1:30PM, U1
"The MOCVD Growth of Novel Materials for Use in Infrared Emitters:" A.A. ALLERMAN, R.M. Biefeld, S.R. Kurtz, Sandia National Laboratory, Dept. 1126, MS 0601, PO Box 5800, Albuquerque, NM 87185-0601
We are developing mid-infrared (3-6 um) lasers and LED's for use in chemical sensor systems and infrared countermeasures. As-rich, InAsSb heterostructures display unique electronic properties that are beneficial to the performance of these midwave infrared emitters. We have demonstrated an electrically injected, 3.5 um laser with a strained and nominally "dislocation free," active region. The laser was grown by metal-organic chemical vapor deposition (MOCVD). The active region consisted of 10, biaxially compressed quantum wells of InAs0.94Sb0.06. InPSb cladding layers were used for optical confinement. To improve the operating characteristics of our lasers we are exploring the growth of Al(As)Sb by MOCVD as confinement layers for these devices. Although devices using Al(As)Sb have been prepared by molecular beam epitaxy, there have been no reports to date of their successful use in devices when prepared by MOCVD. AlSb and AlAsxSb1-x epitaxial films grown by metal-organic chemical vapor deposition were successfully doped p- or n-type using diethylzinc or tetraethyltin, respectively. We have prepared AlSb and AlAsSb using trimethylamine and ethyldimethylamine alane, arsine, and triethylantimony. We examined the growth of AlAsSb using temperatures of 500 to 600deg.C, pressures of 65 to 630 torr, V/III ratios of 1-17, and growth rates of 0.3 to 2.7 um/hour in a horizontal quartz reactor. SIMS showed C and O levels below 2 x1018 cm-3 and 6x1018 cm-3 respectively for undoped AlSb. Similar levels of O were found in AlAs0.16Sb0.84 films but C levels were an order of magnitude less in undoped and Sn-doped AlAs0.16Sb0.84 films. Hall measurements of AlAs0.16Sb0.84 showed hole concentrations between 1x1018 cm-3 for Sn-doped material. Details of the growth procedure and the characterization results on these materials will be presented. We have grown pseudomorphic InAs/InAsSb quantum well active regions on AlAsSb cladding layers for evaluation by photoluminescence and for use in photo-pumping (lasing) experiments. Photoluminescence has been observed up to 300 K. The applicability of these materials to infrared devices will be discussed.
*This work was supported by the US DOE under Contract DE-AC04-94AL85000.
1:50PM, U2
"Nearly Room-Temperature Type-II Quantum-Well Lasers at 4 um:" CHIH-HSIANG LIN, P.C. Chang, S.J. Murry, D. Zhang, Rui Q. Yang, S.S. Pei, Space Vacuum Epitaxy Center, University of Houston, Houston, TX 77204-5507; J.I. Malin, J.R. Meyer, C.L. Felix, J.R. Lindle, L. Goldberg, C.A. Hoffman, F.J. Bartoni, Code 5600, Naval Research Laboratory, Washington, D.C. 20375
We have studied mid-infrared lasers based on type-II InAs/InGaSb/InAs/InGaAlSb quantum wells (QWs). Here, we will report optically pumped lasers emitting at 3.88 - 4.06 um at temperatures from 80 K to 285 K. The active region was composed of 35 periods of undoped InAs/In0.3Ga0.7Sb/InAs/AlSb (21Å/31Å/21Å/43Å) QWs, which were lattice-matched to the AlSb cladding layers. These type-II QW lasers utilize interband transitions between an electron state in the conduction band of InAs layers and a hole state in the valence band of InGaSb layer. It has been predicted theoretically and demonstrated experimentally that the Auger non-radiative recombination can be significantly suppressed in this type-II QW structure. Therefore, the radiative efficiency in these type-II interband transitions will be much higher than that of type-I QW lasers. The AlSb cladding layers provide both good optical and electrical confinement.
The characteristic temperature To of these lasers was 96 K for temperatures less than 170 K, and was 35 K for temperatures between 170 K and 270 K. These lasers demonstrated the record-high characteristic temperature To for wavelengths beyond 3.2 um and the record-high maximum lasing temperature Tmax for wavelengths beyond 2.9 um. For temperatures less than 170 K, the recombination lifetime at threshold [[tau]]th had a nearly constant value of 2 ns, suggesting that Shockley-Read recombination dominated in that range, and was responsible for the larger To of 96 K. For temperatures higher than 170 K, [[tau]]th decreased in a manner consistent with Auger recombination. At 81 K, the maximum peak output power per facet with a 500 um cavity length was 650 mW, and the threshold pumping power was 3 kW/cm2. While at 170 K, the maximum output power per facet was about 200 mW. In this talk, we will discuss the device design, MBE growth and characterization of these type-II QW lasers.
