Ultralow internal optical loss in separate-confinement quantum-well laser heterostructures

Internal optical loss in separate-confinement laser heterostructures with an ultrawide (>1 smm) waveguide has been studied theoretically and experimentally. It is found that an asymmetric position of the active region in an ultrawide waveguide reduces the optical confinement factor for higher-order modes and raises the threshold electron density for these modes by 10–20%. It is shown that broadening the waveguide to above 1 μm results in a reduction of the internal optical loss only in asymmetric separate-confinement laser heterostructures. The calculated internal optical loss reaches ∼0.2 cm⁻¹ (for λ≈1.08 μm) in an asymmetric waveguide 4 μm thick. The minimum internal optical loss has a fundamental limitation, which is determined by the loss from scattering on free carriers at the transparency carrier density in the active region. An internal optical loss of 0.34 cm⁻¹ was attained in asymmetric separate-confinement laser heterostructures with an ultrawide (1.7 μm) waveguide, produced by MOCVD. Lasing in the fundamental transverse mode has been obtained owing to the significant difference in the threshold densities for the fundamental mode and higher-order modes. The record-breaking CW output optical power of 16 W and wallplug efficiency of 72% is obtained in 100-μm aperture lasers with a Fabry-Perot cavity length of ∼3 mm on the basis of the heterostructures produced. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 12, 2004, pp. 1477–1486. Original Russian Text Copyright ? 2004 by Slipchenko, Vinokurov, Pikhtin, Sokolova, Stankevich, Tarasov, Alferov.     

Photoelectric phenomena in ZnO:Al-<Emphasis Type="Italic">p</Emphasis>-Si heterostructures

Al-doped zinc-oxide (ZnO:Al) films are obtained by magnetron sputtering. Based on an investigation of electrical properties of the films, it is shown that the electron density in these films is as high as 5×10²⁰ cm⁻³ and is practically constant in the temperature range 77–300 K, which indicates high efficiency of doping ZnO with an Al impurity. It is found that the deposition of thin films (d≈1 μm) on the p-Si(111) surface leads to the formation of heterostructures with the highest photosensitivity of ∼400 V/W at T=300 K, which oscillates in the spectral range 1.3–3.5 eV. With the oblique incidence of linearly polarized radiation, induced pleochroism emerges in such heterostructures. The magnitude of pleochroism oscillates in the range 5–40% (θ≈75°), which is associated with the interference phenomena in the ZnO films. The prospects of using the heterostructures obtained as highly selective photosensors of natural and linearly polarized radiation are considered. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 11, 2003, pp. 1329–1333. Original Russian Text Copyright ? 2003 by Nikitin, Nikolaev, Polushina, V. Rud’, Yu. Rud’, Terukov.     

InAsSbP double-heterostructure lasers for the spectral range 2.7–3.0 µm (T=77 K)

We have produced, by using liquid-phase epitaxy, 2.7 to 3.0-μm lasers based on InAsSbP double-heterostructures with different phosphorus contents in the active and wide-gap regions. The lasers possess threshold current density ∼0.8 kA/cm² at 77 K and operate in the pulsed mode up to ∼124 K with maximum threshold current density 10–12 kA/cm². The lasers have a low series resistance ∼0.45 Ω. Fiz. Tekh. Poluprovodn. 32, 241–244 (February 1998)     

Internal quantum efficiency of stimulated emission of (λ=1.55 µm) InGaAsP/InP laser diodes

The stimulated emission (η _i ^st ) of InGaAsP/InP separate-confinement double heterostructure lasers operating at λ=1.5–1.6 μm has been studied experimentally and theoretically. Laser heterostructures with a varied design of the waveguide layer were grown by MOCVD. The maximum internal quantum efficiency η _i ^st ≈97% was obtained in a structure with a double-step waveguide characterized by minimum leakage into the p-emitter above the generation threshold. The high value of η _i ^st is provided by low threshold and nonequilibrium carrier concentrations at the interface between the waveguide and p-emitter. The calculation yields η _i ^st values correlating well with the experimental data. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 2, 2003, pp. 243–248. Original Russian Text Copyright ? 2003 by Skrynnikov, Zegrya, Pikhtin, Slipchenko, Shamakhov, Tarasov.     

