This paper summarizes recent in situ x-ray analyses of the growth of GaAs by organometallic vapor phase epitaxy (OMVPE). This growth was carried out using tertiarybutylarsine (TBAs) and trimethylgallium (TMG) as the source materials. Examples of in situ x-ray measurements are given including x-ray absorption studies of gas phase behavior and x-ray scattering studies of layer-by-layer growth.

Real-time control of epitaxial crystal growth is a necessity for the production of advanced materials in order to improve the yields of new generations of digital, RF, and optoelectronic devices. Process tolerances are becoming tighter in terms of both layer stoichiometry and layer thickness. The traditional grow-characterize-grow again technique that has served us well over past decades is no longer a production worthy method of ensuring that wafers grown after calibration meet the design specifications. The day to day drift in most epitaxial growth systems is often as great as the wafer specification window. In this paper we describe a spectroscopic ellipsometer based control system and present results obtained for GaAs-AlGaAs structures grown by organometallic molecular beam epitaxy.

Current difficulties of Atomic Layer Epitaxy (ALE) include relatively low growth rates and narrow process windows. Gas phase reaction, complex behavior of valve switching and purging times are suggested as the major causes. We have used a movable X-shaped mechanical barrier to divide the growth chamber into four zones. Alternate zones either supply source gas or mask the wafer from exposure to source gases. If the barrier is positioned 0.5 - 2 mm from the wafer carrier, it can efficiently shear off the boundary layer and thereby reduce gas phase reactions. The substrate, continuously rotating beneath the barrier, is alternately exposed to group III and group V sources. The result is that process times are significantly reduced. Initial results have shown a saturated growth rate of up to 0.35 micrometers /hour at 525 degree(s)C and a relatively wide process window. Thickness uniformity of +/- 1% over 85% of a 2 inch wafer has been obtained.

The Vapor Transport Epitaxy (VTE) thin film deposition technique for the deposition of III - V and II - VI compound semiconductors and material results are reviewed. The motivation for development of the VTE technique is the elimination of several problems common to molecular beam epitaxy/chemical beam epitaxy and metalorganic chemical vapor deposition systems. In VTE, vapors from sources feed through throttling valves into a common manifold which is located directly below the inverted wafer. A high degree of film uniformity is achieved by controlling the flux distribution from the common manifold. The technique operates in the 10^-4 - 10^-6 Torr range using elemental, metalorganic or gaseous precursors. The system is configurated for 2 inch diameter wafers but the geometry may easily be scaled for larger diameters. Using elemental sources, we have demonstrated oval defect free growth of GaAs on GaAs (100) and (111) 2 degree(s) off substrates, through several microns of thickness at growth rates up to ten microns per hour. GaAs films which were grown without the manifold exhibit classic oval defects. The deposition rate of ZnSe films as a function of elemental flux, VI/II ratio, and growth temperature are described. The ZnSe films exhibited smooth surface morphologies on GaAs (100) 2 degree(s) off substrates. X- ray analysis shows that III - V and II - VI films exhibited crystallinities comparable to films produced by molecular beam epitaxy and metalorganic chemical vapor deposition techniques.

This paper summarizes the breakthrough in III - V multiwafer MOVPE mass production applications using the Planetary Multiwafer Reactor with Gas Foil Rotation (5 X 3 inch/7 X 2 inch or 5 X 4 inch/8 X 3 inch) which was originally developed and patented by LEP for growth of GaAs/AlGaAs heterostructures and has been used successfully since then for HEMT production, laser fabrication, and GaInP deposition. In similar planetary reactors, GaAs and InP based materials for a wide range of optoelectronic applications have been produced. The use of low pressures is not only advantageous for the handling of P- containing compounds and reduction of overall gas consumption, but also allows drastic reduction of the amount of H₂ required for driving the wafer support. The variation of thickness in these multiwafer systems is reduced to the order of 1 - 2% for GaAs, AlGaAs, GaInP, InP, GaInAs, and GaInAsP (1.55, 1.3, 1.05 micrometers ). Thus, one major advantage in comparison to MBE is that these reactors are capable of handling both GaAs and InP based processes with high concentration of phosphorus. For production of visible lasers (AlGaInP), GaInP HBTs, or complex solar cell structures, these processes have also been developed. This finally makes this MOVPE technology by far superior for production application than MBE.

Ternary III - V semiconductor alloys show ordering specially when they grow lattice matched to a substrate. The ordered ternary alloys have different crystal structures and bandgaps from their corresponding disordered ones. We review the advantages of growing these ordered alloys. We also report on our efforts to grow the ordered alloys by the atomic layer epitaxy technique.