2:10PM, U3
"Mid-Infrared Spectroscopy of Carrier Dynamics in Broken-Gap Superlattice Quantum Wells:" THOMAS F. BOGGESS, S.W. McCahon, S,A. Anson, J.T. Olesberg, M.E. Flatté, D.H. Chow, T.C. Hasenberg, C.H. Grein, Department of Physics & Astronomy, University of Iowa, 100 IATL, Iowa City, IA 52242; Hughes Research Laboratories, 3011 Malibu Canyon Rd., Malibu, CA 90265; Physics Department, University of Illinois at Chicago, Chicago, IL 60607
We have used all-optical techniques to generate and subsequently measure the dynamics of dense electron-hole distributions in GaInSb/InAs broken-gap superlattice multiple quantum wells, structures that are of interest for mid-IR lasers and detectors. A typical structure, grown by molecular beam epitaxy on an undoped GaSb substrate, contains 225Å quantum wells, each a Type II GaInSb/InAs superlattice, consisting of five 33 Å Ga0.75In0.25Sb layers and four 15 Å InAs layers, separated with Al0.2Ga0.8Sb barriers. Time-resolved differential transmission measurements were performed at room temperature using 140 fs pump pulses from a mode-locked Ti:sapphire laser operating at 840 nm and 170 fs probe pulses from a synchronously-pumped optical parametric oscillator, which is tunable near the ~ 3.8 um band gap of the samples.
The measurements reveal a rapid saturation of the band edge absorption, which reaches a quasi-steady-state within ~ 10 ps. This behavior is associated with the filling of band edge states as the initially hot carriers are captured in the wells. We analyze the quasi-steady-state saturation using a band-filling model and a density-independent intersubband absorption cross section. On a nanosecond time scale, the saturated absorption recovers to its unperturbed value as the electron-hole pairs recombine. We fit this behavior using the aforementioned saturation model and a rate equation for the carrier recombination dynamics, which are dominated by Shockley-Read-Hall and Auger processes. The band structure parameters required for this analysis are obtained using a finite-superlattice K.p model. A comparison between measured and calculated values for the Auger coefficient and intersubband cross section will be discussed.
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a) Hughes Research Laboratories, 3011 Malibu Canyon Rd., Malibu, CA 90265
b) Physics Department, University of Illinois at Chicago, Chicago, IL 60607
2:30PM, U4
"MBE Growth of InSb/InAlSb Structures for Mid IR LEDs:" A.D. JOHNSON, Defence Research Agency, St. Andrew's Rd, Malvern, Worcestershire, WR14 3PS, UK
There is an increasing requirement for efficient, small, cheap and reliable mid and long wavelength infrared sources for application such as gas sensing. Semiconductor light emitting diodes offer a solution to this requirement, with the potential for very low cost in high volumes as shown by the ubiquitous visible and near infrared devices. In this paper we describe the molecular beam epitaxy (MBE) growth of InSb/InAlSb based LEDs operating in the wavelength range 4 mm to 7 um at room temperature.
The InSb diodes are grown by MBE on to InSb(001) substrates. They comprise four or five layers, including a pseudomorphic In1-xAlxSb region, to ensure effective confinement of both carrier types in the same part of the structure and so maximise the radiative efficiency. The effects on device performance of several growth parameters have been studied and optimized, including substrate temperature and V/III ratio. We also present the effects of growth using an Sb dimer source, which has been shown to give increased luminescent intensity in other antimonide material systems [1].
The emission peaks at 5.5 um to 5.8 um, depending on injection level, and has the width expected from the thermal distribution of the carriers. Radiative efficiencies close to the theoretical value of 5% are achieved at low injection levels. The output power has a sub-linear dependence on current, which is within 10% of the trend expected from the relative magnitudes of the radiative and non-radiative (Auger) recombination rates at elevated carrier densities. A power density of approximately 10 mW/cm2 has been achieved under pulsed high injection conditions.
The structure of the LED ensures that when it is reverse biased a large proportion of electrons and holes are removed from the active region of the device through the processes of minority carrier extraction and exclusion. This reduces both the radiative and Auger recombination rates below their equilibrium values. The suppression of the radiative recombination leads to so called "negative luminescence", where the device emits less radiation than its surroundings and appears to be cold. For many applications this variation below equilibrium can be used as effectively to form an IR source as the normal positive luminescence.
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[1] R. H. Miles, D. H. Chow, T. C. Hasenberg, A. R. Kost, and Y. H. Zhang, "Narrow Gap Semiconductors 1995", Inst. Phys. Conf. Ser. No. 144, pp 31-35, 1995.
