by an electrical current. This results in a higher efficient operation. Power conversion efficiency of ~50% is

commonly for diode lasers, whereas efficiencies for the gas and solid-state lasers are commonly 1~10%.

As depicted in Fig. 2.2, the structure of most VCSELs consist of two parallel reflectors which are distributed Bragg reflectors (DBRs) and a cavity including a multiple quantum wells (MQWs) served as active layer. The reflectivity necessary to reach the lasing threshold should normally be higher than 99.9%. The electrode pads are for injecting electrons and holes effectively into the active region is necessary for a current injection device. Corresponding to the ingredients of a laser, the active layer is the gain medium that amplify the optical radiation in the cavity; the top DBR, bottom DBRs and cavity form a resonant cavity where the radiation can interact with active region and have positive feedback; the electrode pads is for current injecting into the cavity to serve as pumping source.

2.2.2 Obstacles in achieving a GaN-based VCSEL

In the GaN-based VCSEL structure, a micro cavity with a few λ in the optical thickness and a pair of high reflectivity (above 99%) distributed Bragg reflectors (DBRs) are necessary for reducing the lasing threshold. A difficulty in fabricating monolithically grown GaN-based VCSELs is the growth of high reflectivity epitaxial DBRs, which require a large number of AlGaN/GaN pairs to reach a high reflectivity

owing to the small difference in refractive index (~0.1) between the AlGaN/GaN pairs.

In addition, the requirement of high reflectivity and high quality DBRs using AlxGa1-xN and GaN materials is quite formidable since these two materials have large lattice mismatch and difference in thermal expansion coefficients that tends to form cracks in the epitaxially grown DBR structure. These cracks in DBR could result in the reduction of reflectivity due to scattering, diffraction and absorption. The crystalline quality of the multiple quantum wells (MQWs) grown on the DBRs would also be degraded because of the cracks. Recently, several groups have reported optically pumped GaN-based VCSELs mainly using three different kinds of vertical resonant cavity structure forms: (1) monolithically grown vertical resonant cavity consisting of epitaxially grown III-nitride top and bottom DBRs (epitaxial DBR VCSEL), (2) vertical resonant cavity consisting of dielectric top and bottom DBR (dielectric DBR VCSEL). (3) vertical resonant cavity consists of an epitaxially grown III-nitride top DBR and a dielectric DBR (hybrid DBR VCSEL). In 1996, Redwing et al. proposed an all epitaxial DBR VCSEL structure consisting of a 10-μm GaN

microcavity embedded by two epitaxially grown 30-pair Al0.12Ga0.88N/Al0.4Ga0.6N DBRs [2.8]. The reflectivity of top and bottom DBR is 93% and 84%, respectively.

Although the reflectivity of the DBR was not very high, a stimulated emission with a wavelength of 363 nm was observed due to the thick gain layer (10-μm GaN). In

1998, Arakawa et al. grew an InGaN multiple quantum wells (MQWs) on 35-pair Al0.34Ga0.66N/GaN DBR and deposited a 6-pair SiO2/TiO2 on the grown structure forming the hybrid DBR VCSEL structure [2.9]. The stimulated emission with 381 nm in wavelength and linewidth smaller than 0.1 nm were observed as the pumping power was above the threshold condition at 77 K. Cavity quality factor Q of the resonant cavity was estimated to be 165. Thereafter, in 1999, Song et al. demonstrated a dielectric DBR VCSEL structure consisting of InGaN MQWs and 10-pair HfO2/SiO2 top and bottom DBR using laser left-off technology [2.10]. The reflectivity of top and bottom DBRs were 99.5% and 99.9%, respectively. The emission wavelength and linewidth of the resonant cavity tested at room temperature were 437 nm and 0.7 nm, respectively. Because of the high reflectivity DBRs, the cavity had a high Q factor of 600. In 2005, Feltin et al. showed that the Al1-yIn_yN/GaN material system could be well suited to the growth of vertical cavity structures [2.11] because Al1-yIn_yN (y = 0.17) was lattice-matched to GaN thus avoiding the subsequent appearance of additional structural degradation (new dislocations and/or cracks) while presenting a refractive index contrast around 7–8%, where the DBR requires over 40 pairs to reach a reflectivity of 99%. Their VCSEL structure consisted of In_xGa1-xN/GaN (x = 0.15) MQWs and 28 (bottom)/23 (top) AlInN/GaN DBRs and exhibited a Q factor of 800. However there is so far few detailed report of the

performance characteristics of optically pumped GaN VCSELs.

2.2.3 GaN-based VCSELs with two dielectric mirrors

In this study we propose a VCSEL structure consisting two dielectric DBRs and a GaN-based resonant cavity. An epitaxially grown, thick (~4 μm) GaN-based cavity incorporated with InGaN MQWs was separated from the sapphire substrate by using laser liftoff and then embedded the cavity between two dielectric DBRs. By using different dielectric materials with large difference in refractive index (for example, the difference in SiO2 and TiO2 is ~1.58 at 430 nm), a DBR with a high reflectivity and wide stop band could be achieved with less DBR pairs.