Al x Ga 1-x As/GaAs Quantum Well Heterojunction Lasers Grown By Molecular Beam Epitaxy

Abstract AlxGai_xAs/GaAs multiple quantum well (MQW) heterojunction structures have been grown by molecular beam epitaxy. Photopumped structures having quantum well sizes as low as 28 A have shown 300 K and cw performance. MQWT s with 28 X well sizes grown at a substrate temperature of 720°C resulted in an equivalent current density of 2.3 kA/cm2 at a wavelength of about 7,270 A.IntroductionMolecular beam epitaxy, owing to its extreme control of thickness and dopant concentra­ tion, has been extensively used to prepare many types of device structures-*-. These struc­ tures many times involve the use of GaAs/AlxGa]__xAs heterojunctions for lasers^, modula­ tion doped structures3, wide bandgap emitter bipolar transistors4 and detectors utilizing camel diode concepts5 . While the interface properties at the heterojunction should not be overlooked in bulk device structures, they dominate the performance of structures that are extremely thin. For example, the modulation doped structures show mobility enhancement only when the AlxGai_xAs layer is doped with a donor impurity, the GaAs layer is left undoped and the heterointerface is of high quality. Electrons in GaAs are confined to within about 100 A of the interface and can assume extremely high mobilities^ (e.g., 300,000 cm^/Vs at 10 K). Any small irregularities at the interface(s) are detrimental to electron mobility and thus to any device fabricated using these structures. AlxGai_xAs/GaAs mul­ tiple quantum wells (MWQs) intended for visible lasers are strongly dependent on the qual­ ity of the interfaces. Prior to this report, it was not possible to prepare QWHs with MBE having well sizes as low as 28 2 and cw room temperature laser performance. In this report, the growth conditions and performance of AlxGai~xAs/GaAs MQW lasers prepared by MBE will be presented.Experimental procedureThe quantum well heterostructures (QWH) of interest here are grown on (100) Si-doped n-type GaAs substrates. One side of the wafer is polished using a pellon cloth saturated with a Br-CH3OH solution until a mirror-like surface is obtained. The back of the sub­ strate is chemically etched for ten minutes in I^SO^ : H-2&2 '• H2°- T^e wafers are then cleaved into 2.5 cm x 2.5 cm squares and are degreased, in order, in boiling trichloroeth- ylene, trichloroethylene, acetone and methyl alcohol. Following decreasing, the substrate is etched in H2SC>4: ^2^2 : H2° and rinsed in deionized water. Indium is used to mount a substrate on a molybdenum block which is then loaded into the airlock of the MBE system. Following the evacuation of the airlock to a pressure of about 10~8 Torr, the substrate is transferred into the growth chamber. The growth chamber is equipped with a liquid nitro­ gen shroud surrounding the ovens and hemispherical shroud lining the walls of the growth chamber. The effect of such a shroud is to insure that all condensibles, such as water vapor, are frozen out. In addition, any desorption off the walls of any of the beam sources is avoided and the effusion cell apertures are the only hot regions exposed to the substrate. The effusion cells are pyrolytic boron nitride crucibles surrounded by tanta­ lum heating coils and tantalum or molybdenum foil heat sheilds. A dc current, controlled by a temperature controller (WRe 5% - WRe 26% thermocouple), is used to power the effusion cells. The growth chamber is also equipped with an ionization gauge situated behind the substrate block for monitoring the As flux and with a high energy electron diffraction (HEED) system.Prior to the initiation of the epitaxial growth, the liquid nitrogen is fed into the cooling shroud and the effusion cells are then turned on. Generally, a background pres­ sure as low as 10~~10 Torr is easily obtained. Soon after the effusion cells are stabil­ ized at the desired temperatures, the substrate, which is also heated with a dc current, is raised to a surface temperature of 630°C to desorb any native oxide from the GaAs sub­ strate surface. Once the 630°C surface temperature is reached, the shutter for each effu­ sion cell to be employed is opened and then the main shutter is opened to initiate the film deposition. After the main shutter is opened, a HEED pattern is monitored to ob­ serve the proper initiation of growth. The substrate surface temperature during the actual growth of the QWH is generally kept at 700°C or, in some cases, slightly higher. The Ga cell temperature is adjusted to obtain a growth rate of about 0.7 ym/hr (or