Spectral line width of the current-tunable lasers on the base of InAsSb/InAsSbP at low temperature

Current-tunable diode lasers with a narrow emission line (7–10 MHz) were designed on the basis of a InAsSb/InAsSbP heterostructure for low-temperature (15–60 K) operation within the range of 3.2–3.4 μm. It is shown that the fast modulation of radiation frequency can be attained only if the current far exceeds the threshold value. This is due to the fact that a smooth optical waveguide can be formed only if the concentration of nonequilibrium charge carriers at the lateral edges of the cavity is in sufficient excess over the threshold. It is noted that the current-induced increase in the hole concentration near the p region due to a reduction in the effective lifetime of the carriers in the lasing mode additionally extends the frequency tuning range. The absorption spectrum of ammonia is presented within the range from 3232 to 3237 cm⁻¹ as measured with the use of the laser developed. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 5, 2002, pp. 622–628. Original Russian Text Copyright ? 2002 by Imenkov, Kolchanova, Kubat, Tsivish, Yakovlev.     

InAsSb/InAsSbP double-heterostructure lasers emitting in the 3–4 µm spectral range

Our earlier reports concerning the fabrication by liquid-phase epitaxy and investigation of InAsSbP/InAsSb/InAsSbP double heterostructure lasers emitting at 3–4 μm are reviewed. The dependences of spectral characteristics and the spatial distribution of the laser emission on temperature and current are discussed. Lasing modes are shifted by 0.5–1.0 cm⁻¹ to longer wavelengths with increasing temperature. The tuning of the lasing modes by means of current is very fast (10⁻⁸–10⁻¹² s). With increasing current, the modes are shifted to shorter wavelengths by 50–60 ? at 77 K. The maximum mode shift of 104 ? (10 cm⁻¹) is observed at 62 K. The spectral line width of the laser is as narrow as 10 MHz. Abnormally narrow directional patterns in the p-n junction plane are observed in some cases in the spatial distribution of laser emission. The current tuning of lasers, due to nonlinear optical effects, has been modeled mathematically in good agreement with the experiment. Transmittance spectra of OCS, NH₃, H₂O, CH₃Cl, and N₂O gases were recorded using current-tuned lasers. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 12, 2001, pp. 1466–1480. Original Russian Text Copyright ? 2001 by Danilova, Imenkov, Kolchanova, Yakovlev. See [1].     

Pulsed laser deposition and processing of wide band gap semiconductors and related materials

The present work describes the novel, relatively simple, and efficient technique of pulsed laser deposition for rapid prototyping of thin films and multi-layer heterostructures of wide band gap semiconductors and related materials. In this method, a KrF pulsed excimer laser is used for ablation of polycrystalline, stoichiometric targets of wide band gap materials. Upon laser absorption by the target surface, a strong plasm a plume is produced which then condenses onto the substrate, kept at a suitable distance from the target surface. We have optimized the processing parameters such as laser fluence, substrate temperature, background gas pressure, target to substrate distance, and pulse repetition rate for the growth of high quality crstalline thin films and heterostructures. The films have been characterized by x-ray diffraction, Rutherford backscattering and ion channeling spectrometry, high resolution transmission electron microscopy, atomic force microscopy, ultraviolet (UV)-visible spectroscopy, cathodoluminescence, and electrical transport measurements. We show that high quality AlN and GaN thin films can be grown by pulsed laser deposition at relatively lower substrate temperatures (750–800°C) than those employed in metal organic chemical vapor deposition (MOCVD), (1000–1100°C), an alternative growth method. The pulsed laser deposited GaN films (∼0.5 μm thick), grown on AlN buffered sapphire (0001), shows an x-ray diffraction rocking curve full width at half maximum (FWHM) of 5–7 arc-min. The ion channeling minimum yield in the surface region for AlN and GaN is ∼3%, indicating a high degree of crystallinity. The optical band gap for AlN and GaN is found to be 6.2 and 3.4 eV, respectively. These epitaxial films are shiny, and the surface root mean square roughness is ∼5–15 nm. The electrical resistivity of the GaN films is in the range of 10⁻²–10² Θ-cm with a mobility in excess of 80 cm²V⁻¹s⁻¹ and a carrier concentration of 10¹⁷–10¹⁹ cm⁻³, depending upon the buffer layers and growth conditions. We have also demonstrated the application of the pulsed laser deposition technique for integration of technologically important materials with the III–V nitrides. The examples include pulsed laser deposition of ZnO/GaN heterostructures for UV-blue lasers and epitaxial growth of TiN on GaN and SiC for low resistance ohmic contact metallization. Employing the pulsed laser, we also demonstrate a dry etching process for GaN and AlN films.     

