Comparison of Long-Time Delay in Lasing in Homo- and Heteroepitaxially Grown II-VI Laser Diodes

Abstract

Gain guided laser diodes exhibit unexpected low threshold current densities. Under these conditions, lasing only occurs under a current dependent long-time delay, which is three orders of magnitude larger than the time needed to reach population inversion. This effect is attributed to a thermally induced index guiding. The change in temperature of the quantum well region can be estimated using the shift in the wavelength of emission to be up to 70 K. As a further consequence, the threshold current density can be reduced by a factor of 4 simply by changing the pulse width of the applied current.PACS numbers: 42.55.ΡχZnSe laser diodes (LD) grown on GaAs have been investigated for the past six years. The quality of heteroepitaxial devices could be improved to reach lifetimes of up to 400 h [1]. Recently it has been shown that also ZnSe-substrates offer the potential to realize devices for cw-operation at room temperature [2].The aim of this paper is to compare the dynamic behavior of heteroepitaxially grown laser diodes to homoepitaxially grown ones. Both investigated samples were double heterostructure separate confinement laser diodes. The heteroepitaxial structure was grown on GaAs-substrate and consists of MgZnSSe claddings of 1000 nm,. ZnSSe waveguides of 100 nm to both sides of the quantum well (QW), a 4 nm thin CdZnSSe quantum well and a conventional ZnSeTe contact layer. For the homoepitaxial laser diode, an aluminum-doped ZnSe-substrate was used. This diode was identical to the heteroepitaxial device except for the quaternary cladding layer which was lattice matched to ZnSe and the waveguide which was grown from ZnSe instead of ZnSSe. The CdZnSe quantum well was 4 nm thin, as well. The contact layers consisted of a low resistivity ZnSeTe-multilayer. Details of growth can be found elsewhere [3,4]. A palladium-gold contact was evaporated to the p-side and Al2O3 was used as insulating material to form stripe widths of 10 m and 20 m, respectively. No etching beside the contact was performed. The lasers with a cavity length of 590 m and 755 μm, respectively, were mounted episide up, i.e. with the substrate side onto copper heat sinks.(355)

High-Power Operation of ZnSe-Based cw-Laser Diodes

“…To get an first impression of the heat generated during operation we measured the shift of emission wavelength between pulsed and dc current injection. Earlier results showed a temperature increase of 70 K [9]. 2 where the emission spectrum of a LD in pulsed operation (0.5% duty cycle, 1 ms pulse width) and in cw-operation is shown.…”