Effect of substrate annealing and V: III flux ratio on the molecular beam epitaxial growth of AlGaAs-GaAs single quantum wells

Photoluminescence spectra are presented on single quantum well (SQW) structures on thick (1.0 μm) A1_0.20Ga_0.80As buffers grown by molecular beam epitaxy (MBE). Through substrate preparation and careful control of growth conditions, full width half maximum (FWHM) luminescence line-widths of 0.7 meV have been achieved for the n= l confined electron-heavy hole transition. This is the narrowest luminescent linewidth ever reported for any epitaxially grown material. Annealing of substrates with the subsequent removal of the converted surface prior to MBE growth is shown to improve SQW luminescence. Luminescence quality is shown to vary strongly with V: 111 flux ratio during well growth. Improved SQW luminescence is observed for SQW grown using lower V: 111 flux ratios. Growth of a GaAs SQW at 680°Cusing a V:III flux ratio one-half of that required for thick GaAs epilayers while maintaining the same surface reconstruction is demonstrated. The techniques reported are important for the growth of heterojunction interfaces and devices in AlGaAs-GaAs.     

Study of transient processes in a <Emphasis Type="Italic">p-i-n</Emphasis> photodetector using the nonstationary physical-topological model

We developed a nonstationary diffusion-drift physico-topological model of a GaAs p-i-n photodetector for operation as an element of an optical switch integrated circuit, together with a high-speed injection laser based on a double heterostructure with a functionally integrated radiation modulator.     

Luminescent properties of AlGaAs grown by transient-mode liquid epitaxy

Transient-mode liquid epitaxy (TMLE), a method of non-equilibrium LPE growth, has been used to prepare thin n-type single-crystal layers of AlGaAs on GaAs substrates for the fabrication of infrared-emitting LED’s. It is well known that the concentration of Al in an epitaxial layer grown from the liquid phase is primarily dependent on solution composition, growth temperature and growth rate. During conventional LPE, the growth rate is usually made low enough that the equilibrium distribution coefficient can be assumed to prevail and the Al concentration for direct bandgap compositions will decrease as a layer grows. This, however, is not the case for material produced by TMLE. Since the growth rate during TMLE continually drops from an initially high value towards zero, the amount of Al incorporated in the epitaxial layer, and hence the bandgap energy, is not reduced as the layer is generated. The luminescence created at a diffused p-n junction is emitted through the p-side with very little absorption. Photoluminescence measurements using an argon-ion laser source (4579 Å) were made to determine the bandgap energies of AlGaAs epitaxial layers grown by TMLE for various lengths of time. Diodes fabricated from from this material had peak electroluminescence between 7900 and 8250 Å With half-intensity bandwidths of 200 A. In addition to a discussion of diode preparation, the spectral characteristics of diodes are compared with results obtained from photoluminescence measurements.     

InGaAs-GaAs-InGaP distributed feedback buried heterostructurestrained quantum-well lasers for high-power operation at 0.98 μm

Data are presented on device results from InGaAs-GaAs distributed feedback buried heterostructure (DFB BH) strained-quantum-well lasers with InGaP cladding layers. DFB BH lasers with a p-n InGaP current blocking junction entirely grown by a three-step MOVPE on GaAs substrates show a low laser threshold of 3.2 mA and a high output power of 41 mW with single-longitudinal-mode operation, both measured CW at RT. The monomode oscillation is obtained even at the injection current of 140 mA (44 times the laser threshold) with the side-mode suppression ratio of 35 dB and the temperature sensitivity of Bragg modes being 0.5 Å/°C measured between 20 and 40°C     

Effects of mesa area and orientation on defects in strained InxGa1−xAs films grown by LPOMCVD

We have selectively grown In_xGa_1?xAs (0.04 <x < 0.20, 200 ? 7000Å) on rectangular growth areas (100 μm by 200 mil) patterned on GaAs substrates with very low etch pit densities (～200 cm^?2). The edge orientations of the growth areas were varied on the substrate resulting in distinct facet formation on the deposited mesa structures. Layers were grown using Low-Pressure Organometallic Chemical Vapor Deposition (LPOMCVD) and all samples were annealed for 1 hr at 700° C. Indium content and film thicknesses were very uniform across a wafer so that only the facet shape was varied. Observation of Crosshatch defect densities as a function of growth window orientation showed that 2900Å In_0.08Ga_0.92As mesas with [010] edge orientations exhibited no Crosshatch defects. Identical mesas grown in windows with other edge orientations exhibited varying crosshatch defect densities. Large unpatterned areas of growth were heavily crosshatched. This variation in Crosshatch defect density as a function of mesa orientation appears to be associated with non-area related dislocation nucleation mechanisms at the mesa edges. Proper choice of window orientation for patterned substrate epitaxy will allow the mesa facets to be controlled thereby reducing misfit dislocations in the heterostructure interface.     