The growing request for advanced structures in the opto- and microelectronic field has led to the development of epitaxial growth techniques capable for atomic layer sharp transitions in crystal composition. A very important role under these techniques is played by the MOVPE. The deposition of semiconductor materials from metal-organyl complex compounds of group III elements and hydride compounds of group V elements has been refined by means of developments in reactor technology and improvement of the source materials. An exceptional improvement was reached by introducing reduced pressure operation of the described process. The present work is an overview of the process performance and optimization in Low Pressure MOVPE for a selection of compound semiconductors from the Al-Ga-In-As-P alloy system lattice matched to either InP or GaAs. Conventional sources are used to grow the materials from the described alloy system. The influence of various process parameters on important material properties are discussed.

We report on the recent growth by Atomic Layer Epitaxy (ALE) of device quality Al_0.3Ga_0.7As in a modified commercial reactor. A standard Emcore reactor was altered by the installation of baffles to prevent mixing of the reactant gas streams and a computer controlled servo motor to allow for a nonlinear rotation cycle. By varying the V/III ratio and the exposure time to the reactant gases it is possible to control the background carbon doping from high resistivity to p equals 1 X 10²⁰ cm^-3, without the need for an additional p-type source. Since low background doping was also achieved, silane was used to obtain n-type Al_0.3Ga_0.7As as high as n equals 1 X 10¹⁸ cm^-3. The room temperature Hall mobility of the n-type Al_0.3Ga_0.7As films varied from 1200 to 3700 cm²/V(DOT)sec. Photoluminescence and preliminary doping results are presented and discussed.

The evolution of ordered surface and interface structures on (111), (211), and (311) GaAs during molecular beam epitaxy offers the unique possibility to directly synthesize GaAs quantum wires and quantum dots in an AlAs matrix. We show that well ordered alternating thicker and thinner regions of GaAs and AlAs form symmetric and asymmetric quantum-dot structures on (111) and (211) substrates, respectively, and quantum-wire structures on (311) substrates. The observed optical properties confirm the lateral size quantization in these GaAs/AlAs multilayer structures.

We review a new molecular beam epitaxy (MBE) technique we call cleaved edge overgrowth (CEO), which makes possible fabrication of quantum wires or other lower dimensional quantum structures with atomic precision. CEO is accomplished by performing two separate MBE overgrowths separated by an in situ cleave of the substrate sample. We review our development of this novel method and give three examples of new structures recently fabricated using it.

Selective area epitaxial growth of gallium arsenide by laser assisted chemical vapor deposition (LCVD) offers a promising approach to in situ device fabrication and integration. By scanning a focussed Ar ion laser beam across a thermally biased substrate in the presence of arsine and an organometallic source, GaAs is selectively deposited only on areas of the substrate exposed to the laser beam at substrate temperatures in the range of 300 - 400 degree(s)C. Laser assisted chemical vapor deposition of undoped and n-type doped device quality GaAs has been demonstrated. The laser-grown undoped GaAs films are highly resistive and exhibit 77 degree(s)K PL spectra with FWHMs of < 10 meV. N-type GaAs, using silane as a dopant source, has been deposited having controllable carrier concentrations ranging from 1 (DOT) 10¹⁷ - 7 (DOT) 10¹⁸ cm^-3 and room temperature mobilities between 600 - 5100 cm²/V(DOT)sec. GaAs MESFET structures have been selectively deposited using the LCVD growth technique. These devices have performance characteristics comparable to devices of similar dimensions grown by conventional techniques.

Over the last few years, surface emitting injection lasers have been widely studied in various laboratories around the world.' The principal advantages of these lasers over regular cleaved facet semiconductor lasers are (i) single frequency operation by design, (ii) narrow circular beam pattern and (iii) two-dimensional array fabrication. The latter is very important for two dimensional parallel optical interconnection for switching and computing applications. The schematic of a surface emitting laser structure is shown in Fig. 1 . Thestructure has multilayer stacks of AlAs and Al0 1Ga09As on either side of a GaAs active region. The top mirror is doped p-type and the bottom mirror is doped n-type. The entire layer structure is grown by molecular beam epitaxy over a n-GaAs substrate.

We report on Molecular Beam Epitaxial growth and properties of strained In_0.2Ga_0.8As-GaAs quantum well (QW) structures suitable for 980 nm lasers. The QW width was maintained at 80 A and the barrier thickness was varied from 50 A to 300 A. The effects of increasing the barrier width on the structural and optical properties of the QW were examined using double crystal x-ray diffraction rocking scans (DCXRD) and photoluminescence measurements. DCXRD rocking scans revealed satellite peaks from the strained layer quantum wells (SLQW). The linewidth of the peaks decreased as the barrier width was increased. Optical measurements indicate significant improvements in the internal luminescence efficiency in the thick barrier structures. We assign the improvements in the luminescence properties to the reduction of non-radiative centers in the thick barrier structures. The sources of the non-radiative centers are ascribed to structural defects that are generated as a result of strain relaxation in the thin barrier structures. A new broad photoluminescence feature at 0.9 eV was also observed and believed to originate in the AlGaAs:Si cladding region. We shall present these results and discuss the implications of increasing the barrier thickness of In_0.2Ga_0.8As-GaAs QW on the performance of 980 nm lasers.