VCSELs based on arrays of sub-monolayer InGaAs quantum dots

Vertical-cavity surface-emitting lasers (VCSELs) with an active region based on sub-monolayer InGaAs quantum dots and doped AlGaAs/GaAs distributed Bragg reflectors were grown by MBE. VCSELs with current aperture of 3 μm in diameter demonstrate single-mode lasing in 980-nm range with the threshold current of 0.6 mA, maximum output power up to 4 mW, and external differential efficiency of 68%. Multimode VCSELs with a (10–12)-μm aperture demonstrate ultralow internal optical loss of 0.09% per pass, which compares favorably with the best results obtained in similar lasers with undoped distributed Bragg reflectors. Original Russian Text ? S.A. Blokhin, N.A. Maleev, A.G. Kuz’menkov, Yu. M. Shernyakov, I.I. Novikov, N.Yu. Gordeev, V.V. Dyudelev, G.S. Sokolovskiĭ, V.I. Kuchinskiĭi, M.M. Kulagina, M.V. Maximov, V.M. Ustinov, A.R. Kovsh, S.S. Mikhrin, N.N. Ledentsov, 2006, published in Fizika i Tekhnika Poluprovodnikov, 2006, Vol. 40, No. 5, pp. 633–638.     

Single-mode InAsSb/InAsSbP laser (λ≈3.2 μm) Tunable over 100 Å

A report is presented on the development of a single-mode laser based on the InAsSb/InAsSbP double heterostructure and operating at wavelengths of 3.2–3.3 μm in a temperature range of 12–90 K. The single-mode regime is assumed to be realized due to a smooth optical waveguide formed across the laser cavity in which the radiation flux oscillates and maintains its oscillations and intensity. An analysis is made of the effect of the current-induced shifts of the lasing frequency and the peak of the gain spectrum on the probability of single-mode lasing. Experiments were made on the scanning of OCS, NH₃, CH₃Cl, and H₂O gas media with radiation of the given laser in the frequency range with a record width of 10 cm⁻¹ (104 ?). __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 2, 2000, pp. 243–248. Original Russian Text Copyright ? 2000 by Danilova, Imenkov, Kolchanova, Civis, Sherstnev, Yakovlev.     

Impact of growth rate on the quality of ZNS-MQW InGaAsP/InP laser structures grown by LP-MOVPE

We investigated the influence of the growth rate on the quality of zero-net-strained InGaAsP/InGaAsP/InP multiquantum well structures for 1.55 μm emission grown by low pressure metalorganic vapor phase epitaxy. The samples consisted of fixed compressive strained wells (ɛ=+1%) and tensile strained barriers (ɛ=−0.5%) grown with different quaternary bandgap wavelengths (λ_B=1.1–1.4 μm). Using higher growth rates, we obtained for the first time high quality zero net strained multi quantum well structures, regardless having constant group V composition in the well and barriers. The samples were analyzed by x-ray diffraction, photoluminescence and atomic force microscopy techniques. The amplitude of surface modulation roughness along [011] direction decreased from 20 nm to 0.53 nm with increasing growth rate and/or quaternary compositions grown outside the miscibility gap. A new deep PL broad emission band strongly correlated with the onset of wavy layer growth is also reported. Broad area and ridge waveguide lasers with 10 wells exhibited low losses (34 cm⁻¹) and low threshold current densities at infinite cavity length (1020 A·cm⁻² and 1190 A·cm⁻², respectively).     

