ZnSe-based laser diodes with quaternary CdZnSSe quantum wells as active region: { Chances and limitations { Matthias Klude A dissertation submitted in partial satisfaction of the requirements for the degree of Doktor der Naturwissenschaften. Bremen 2002 Contents Introduction v 1 Background and prerequisites 1 1.1 Physics of semiconductor lasers . 1 1.1.1 Electromagnetic wave inside a crystal . 1 1.1.2 Fabry-Per´ ot cavity filled with a gain medium: condition for lasing 4 1.2 Design of a semiconductor laser diode . 7 1.3 The II-VI material system ZnSe . 10 1.3.1 Physical properties . 11 1.3.2 Epitaxy . 12 1.3.3 Defects . 13 1.3.4 Alloys . 15 1.3.5 Doping . 19 1.4 State of the art and solved problems . 20 1.4.1 History of ZnSe-based laser diodes . 20 1.4.2 GaAs/ZnSe heterointerface: growth start . 23 1.4.3 p-type doping . 24 1.4.4 p-side contact . 25 2 Experimental techniques and standard devices 29 2.1 Molecular beam epitaxy . 29 2.1.1 Principle of operation . 30 2.1.2 The Bremen MBE system . 30 2.1.3 Calibration of the growth parameters . 32 2.1.4 Standardization of the growth process . 35 2.2 Device processing . 36 2.3 Standard laser structure and characterization scheme . 38 2.3.1 Layer sequence . 38 2.3.2 Growth . 40 2.3.3 Structural and optical characterization . 42 2.3.4 Electrical and electro-optical characterization . 49 2.3.5 Reproducibility . 58 3 Degradation in ZnSe-based laser diodes 63 3.1 Initial observations . 63 3.2 Dark defects . 65 3.2.1 Experimental observations . 65 i Contents 3.2.2 Microscopic nature and generation of new defects . 67 3.3 Dynamics of recombination enhanced defect reaction . 70 3.3.1 Defect generation mechanisms and long-term behavior . 70 3.3.2 Experimental verification . 71 3.3.3 Analytical solution . 72 3.4 Driving forces of the degradation mechanism . 74 3.4.1 p-type doping . 74 3.4.2 Strained quantum well and Cd diffusion . 76 3.4.3 Current injection and accumulation of heat . 80 3.5 Improving the quantum well stability . 83 3.5.1 Alternative growth modes . 84 3.5.2 Low-temperature growth . 85 3.5.3 Strain reduction in the active region . 87 3.5.4 Sony's approach . 90 3.6 High-power operation . 92 3.7 Summary . 96 4 Advanced processing technologies 97 4.1 Overview . 97 4.2 Top-down mounting . 98 4.2.1 Idea and background . 98 4.2.2 Pelletizing of laser bars . 99 4.2.3 Design of the heat sink . 101 4.2.4 Solder . 101 4.2.5 Results and outlook . 103 4.3 High-reflectivity facet coating . 104 4.3.1 Dielectric mirrors . 104 4.3.2 Influence on the threshold current density . 106 4.4 Summary . 107 5 Exploring the limits and possibilities of Cd-rich quantum wells 109 5.1 Motivation . 109 5.2 Growth optimization . 111 5.3 Quantum wells . 114 5.4 First laser diodes emitting at 560 nm . 115 5.5 Operational characteristics in comparison: 500 nm vs. 560 nm emission . 117 5.6 Degradation . 121 5.7 Summary . 123 6 An alternative approach: CdSe quantum dots 125 6.1 Quantum dot laser . 125 6.2 Self-organized growth of CdSe quantum dots . 127 6.3 CdSe quantum dot stacks as active region . 128 6.3.1 Design of the active area and structural characterization . 128 6.3.2 Electroluminescence . 131 6.4 Lasing operation . 134 6.5 Degradation . 136 ii Contents 6.6 Outlook . 138 Summary and conclusion 141 A Externally processed devices 145 A.1 Ridge waveguide by ion implantation . 145 A.1.1 Lateral index guiding . 145 A.1.2 Implantation-induced disordering . 147 A.1.3 Technological realization . 148 A.1.4 Results . 149 A.2 Novel ex-situ p-side contacts . 151 A.2.1 Idea . 151 A.2.2 Results and discussion . 152 A.3 Distributed feedback laser . 154 A.3.1 Longitudinal mode control . 154 A.3.2 DFB laser fabrication . 155 A.3.3 Characterization . 157 A.4 Short summary and outlook . 158 B Index of laser structures 161 C Publications 163 Bibliography 171 iii Contents iv Introduction The beginning of the 21st century is marked by a tremendous change in communication technology. With the development of data communication technologies, a new area has be- gun – connecting the mass communication systems radio, television, and telecommuni- cation with computer technology. Thus, information becomes available – for everyone, always, and everywhere. The medium that provides this new flexibility is the internet; the backbone of the internet is data communication. Given the vast amount of infor- mation that is transfered, the transmission has to be as fast as possible. Consequently, optical data transmission provides the foundation of this new form of communication, so that the new century is sometimes referred to as the century of the photon [1]. But the transmission of data is only one aspect – it is of equal importance to store the data. Again, light provides the solution. Optical data storage