A single-step metal organic chemical vapor deposition (MOCVD) growth has been used to fabricate a buried heterostructure InGaAs/GaAs multi-quantum well laser over a patterned GaAs substrate. The pattern used here is a re-entrant mesa formed by wet chemical etching oriented along [011] direction. Growth over the mesa results in isolated buried heterostructures. The 250 micrometers long lasers have threshold currents of 30 mA and emit > 100 mW/facet at room temperature. The external differential quantum efficiency is found to be almost independent of temperature in the temperature range of 10 degree(s)C to 90 degree(s)C which suggests a low temperature dependence of leakage current. The threshold current of the laser as a function of temperature can be represented by the usual expression I_th approximately I_o exp(T/T_o) with a characteristic temperature (T_o) of about 120 K in the temperature range 10 degree(s)C to 90 degree(s)C.

New AlGaAs buried heterostructure (BH) formation processes based on low temperature (T < 600 degree(s)C) in-situ mesa melt-etching and liquid phase epitaxial (LPE) regrowth have been studied and applied to the fabrication of BH laser diodes. It was shown that the BH formation process on Al_xGa_1-xAs laser structures with x equals 0.2...0.5 of cladding layers and masking stripes oriented along [011] or [011] on (100) planes depends on melt-etching rate anisotropy, while in a similar process for x > 0.6, or masking stripe orientations other than [011] or [011] the most important factors is the melt-etching material selectivity. Difficult AlGaAs nucleation on A-type planes is a distinct feature of LPE regrowth at temperatures lower than 600 degree(s)C. Low temperature melt-etching and regrowth produced single mode AlGaAs BH laser diodes emitting at 800 nm with the maximum optical power of 120 mW at 106 mA.

We report the first band offset measurement of GaAs/Ga_0.51In_0.49P multiquantum wells and superlattices by electrolyte electroreflectance spectroscopy. The conduction and valence band discontinuities (Delta) E_c equals 159 +/- 4 meV and (Delta) E_v equals 388 +/- 6 meV have been measured. The values found for the conduction band, heavy-hole and light-hole masses in the GaInP barriers and GaAs wells and for the split-off well mass are in excellent agreement with the literature. The intraband, intersubband transition energies, which are important for III - V infrared detection devices, also were directly measured.

Carbon is a very suitable acceptor in III - V compounds for device applications requiring thin layers with very low resistivity due to its low diffusion coefficient. We have used a modified, atmospheric pressure MOCVD reactor to grow carbon doped GaAs films by atomic layer epitaxy (ALE) using trimethylgallium (TMGa) as the carbon source. The hole density was controllable from high resistivity to 10²⁰ cm^-3. These results were then used as the basis for the study of carbon doping of InGaAs in a layer-by-layer technique using TMGa, triethylindium (TEIn) and arsine (AsH₃).

We report on the fabrication and performance characteristics of (GaAs)₃/(AlAs)₁ short-period superlattices (SPSs) quantum well lasers emitting at 737 nm. The SPSs consists of eight periods of 3 and 1 ML of GaAs and AlAs, respectively. The (GaAs)_m/(AlAs)_n SPSs have many advantages over their equivalent AlGaAs alloy counterparts. The broad area threshold current density, J_th, for 500 micrometers long lasers is 510 A cm^-2. The 500 micrometers -long ridge waveguide lasers have a threshold current of 48 mA with a characteristic temperature of 68 K in the temperature range 19 to 60 degree(s)C. The external differential quantum efficiency near threshold is 0.58 mW/mA/facet. The devices lase in a single mode with spectral width within the resolution limit of the spectrometer.

Quantum well wire structures in metalorganic vapor phase epitaxy (MOVPE) grown Ga_.53In_.47As/InP and in Ga_.85In_.15As/GaAs have been fabricated by electron beam lithography and subsequent metalorganic reactive ion etching (MORIE) and/or wet etching. The dry etching was optimized for low-damage conditions and for mask-to-wafer pattern transfer. In the wet etching process, an underetching was implemented in order to reduce the linewidth defined by the etching mask. A wet etching step has been used after the dry etching for removal of the partly damaged surface region and for smoothing of the sidewalls of the wires. Differently processed areas were excited selectively by low- temperature cathodoluminescence (CL) from which the optical quality of the wire material was evaluated and blue shifts for the wires as large as 10 meV were observed. Individual wires have also been imaged and effects of one-dimensional exciton diffusion have been probed.