Design of edge termination for GaN power Schottky diodes

The GaN Schottky diodes capable of operating in the 300–700-V range with low turn-on voltage (0.7 V) and forward conduction currents of at least 10 A at 1.4 V (with corresponding forward current density of 500 A/cm²) are attractive for applications ranging from power distribution in electric/hybrid electric vehicles to power management in spacecraft and geothermal, deep-well drilling telemetry. A key requirement is the need for edge-termination design to prevent premature breakdown because of field crowding at the edge of the depletion region. We describe the simulation of structures incorporating various kinds of edge termination, including dielectric overlap and ion-implanted guard rings. Dielectric overlap using 5-μm termination of 0.1–0.2-μm-thick SiO₂ increases the breakdown voltage of quasi-vertical diodes with 3-μm GaN epi thickness by a factor of ∼2.7. The use of even one p-type guard ring produces about the same benefit as the optimized dielectric overlap termination.     

A study of thick 3C-SiC epitaxial layers grown on 6H-SiC substrates by sublimation epitaxy in vacuum

3C-SiC epitaxial layers with a thickness of up to 100 μm were grown on 6H-SiC hexagonal substrates by sublimation epitaxy in vacuum. The n-type epitaxial layers with the area in the range 0.3–0.5 cm² and uncompensated donor concentration N _d − N _a ∼ (10¹⁷–10¹⁸) cm⁻³ were produced at maximum growth rates of up to 200 μm/h. An X-ray analysis demonstrated that the epitaxial layers are composed of the 3C-SiC polytype, without inclusions of other polytypes. The photoluminescence (PL) spectrum of the layers was found to be dominated by the donor-acceptor (Al-N) recombination band peaked at hv ≈ 2.12 eV. The PL spectrum measured at 6 K was analyzed in detail. It is concluded that the epitaxial layers obtained can serve as substrates for 3C-SiC-based electronic devices. Original Russian Text ? A.A. Lebedev, V.V. Zelenin, P.L. Abramov, E.V. Bogdanova, S.P. Lebedev, D.K. Nel’son, B.S. Razbirin, M.P. Shcheglov, A.S. Tregubova, M. Suvajarvi, R. Yakimova, 2007, published in Fizika i Tekhnika Poluprovodnikov, 2007, Vol. 41, No. 3, pp. 273–275.     

Optically active layers of silicon doped with erbium during sublimation molecular-beam epitaxy

A study is made of the electrical, optical, and structural properties of Si:Er layers produced by sublimation molecular-beam epitaxy. The Er and O contents in the layers, grown at 400–600°C, were as high as 5×10¹⁸ and 4×10¹⁹ cm⁻³, respectively. The electron concentration at 300 K was ∼10% of the total erbium concentration and the electron mobility was as high as 550 cm²/(V·s). Intense photoluminescence at 1.537 μm was observed from all the structures up to 100–140 K. The structure of the optically active centers associated with Er depended on the conditions under which the layers were grown. Fiz. Tekh. Poluprovodn. 33, 156–160 (February 1999)     

Growth of fully doped Hg1−xCdxTe heterostructures using a novel iodine doping source to achieve improved device performance at elevated temperatures