2:50PM, U5
"Growth of InSb/AlxIn1-xSb Strained Layer Structures by Molecular Beam Epitaxy:" M.B. SANTOS, W.K. Liu, Weiluan Ma, Xuemei Zhang, K. Goldammer, Department of Physics and Astronomy and Laboratory for Electronic Properties of Materials, University of Oklahoma, Norman, OK 73019; R.J. Hauenstein, M.L. O'Steen, Department of Physics, Oklahoma State University, Stillwater, OK 74078
InSb has the smallest energy gap and highest electron mobility of all binary III-V compounds. These characteristics have led to applications of InSb thin films in infrared detection and emission and in position sensing. The flexibility afforded by an electron barrier material, such as AlxIn1-xSb, can lead to improved device performance. For example, room temperature operation of InSb infrared LEDs has recently been realized through the incorporation of a 200Å Al0.17In0.83Sb barrier that increases the rate of radiative recombination in the p-region [1].
To further explore the potential of these materials, we have grown a variety of InSb/AlxIn1-xSb (x<0.20) strained layer structures by molecular beam epitaxy. Since AlSb has a much larger energy gap than InSb, only a small Al concentration is required in AlxIn1-xSb barrier layers. High resolution x-ray diffraction measurements demonstrate that high quality epilayers (246 arcsec FWHM for a 3 um Al0.10In0.90Sb layer, for example) can be achieved through use of buffer layers that reduce the effects of the ~14% lattice mismatch with the GaAs(100) substrate. InSb/AlxIn1-xSb interface sharpness is demonstrated through multiple satellite peaks from superlattice structures.
By selectively doping AlxIn1-xSb barrier layers with Si, we have realized two-dimensional electron systems in InSb quantum wells [2]. Hall effect measurements indicate that electron densities between 0.9x1011 cm-2 and 4x1011 cm-2 with mobility as high as 97,000 cm2/Vs at 4.2K have been achieved. A decrease in the electron density and an increase in the electron mobility are observed when the distance between the quantum well and the dopants is increased. The effects of quantum-well strain and MBE growth conditions on electrical properties are also investigated.
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[1] T. Ashley, C. T. Elliot, N. T. Gordon, R. S. Hall, A. D. Johnson, and G. J.
Pryce, Appl. Phys. Lett. 64, 2463 (1994).
[2] W. K. Liu, Xuemei Zhang, Weiluan Ma, J. Winesett, and M. B. Santos, J. Vac.
Sci. Technol. B, in press.
3:30PM, U6
"Infrared Light Sources for Wavelenghts >2um:" A. KRIER, Advanced Materials & Photonics Group, School of Physics & Chemistry, Lancaster University, Lancaster, LA1 4YB, UK
There is increasing interest in mid-infrared optoelectronic materials and devices due to their huge potential in applications such as: environmental monitoring, thermal imaging, remote sensing, telecommunications and laser surgery, etc. In particular, at the moment there is perceived to be a growing requirement for efficient, small, low-cost and reliable mid-infrared light sources for use in the next generation of optical gas sensor instrumentation. Semiconductor light emitting diodes have the potential to fulfill this need since they offer the instrument designer the capability of portable solid state sensor development with no moving parts and reduced cost of ownership derived from lower on-site maintenance.
This paper describes a range of mid-infrared light emitting diodes which have been developed using liquid phase epitaxial growth of bulk ternary and quaternary III-V alloys. Room temperature electroluminescence has been obtained at several of the key wavelengths for optical gas monitoring, including 3.3 um (methane) and 4.2 um (carbon dioxide). Some of the LPE growth parameters and LED operating characteristics are presented for InAsSbP/InGaAs and InAsSb/GaSb p-i-n LED structures.
LPE growth at thermal equilibrium produces material of the highest quantum efficiency and with few point defects. The results indicate that these devices are operating up to the theoretical limit of internal quantum efficiency imposed by non-radiative Auger quenching in these narrow gap semiconductors. Furthermore, it appears that the performance of the bulk LPE material is so far still superior to low-dimensional emitters grown either by MBE or MOVPE for LED applications.
3:50PM, U7
"Suppression of Auger Recombination in Arsenic-Rich InAs1-xSbx Strained Layer Superlattices at 300 K:" C.R. PIDGEON, C.M. Ciesla, B.N. Murdin, R.A. Stradling, C.C. Phillips, M. Livingstone, I. Galbraith, D.A. Jaroszynski, C.J.M. Langerak, Physics Department, Heriot Watt University, Edinburgh EH14 4AS, UK; FOM Institute for Plasma Physics, "Rijnhuizen", PO Box 1207, 3420 BE Nieuwegein, The Netherlands; Physics Department, Imperial College, London SW7 2AZ, UK; LURE, Batiment 209d, Universite de Paris-Sud, 91405 Orsay Cedex, France
Room temperature pump-probe transmission experiments have been performed on an arsenic-rich InAs/InAs1-xSbx strained layer superlattice (SLS) above the fundamental absorption edge near ten microns, using a ps far-infrared free electron laser. Measurements show complete bleaching at the excitation frequency, with recovery times which are found to be strongly dependent on the pump photon energy. At high excited carrier densities, corresponding to high photon energy and interband absorption coefficient, the recombination is dominated by Auger processes. A direct comparison with identical measurements on epilayers of InSb, of comparable room temperature bandgap, shows that the Auger processes have been substantially suppressed in the superlattice case as a result of both the quantum confinement and strain splitting in the SLS structure. In the non-degenerate regime, where the Auger lifetime scales as [[tau]]= C1 N, a value of C1 some 100 times smaller is obtained for the SLS structure. The results have been interpreted in terms of an 8x8 k.p energy band calculation, including the full dispersion for both k in-plane and k parallel to the growth direction. This is the strongest example of room temperature Auger suppression observed to date for these long wavelength SLS alloy compositions and implies that these SLS materials may be attractive for applications as room temperature mid-IR diode lasers.