We present a Monte Carlo analysis of a `true' GaAs-based quantum wire, whose dimensions correspond to present state-of-the-art technology. Intrasubband and intersubband scattering rates for the electron-polar optical phonon interaction are included in the simulation as well as electron-electron interaction. We have studied the nonequilibrium transport characteristics of the one-dimensional system in two different situations: the response of the electron gas to an external electric field applied along the wire direction, and the cooling dynamics following laser photoexcitation. With respect to 3-D and 2-D systems, we can show that the electron- phonon interaction is not substantially modified, while a strong reduction in the electron- electron scattering rate of the wire is found.

We present experimental and theoretical results on the low temperature luminescence intensity of dry etched GaAs-Al_xGa_1-xAs quantum dots. The luminescence intensity was found to decrease by two orders of magnitude with the decrease of dot sizes from 1 micrometers to 60 nm. Our intrinsic model of the emission yield invokes slower momentum and energy relaxation mechanisms as the lateral dimensions decrease. The extrinsic effect, which we include in our interpretation of the luminescence intensity, involves carrier diffusion with a surface nonradiative recombination velocity. The combined effect (intrinsic and extrinsic) gives a very good fit to our data. The surface recombination velocity needed for the fit was approximately 10⁵ cm/s. Raman studies on the quantum dots showed enhanced surface phonons with the decrease of the nanostructure sizes. `GaAs'-like and `AlAs'-like surface phonons were also observed for the first time in etched nanostructures, in good agreement with the theoretical predications.

The lateral dimensions of resonant tunneling AlGaAs-GaAs double barrier heterostructures have been restricted by hydrogen plasma exposure. Ohmic contacts to the submicron diodes have been made by solid phase epitaxial growth of Ge on GaAs. The current-voltage characteristics show a fine structure splitting that is inversely proportional to the lateral size of the diode. The results are interpreted as resonant tunneling through zero-dimensional states in the quantum box confined by the AlGaAs barriers and a harmonic lateral confining potential.

We have calculated dc I - V curves of the semiconductor superlattices of a very small (practically, submicron) cross-section. The I - V curves exhibit periodic oscillations with a voltage period e/C. These oscillations are caused by quantization of electric charge Q of the walls of static high-field domains.

Quantum wires with double bend discontinuities have been fabricated in modulation-doped field-effect transistors. The low temperature conductance shows resonant peaks in the lowest quantized conductance plateau. The double bend constitutes an electron cavity where the number of peaks is directly related to the cavity length. This view is supported by comparison to the theoretical conductance calculated from a generalized mode-matching theory. The experimental peak conductivity decreases with cavity length, which is consistent with elastic scattering due to random disorder in the quantum wire. Magnetic field studies show quenching of the resonance structure when the cyclotron radius approaches the one-dimensional channel width.

We report a novel, three-step dry etching process for the fabrication of first-order diffraction gratings in III - V semiconductors. The process takes advantage of the etching of thin films of SiO₂ and an organosilicon electron-beam resist (poly(3-butenyltrimethylsilane sulfone)) by the H₂/CH₄ plasma that is used to etch InP and its alloys. The new method significantly reduces wafer handling and eliminates the wet resist stripping step. The gratings were imaged with a scanning tunneling microscope.

Processes for the fabrication of nanometer-scale geometries in InP and related materials are discussed. Special emphasis is directed to pattern transfer using reactive ion etching in methane-hydrogen plasmas. Using these processes, 70 nm period gratings etched 900 nm deep in InP resulting in an aspect ratio of 25 is demonstrated. It is shown that these processes can be applied successfully to the fabrication of quantum wires in InP/InGaAs heterostructures. The high luminescence efficiency of these wires even at dimensions down to 40 nm shows that CH₄/H₂ reactive ion etching does not severely degrade the optical properties of quantum wires.

MBE grown calcium fluoride has been directly patterned using an 100 keV electron beam. An array of dots 4 nm in diameter with a period of 15 nm and lines 7 nm wide are reported. The dependence of the damage process on beam current showed that there is a simple dose requirement of 9 X 10³ C.cm^-2, which corresponds to 5 X 10⁸ e^-nm^-2. Complete holes were not observed. Electron energy loss spectroscopy studies indicate that fluorine has been removed and metallic calcium is left in the damaged region. The damage process was studied with the sample orientated along the <111> direction and tilted away from this axis by up to 10 degrees. No orientation dependence in the hole size or dose required was observed.