Localization of holes in an InAs/GaAs quantum-dot molecule

Deep-level transient spectroscopy is used to study the emission of holes from the states of a vertically coupled system of InAs quantum dots in p-n InAs/GaAs heterostructures. This emission was considered in relation to the thickness of a GaAs interlayer between two layers of InAs quantum dots and to the reversebias voltage U_r. It is established that hole localization at one of the quantum dots is observed for a quantum-dot molecule composed of two vertically coupled self-organized quantum dots in an InAS/GaAs heterostructure that has a 20-Å-thick or 40-Å-thick GaAs interlayer between two layers of InAs quantum dots. For a thickness of the GaAs interlayer equal to 100 Å, it is found that the two layers of quantum dots are incompletely coupled, which results in a redistribution of the hole localization between the upper and lower quantum dots as the voltage U_r applied to the structure is varied. The studied structures with vertically coupled quantum dots were grown by molecular-beam epitaxy using self-organization effects.     

Low drive voltage GaAs quantum well electroabsorption modulatorsobtained with a displaced junction

A reduction in the drive voltage of an electroabsorption waveguide modulator based on an AlGaAs/GaAs quantum well laser heterostructure is demonstrated by a simple change in the position of the p-n junction relative to the quantum well. This is accomplished by adjusting the doping profile and does not significantly alter the operation of the structure as a laser or degrade other aspects of device performance such as reverse breakdown voltage and series resistance. A contrast ratio of 25 dB (5 dB/100 μm) was obtained with a bias change of -2 V     

The two-dimensional lateral injection in-plane laser

In this paper, a two-dimensional (2-D) p-n junction was used for population inversion in a GaAs quantum-well laser. The device, incorporating modulation doping within the core of a separate confinement heterostructure, was designed to exploit the amphoteric behavior of silicon in GaAs [doping p-type on (311)A facets and n-type on (100)]. It is believed to be the first lasing device to use an amphoterically doped junction for population inversion. In the first attempted design (described here), CW lasing was achieved at temperatures up to 90 K. The factors affecting the temperature dependence of threshold are discussed in the context of possible design improvements. The device may eventually show improved modulation bandwidth over conventional vertical injection lasers with bulk contacts, since its geometry and the 2-D nature of the injection offer reduced capacitance, HEMT integration, and an elimination of carrier capture problems     

Otpically illuminated dielectric interfaces as high-speed far-infrared modulators

A new device for the gigahertz modulation of far-infrared radiation is analytically and numerically analyzed. It consists of a thin layer of a high-mobility, direct-bandgap semiconductor, such as GaAs, in which a high-density electron-hole plasma is rapidly created and destroyed, thereby rapidly changing the free-carrier reflectivity of the active layer. Illumination by a high-power, near-infrared laser diode array generates the plasma through intrinsic photoconduction. It is shown that this device acis primarily as an amplitude modulator, and that its efficiency increases sharply with increasing far-IR frequency, in contrast to a Schottky diode, which acts primarily as a phase modulator, and whose efficiency falls off sharply with far-IR frequency. The breakeven frequency lies at about 1.5 THz, depending slightly on the assumed device parameters. The relative advantage of the new device increases rapidly with increasing far-infrared frequency. At an operating frequency of 2.5 THz (119 μm), for example, a 1 GHz modulation bandwidth may be achieved with a single-sideband conversion loss of only-21 db, versus a Schottky's loss of-39 db, assuming a laser diode power of 1 W, which is readily available from recently developed laser diode arrays.     

Photoelectric and photovoltaic properties of structures based on mesoporous silicon passivated with iron

The photoelectric and photovoltaic properties of structures based on mesoporous silicon passivated with iron (SiMP:Fe) are studied. It is shown that these properties ambiguously depend on the iron concentration. In the case of a sample with space-charge-limited currents (SCLCs), the charge-transport mechanism in the Al-SiMP:Fe-p-Si-Al heterostructure changes under illumination from the SCLC type to the barrier type. Passivation with 0.1–0.2 at % iron stabilizes not only the electrical, but also the photoelectric and photovoltaic properties of the structures. A further increase in the Fe concentration gives rise to new traps caused by the appearance of iron and silicon oxides, which leads to instability of the properties. The structures exhibit high sensitivity under low-level illumination. The open-circuit voltage is 16 mV under AM-1 irradiation (～2 mW/cm²).     