Band gap engineered Hg_1−xCd_xTe (MCT) heterostructures should lead to detectors with improved electro-optic and radiometric performance at elevated operating temperatures. Growth of such structures was accomplished using metalorganic vapor phase epitaxy (MOVPE). Acceptor doping with arsenic (As), using phenylarsine (PhAsH₂), demonstrated 100% activation and reproducible control over a wide range of concentrations (1 × 10¹⁵ to 3.5 × 10¹⁷ cm⁻³). Although vapor from elemental iodine showed the suitability of iodine as a donor in MC.T, problems arose while controlling low donor concentrations. Initial studies using ethyliodide (EtI) demonstrated that this source could be used successfully to dope MCT, yielding the properties required for stable heterostructure devices, i.e. ≈100% activation, no memory problems and low diffusion coefficient. Cryogenic alkyl cooling or very high dilution factors were required to achieve the concentrations needed for donor doping below ≈10¹⁶cm⁻³ due to the high vapor pressure of the alkyl. A study of an alternative organic iodide source, 2-methylpropyliodide (2 MePrI), which has a much lower vapor pressure, improved control of low donor concentrations. 2 MePrI demonstrated the same donor source suitability as EtI and was used to control iodine concentrations from ≈ 1 × 10¹⁵ to 5 × 10¹⁷cm⁻³. The iodine from both sources only incorporated during the CdTe cycles of the interdiffused multilayer process (IMP) in a similar manner to both elemental iodine and As from PhAsH₂. High resolution secondary ion mass spectroscopy analysis showed that IMP scale modulations can still be identified after growth. The magnitude of these oscillations is consistent with a diffusion coefficient of≈7 × 10⁻¹⁶cm²s⁻¹ for iodine in MC.T at 365°C. Extrinsically doped device heterostructures, grown using 2 MePrI, have been intended to operate at elevated temperatures either for long wavelength (8–12 smm) equilibrium operation at 145K or nonequilibrium operation at 190 and 295K in both the 3–5 μ and 8–12 μ wavelength ranges. Characterization of such device structures will be discussed. Linear arrays of mesa devices have been fabricated in these layers. Medium wave nonequilibrium device structures have demonstrated high quantum efficiencies and R₀A = 37 Ωcm² for λ_co = 4.9 μ at 190K.     

Properties of p +-n structures with a buried layer of radiation-induced defects

The p ⁺-n structures based on n-type Si with dopant density 1.7×10¹³–1.2×10¹⁴ cm⁻³ were irradiated with ²³⁸Pu α particles. A layer containing radiation-induced defects with a density of the order of 3×10¹³ cm⁻³ was produced at a depth of 20 μm. This defect density gave rise to intense draining of nonequilibrium carriers in the injection-extraction regime with stationary injection as well as with pulsed generation by single particles. This makes it possible to treat the damaged layer as a plane, introduced into the bulk, with an infinite surface recombination rate. The radiation-induced defects also participated in decreasing the conductivity. A characteristic space charge distribution and, correspondingly, a bias dependence of the capacitance are observed in the structure under reverse bias. Despite the presence of formally three charge regions, four sections appear on the capacitance curve. This latter effect is due to the “additional” charge step arising in the contact potential difference field and is characteristic of compensated deep levels in semiconductors. Fiz. Tekh. Poluprovodn. 32, 359–365 (March 1998)     

Absorption and photoluminescence spectroscopy of diffusion-doped ZnSe:Cr2+

ZnSe:Cr²⁺ is an attractive candidate as a room-temperature tunable solid-state laser with output in the 2–3 μm range. Passive absorption losses in this emission range currently limit laser performance. In this study, we use absorption and photoluminescence spectroscopies at 5 and 296K to address the origin of these optical losses. A series of diffusion-doped ZnSe:Cr single-crystal samples with Cr²⁺ concentrations in the range from 2×10¹⁷ cm⁻³ to 9×10¹⁹ cm⁻³ were obtained using CrSe powder as the dopant source. We find that trace amounts of Fe²⁺ produce absorption in the 2–3 μm range. Also, we have obtained data on a 680 nm absorption band observed in ZnSe:Cr which has been assigned to an internal transition of Cr²⁺. In our series of samples, the relative intensities of the 680 nm absorption band do not track the relative intensities of the 1.8 μm band (known to be due to Cr²⁺), although excitation near 680 nm does produce weak Cr²⁺ luminescence. Our absorption data do not support the current assignment of the 680 nm absorption as being an internal transition of the Cr²⁺ ion.     