systems allow to store information with a high density and fast access. Both applications have in common that they pose special requirements to the light source. Its light has to be as monochromatic as possible and of defined color, of high intensity, and easy to focus – even over long distances. In addition, the source should be small, efficient, robust, and cheap. A light source that satisfies all these conditions is the semiconductor laser diode: lasers provide light amplification by stimulated emission of radiation, which results in monochromatic, coherent light of high intensity; semiconductor technology allows mass-production of small devices with high reliability. Thus, the semiconductor laser diode is the ultimative light source of the communication age. The use of semiconductor lasers is not limited to communication systems. Other ap- plications include systems for environmental detection, health science, biotechnology, medicine, and even production [1]. The most attractive and fascinating application, however, is reserved for the special set of laser diodes – devices that produce the three primary colors red, green, and blue: display technology. Using lasers, new, sharp, and brilliant displays with a color spectrum, never seen before, can be fabricated – in a size smaller than a box of cigars. The first electrically pumped semiconductor laser diodes were realized in 1962. In the last 40 years, these devices transformed from pure academic curiosities to commer- cial products that can be found in almost every household nowadays. Commercially available laser diodes cover an emission spectrum from the red to the infrared. Recently, blue-violet emitting laser diodes based on galliumnitride (GaN) were commercialized. The emission wavelength of these devices is specially optimized for optical data stor- age systems, where data density is mainly determined by the emission wavelength – the shorter the emission wavelength, the higher the density. Thus, the emission of these de- vices is not true-blue (around 480 nm), as required for display application. Such lasers are expected for the near future – at present, the longest emission wavelength obtained v Introduction from a GaN-based laser diode is 465 nm [2]. That leaves a green-emitting semiconductor laser diode as remaining task. Until today, there exists only one semiconductor material system that enables the re- alization of electrically pumped lasers in the green part of the visible spectrum: the II-VI material system based on zincselenide (ZnSe). Initially, the research activities on these devices were motivated by the demand to develop a short-wavelength semiconductor laser source for optical data storage systems. In the recent years, the research intensity cooled down considerably due to the overwhelming success of the GaN-based emitters. Nowadays the motivation to continue the research on ZnSe-based laser diode devices is directly connected to their unique possibility to emit green laser light. Naturally, the display technology dominates this motivation – yet, there is a broad spectrum of appli- cations beyond this. Especially the fact, that the human eye has its highest sensitivity in the green spectral region, gives rise to numerous applications, mainly connected to optical feedback devices, such as laser pointers or guiding-lasers for medical surgery systems. The research on ZnSe-based laser devices was stimulated by the development of a reliable p-type doping technique in 1990 [3, 4]. In the following years, these devices have been brought from short-lived pulsed-operation at low-temperatures (77 K) to continuous wave (cw)-operation at room-temperature, with a record-lifetime of 389 h at present [5]. This limited lifetime is the primary reason that ZnSe-based lasers have not been commercialized, yet, and are still in the research-stage. Obviously, the most in- teresting and pressing problem here is to develop an understanding for the underlying degradation mechanism that causes the insufficient stability, which is closely connected to the active region of the laser diode – the Cd-containing quantum well. Consequently, a significant part of this thesis is concerned with the limitations of quaternary CdZnSSe quantum wells in terms of degradation. However, the investigations will not be restricted to the characterization of the process – novel approaches to slow it down or to minimize its effect will also be presented – revealing the unique possibilities of this special material. The work of this thesis was performed in the Halbleiterepitaxie group of the Institut fur¨ Festkorperphysik¨ of the Universitat¨ Bremen, which took part in the research on ZnSe- based devices as soon as the epitaxy system was installed in the beginning of 1996..