Characterization of oxidized a-SiHx surfaces used in mis structures

Atomically clean and oxidized surfaces of a-SiH_x films, prepared by plasma decomposition of SiH₄ in a capacitative reactor, were studied using Auger and UV photoemission spectroscopy. The oxides were characterized in terms of the fine structure of the Si_LVV Auger transition. We also studied the electrical properties of MIS solar cell structures in which the a-SiH_x film was given the same treatment as the samples for surface studies and correlated the results of the two kinds of measurements. For thin (< 20 Å) oxides the photodiode open circuit voltage increases linearly with oxide thickness. This increase is found to be due to an increase in the barrier height and the built-in potential upon oxidation.     

Structural,electrical and optical characterization of singlecrystal ErAs layers grown on GaAs by MBE

A detailed study is presented of the structural, electrical, and optical properties of ErAs films grown on GaAs by molecular beam epitaxy (MBE). ErAs layers 1500Å thick were grown successfully over a relatively wide range of substrate temperatures (420-580° C), although overgrowth of GaAs on ErAs was found to be difficult. In-situ reflection highenergy electron diffraction (RHEED), x-ray diffraction, and Rutherford backscattering (RBS) measurements all indicate single crystal growth. Analysis of X-ray rocking curves reveals that, over the range of substrate temperatures studied, strain due to the lattice mismatch between ErAs and GaAs is completely inelastically relieved in the 1500Å thick ErAs layers. Variable-temperature Hall measurements reveal metallic behaviour in all samples, with no pronounced dependence on substrate temperature. Spectrally narrow (0.6 meV) intra 4f-shell transitions of Er³⁺ (4f¹¹), at 1.54 μm, have been observed in ErAs epitaxial layers both in absorption (by Fourier transform infra-red spectroscopy, FTIR) and in emission (by cathodoluminescence). The crystal-field splittings observed in the FTIR spectra are consistent with the cubic(O _h)symmetry expected for the Er lattice site in unstrained ErAs, in good agreement with the x-ray analyses.     

850-nm diode lasers based on AlGaAsP/GaAs heterostructures

Laser heterostructures with waveguides and emitter layers made of AlGaAs and AlGaAsP alloys are grown by hydride metal-organic vapor-phase epitaxy on a GaAs substrate and studied. It was shown that the heterostructure containing AlGaAsP layers has a large curvature radius in comparison with the structure consisting of AlGaAs layers. Ridge waveguide lasers with an aperture of 100 ??m are fabricated based on the AlGaAsP/GaAs laser heterostructure and studied. The internal optical loss of the lasers is 0.75 cm^?1; the characteristic parameter T ₀ = 140 K in the temperature range of 20?C70°C. The maximum optical output power per mirror reached 4.1 W; cw lasing was retained at a heat sink temperature of 120°C.     

Temperature dependence of the lasing characteristics of the 1.3 µm InGaAsP-InP and GaAs-Al0.36Ga0.64As DH lasers

We report here our experimental observations on the temperature dependence of threshold current, carrier lifetime at threshold, external differential quantum efficiency, and gain of both the 1.3 μm InGaAsP-InP and GaAs-AlGaAs double heterostructure (DH) lasers. We find that the gain decreases much faster with increasing temperature for a 1.3 μm InGaAsP DH laser than for a GaAs DH laser. Measurements of the spontaneous emission observed through the substrate shows that the emission is sublinear with injection current at high temperatures for the 1.3 μm InGaAsP DH laser. Such sublinearity is not observed for GaAs DH lasers in the entire temperature range 115-350 K. The experimental results are discussed with reference to the various mechanisms that have been proposed to explain the observed temperature dependence of threshold of InGaAsP DH lasers. We find that inclusion of a calculated nonradiative Auger recombination rate can explain the observed temperature dependence of threshold current, carder lifetime at threshold, gain, and also the sublinearity of the spontaneous emission with injection current of the 1.3 μm InGaAsP-InP DH laser. Measurement of the nonradiative component of the carrier lifetime (τA) as a function of injected carrier density (n) shows thattau_{A}^{-1} sim n^{2.1}which is characteristic of an Auger process.     

