This paper discusses the issues involving the design and fabrication of vertical-cavity surface-emitting lasers (VCSELs). A review of the basic experimental structures is given, with emphasis on recent developments in distributed Bragg reflectors, gain media, as well as current and optical confinement techniques. The paper describes present VCSEL performance, in particular, those involving selective oxidation and visible wavelength operation.

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Design, fabrication, and performance of infrared and visible vertical-cavity surface-emitting lasers

Thermodynamically stable, low D/sub it/ amorphous Ga/sub 2/O/sub 3/-(100) GaAs interfaces have been fabricated by extending molecular beam epitaxy (MBE) related techniques. We have investigated both in situ and ex situ Ga/sub 2/O/sub 3/ deposition schemes utilizing molecular beams of gallium oxide. The in situ technique employs Ga/sub 2/O/sub 3/ deposition on freshly grown, atomically ordered (100) GaAs epitaxial films in ultrahigh vacuum (UHV); the ex situ approach is based on thermal desorption of native GaAs oxides in UHV prior to Ga/sub 2/O/sub 3/ deposition. Unique electronic interface properties have been demonstrated for in situ fabricated Ga/sub 2/O/sub 3/-GaAs interfaces including a midgap interface state density D/sub it/ in the low 10/sup 10/ cm/sup -2/ eV/sup -1/ range and an interface recombination velocity S of 4000 cm/s. The existence of strong inversion in both n- and p-type GaAs has been clearly established. We will also discuss the excellent thermodynamic and photochemical interface stability. Ex situ fabricated Ga/sub 2/O/sub 3/-GaAs interfaces are inferior but still of a high quality with S=9000 cm/s and a corresponding D/sub it/ in the upper 10/sup 10/ cm/sup -2/ eV/sup -1/ range. We also developed a new numerical heterostructure model for the evaluation of capacitance-voltage (C-V), conductance-voltage (G-V), and photoluminescence (PL) data. The model involves selfconsistent interface analysis of electrical and optoelectronic measurement data and is tailored to the specifics of GaAs such as band-to-band luminescence and long minority carrier response time /spl tau//sub R/. We will further discuss equivalent circuits in strong inversion considering minority carrier generation using low-intensity light illumination.

Low D/sub it/, thermodynamically stable Ga/sub 2/O/sub 3/-GaAs interfaces: fabrication, characterization, and modeling

The surface structures formed upon deposition of O2 and Ga2O onto the technologically important arsenic-rich GaAs(001)-c(2×8)/(2×4) surface have been studied using scanning tunneling microscopy and spectroscopy, and the results are compared to density functional theory calculations. O2 chemisorbs by displacing first layer arsenic atoms bonded to second layer gallium atoms. Oxygen chemisorption pins the Fermi level at less than 5% monolayer coverage by creating a donor and acceptor site within the band gap originating from the gallium atom bonded between the two O atoms. In contrast, Ga2O chemisorbs by inserting into arsenic dimer pairs at elevated surface temperatures. A monolayer of Ga2O forms a (2×2) surface structure with a crystalline interface that is electronically unpinned: there are no states within the band gap. The unpinned interface results from Ga2O restoring the surface arsenic and gallium atoms to near-bulk charge.

Scanning tunneling microscopy and spectroscopy of gallium oxide deposition and oxidation on GaAs(001)-c(2×8)/(2×4)

High-dielectric-constant Ta2O5 has been grown on the n-GaN epifilm by rf magnetron sputtering Photoluminescence measurement has been performed to compare the luminescence intensity with and without the dielectrics Threefold increase in intensity is obtained, and a surface recombination velocity is estimated to be 3×104 cm/s as an upper limit using a modified dead-layer model A metal-oxide-semiconductor structure has been fabricated with Al on n-GaN as the ohmic contact and on Ta2O5 as the gate metal Capacitance-versus-voltage characteristics have been measured The doping concentration obtained from the depletion regime is compared with the result of Hall measurement, which is 70×1016 cm−3 The flat-band voltage is obtained from the high-frequency data, and the effective oxide charge number density is calculated as 41×1012 cm−2 Indication of strong inversion appears at low reverse bias due to the high dielectric constant of Ta2O5, and matches closely with calculated values Hysteresis is observed a

High-dielectric-constant Ta2O5/n-GaN metal-oxide-semiconductor structure

Gallium oxide thin films have been deposited by atomic layer epitaxy (ALE) using Ga(acac)3(acac = pentane-2,4-dionate) and either water or ozone as precursors. Films were grown on silicon (100), soda lime and Corning glass substrates. The influence of the deposition parameters (e.g. pulse duration, growth and source temperatures) on film growth were studied and by a proper choice of the parameters a self-controlled growth was demonstrated around 370°C. Spectrophotometry, X-ray diffraction (XRD), Rutherford back-scattering spectroscopy (RBS) and X-ray photoelectron spectroscopy (XPS) were used to determine the refractive index, thickness, crystallinity and stoichiometry of the films. All the films were amorphous and highly uniform with only small thickness variations. The films deposited with water contained a considerable amount of carbon as an impurity whereas ozone as an oxidizer gave stoichiometric Ga2O3 films.

Growth of gallium oxide thin films from gallium acetylacetonate by atomic layer epitaxy

Amorphous Ga2O3 films have been deposited in situ on (100) GaAs layers grown by molecular beam epitaxy in ultrahigh vacuum. The Ga2O3–GaAs interface is stable during photoexcitation and the photoluminescence (PL) intensity, measured at 514.5 nm excitation wavelength, is enhanced drastically by a factor of 420 as compared to a corresponding bare GaAs surface. The Ga2O3–GaAs interface recombination velocity derived from a modified dead layer model is below 104 cm/s. Furthermore, the PL intensity of Ga2O3–GaAs structures approaches that of a very low interface state density (2×109 eV−1 cm−2) AlGaAs–GaAs reference structure.

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In situ fabricated ga2o3-gaas structures with low interface recombination velocity

An in situ process which includes electron cyclotron resonance plasma etching and molecular beam epitaxial regrowth is applied to the fabrication of buried heterostructure vertical cavity surface‐emitting laser (SEL) diodes and edge‐emitting laser (EEL) diodes. The buried SEL with a 7.5 μm diameter has a pulsed laser threshold current of 1 mA, and a threshold voltage of 4 V with a peak power of 0.9 mW. The buried EEL with 2.5 μm stripe width and 800 μm cavity length has a threshold current density of 500 A/cm2.

Buried heterostructure laser diodes fabricated using in situ processing

Very low resistivity, nonalloyed ohmic contacts to p‐GaAs are realized by in situ molecular beam epitaxy (MBE). Heavily Be‐doped GaAs either in a uniform doping or with a δ‐doping scheme was grown using MBE. Three different metals of Au, Ag, and Nb were in situ deposited on p‐GaAs, yielding contact resistance RC less than (1–4)×10−7 Ω cm2.

F. A. Thiel

Papers

In situ fabricated ga2o3-gaas structures with low interface recombination velocity

Buried heterostructure laser diodes fabricated using in situ processing

In situ nonalloyed ohmic contacts to p-GaAs