Gain and internal losses in InGaAsSb/InAsSbP double-heterostructure lasers

We report on a study characterizing internal losses and the gain in InGaAsSb/InAsSbP diode-heterostructure lasers emitting in the mid-infrared (3–4 μm). Numerical simulations of the current dependence of the intensity of spontaneous emission above the laser threshold and of the differential quantum efficiency allowed us to determine the intraband absorption k ₀ ≈5.6×10⁻¹⁶ cm². The cavity-length dependence of the threshold current is used to estimate the internal losses at zero injection current α ₀≈5 cm⁻¹. Calculations of the internal losses at laser threshold showed that they increase more than fourfold when the cavity length is decreased from 500 μm to 100 μm. The temperature dependence of the differential quantum efficiency is explained on the assumption that intraband absorption with hole transitions into a split-off band occurs. It is shown that the maximum operating temperature of “short-cavity” lasers is determined by the intraband absorption rather than by Auger recombination. The internal losses are shown to have a linear current dependence. The separation of the quasi-Fermi levels as a function of current demonstrates an absence of voltage saturation of the p-n junction above threshold. Fiz. Tekh. Poluprovodn. 33, 759–763 (June 1999)     

Transition-metal-doped chalcogenide lasers

Broadly tunable mid-infrared lasers are desirable for a number of scientific, remote-sensing, and military applications. Recently, Cr²⁺-doped chalcogenide lasers have emerged as an attractive source of tunable laser radiation in the 2–3.4-μm region. This paper provides a brief introduction to this field, summarizes key laser work to date, and provides examples of transmission-metaldoped lasers that operate in this region. In particular, recent results with the Cr²⁺:ZnSe laser material are highlighted.     

Current-voltage characteristics of Si:B blocked impurity-band structures under conditions of hopping-transport-limited photoresponse

The photoconductivity of Si:B blocked-impurity-band (BIB) structures with boron concentration in the active layer ∼10¹⁸ cm⁻³ has been studied. Measurements were performed in the temperature range 4.2–10 K at different intensities of the exciting radiation 10¹⁰–10¹⁵ photons/cm²·s. Photoexcitation at 5.5 μm was realized using a semiconductor laser. At temperatures below 6 K and low bias voltages (<0.5 V) the current-voltage characteristics were found to have a threshold-like character. The threshold voltage rises as the temperature is lowered and the radiation intensity is increased. A model based on the Frenkel’-Poole effect in the impurity band has been developed. This model can be used to numerically describe the current-voltage characteristics with accuracy better than 5%. As a result, it is found that the photoconductivity rises and then reaches a plateau as the radiation intensity increases. Under these conditions, as under equilibrium conditions (in darkness), the hopping conductivity also depends exponentially on the electric field. This fact is explained in terms of the destruction by the electric field of (A ⁺-A ⁻) impurity complexes which appear under nonequilibrium conditions. Fiz. Tekh. Poluprovodn. 32, 192–199 (February 1998)     

Current-tunable lasers with a narrow emission line operating at 3.3 µm

Current-tunable diode lasers with narrow emission lines for laser spectroscopy in the 3.2–3.4 μm wavelength range are developed. The lasers, based on an InAsSb/InAsSbP double heterostructure, have a wide-stripe cavity. The wave number increases from 3030 to 3034 cm⁻¹ as the current is raised from 1.5 to 3 times the threshold value at 70 K, while the full width at half-maximum of the laser line decreases from 18 to 10 MHz. It is demonstrated that the linewidth is determined by fluctuations of the cavity resonance frequencies as a result of fluctuations in the concentration of nonequilibrium charge carriers. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 3, 2001, pp. 375–379. Original Russian Text Copyright ? 2001 by Imenkov, Kolchanova, Kubat, Moiseev, Civiš, Yakovlev.