Investigation of GaAs Schottky barrier diodes in the THz range

The properties of GaAs Schottky barrier diodes as video detectors and mixing elements were investigated in the frequency range from 0.8–2.5 THz. For the most sensitive diode, the video responsivity and system noise temperature were measured as a function of incident laser power. The highest video responsivity was 2,000 V/W at 214μm and 60 V/W at 118μm. For five diodes differing in doping, capacitance, series resistance and anode diameter, the system noise temperature was measured at 214μm and 118μm. The best single sideband (SSB) values are 12,300 K and 24,200 K at 214μm and 118μm, respectively. The system noise temperature versus frequency is given over the range from 0.5–3 THz for two specific diodes demonstrating that the sharpness of the I–V characteristics is only of secondary importance for mixer perfomance at such high frequencies.     

An AlGaAs/GaAs short-cavity laser and its monolithic integration using microcleaved facets (MCF) process

This paper describes the microcleaved facets (MCF) process developed for AlGaAs/GaAs laser fabrication which can avoid the previous substrate cleavage and therefore be applicable to preparing both discrete short-cavity lasers and optoelectronic integrated circuits (OEIC's). An AlGaAs/GaAs double heterostructure ridge-waveguide laser with an extremely short cavity length, namely 20 μm, was first realized by this process. A threshold current as low as 20 mA and a single longitudinal mode lasing were achieved. The usefulness of this process for integrating a laser and a monitoring photodiode on a GaAs substrate is also demonstrated.     

Temperature Characteristics of Gain Profiles in 1.3-$ muhbox{m} p$-Doped and Undoped InAs/GaAs Quantum-Dot Lasers

The modal gain and differential gain of 1.3-$muhbox{m}break p$ -doped and undoped InAs/GaAs quantum-dot (QD) lasers have been investigated as a function of injection current under different operation temperatures. The results show that $p$ -doping improves the modal and differential gains in QD lasers at high temperatures. Exponential decrease in the differential gain profiles were observed in both types of lasers from 20 $^{circ}hbox{C}$ to 80 $^{circ}hbox{C}$. Theoretical calculations based on the rate equation model for the undoped QD laser gain at different temperatures are presented.      

Fabrication and analysis of GaAs Schottky barrier diodes fabricated on thin membranes for terahertz applications

The GaAs Schottky diode is predominantly used as the critical mixer element in heterodyne receivers in the frequency range from 300 GHz to several THz[1]. At operating frequencies above one THz the skin effect adds significant parasitic resistance to the diode which degrades the receiver sensitivity. A novel diode structure called the Schottky barrier membrane diode is proposed to decrease the skin effect resistance by reducing the current path between the Schottky and ohmic contacts. This is accomplished by fabricating the diode on a very thin membrane of GaAs (about 1 μm thickness). A theoretical analysis has shown that this will reduce the substrate resistance by 60% at 3 THz. This reduction in resistance corresponds to a better frequency response which will improve the device's performance as a mixer element.     

Metamorphic lasers for 1.3-µm spectral range grown on GaAs substrates by MBE

A new method for the epitaxial formation of 1.3-μm injection lasers on GaAs substrates is reported. A metamorphic heterostructure with an In content of about 20% is deposited onto an intermediate buffer layer intended for mismatch strain relaxation. The laser active region is formed by quantum wells with a higher In content (about 40%). Lasers with 100-μm-wide stripes demonstrate room-temperature lasing at 1.29 μm with a minimum threshold current density of 3.3 kA cm² (0.4 kA cm² at T=85 K). __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 9, 2003, pp. 1143–1147. Original Russian Text Copyright ? 2003 by Zhukov, Kovsh, Mikhrin, Semenova, Maleev, Vasil’ev, Nikitina, Kryzhanovskaya, Gladyshev, Shernyakov, Musikhin, Maksimov, Ledentsov, Ustinov, Alferov.     

MOCVD-grown InGaAs/GaAs/AlGaAs laser structures with a broad-area contact

A metal-organic chemical vapor deposition (MOCVD) technique is developed for a diode laser heterostructure in a system of InGaAs/GaAs/AlGaAs solid solutions; the optimal sizes and the doping profile of the structure are determined to minimize the internal optical losses. Mesa-strip diode lasers with a threshold density of current J _th=150–200 A/cm², internal optical loss factor α_i=1.6–1.9 cm^?1, and an internal quantum yield η_i=85–95% were fabricated. In the continuous lasing mode of a diode laser with a 100-µm-wide aperture and a wavelength of 0.98 µm, the optical power output was as high as 6.5 W and was limited by the catastrophic optical degradation of mirrors. The radiation divergence in the plane normal to the p-n junction amounts to θ_⊥. The use of wide-gap waveguide layers, which deepens the potential electron well in the active region, is shown to reduce the temperature sensitivity of the InGaAs/GaAs/AlGaAs laser heterostructures in the temperature range from 0 to 70°C.