4:10PM, U8
"Assessment of InAs/GaInSb and InAs/InAsSb Heterostructures for Infrared Detection and Emission:" F. FUCHS, J. Schmitz, N. Herres, J. Wagner, G. Tränkle, P. Koidl, Fraunhofer-Institut für Angewandte Festk/rperphysik, Tullastr. 72, D-79108 Freiburg, Germany; Y.H. Zhang, Hughes Research Laboratories, Malibu, CA 90265
Antimonide based III-V heterostructures are of current interest because of their potential in infrared optoelectronic applications. We present a detailed study of the structural and optical properties of InAs/GaSb SLs with special emphasis on the type of interface bonds formed. The samples were grown by MBE on GaAs substrates, using GaSb buffers for strain accommodation. Structural characterization has been carried out using X-ray diffraction space mapping, Fourier transform far infrared spectroscopy, and Raman scattering. The electronic properties were assessed by infrared photoluminescence and photocurrent spectroscopy. The results obtained on binary InAs/GaSb SLs are compared with InAs/GaInSb SLs. Besides the expected red-shift of the onset of the photoresponse, at photon energies around 0.2 eV above the effective band gap a sign-reversal of the in-plane photocurrent is observed in the ternary SLs. If the photon energy exceeds the hh-lh splitting of the valence bands light holes with mobilities comparable to the electron mobility are photogenerated.
InAs/InAsSb heterostructures grown by modulated-molecular beam epitaxy have been demonstrated to be promising candidates for mid infrared laser devices. The band gap of this materials system is strongly affected by composition induced shifts and the strain introduced via the ternary InAsSb layers. In addition, depending on the growth conditions and composition, ordering can also influence the electronic band structure. Using Fourier transform photoluminescence excitation spectroscopy the precise location of the optical band gap has been measured. The results provide evidence for the type II band alignment of this materials system.
4:30PM, U9
"Band Alignments & Offsets in In(As,Sb)/InAs Superlattices:" R.A. STRADLING, Y.B. Li, P.J. Tang, C.M. Ciesla, M. Livingstone, M. Pullin, I. Galbraith, C.R. Pidgeon, C. C. Phillips, W.T. Yuen, Department of Physics, Imperial College, London, SW7 2BZ, UK; Department of Physics, Heriot-Watt University, Riccarton, Edinburgh, UK EH14 4AS
The band alignments and offsets are investigated for MBE grown InAs/InAs1-xSbx superlattices of various periods and compositions (x <= 0.4). Photoluminescence, by ourselves and also by other groups, shows that the photon energy emitted falls extremely rapidly with increasing x with the effective band gap of the superlattice decreasing from 0.4 eV for x = 0 to 0.1 eV for x = 0.4 .
The reason for this rapid narrowing of the superlattice energy gap has been controversial with different groups suggesting two conflicting schemes; either extreme "type II" band alignments with the valence band offsets changing rapidly with x (in which case the photon emission is spatially indirect) or alternatively a spatially direct band gap in which the band gap of the alloy is anomalously narrowed by microstructural effects such as atomic ordering.
Magnetoabsorption experiments are used here to determine transition energies and in-plane reduced masses. Using a k.p band structure calculation which takes account of non-parabolicity, valence subband mixing and strain, we attempt to fit experimental transition energies and reduced masses using three distinct models for the band alignments: i) type I, ii) type IIA with the alloy conduction band lying lower than InAs, and iii) type IIB where the electrons are located in the InAs conduction band and the holes in the alloy. The type I model cannot fit the measured reduced masses and requires an implausibly large narrowing of the band gap (~80 meV) compared with literature values and values for the bulk alloy measured by ourselves. Type IIB provides a poor fit for both the measured transition energies and the reduced masses.
A good fit is obtained only for type IIA alignment. The fractional conduction band offset parameter, Qo, which for a type II heterostructure can be greater than one, is found to be 1.62 +/- 0.06 and independent of x for x <= 0.4.
4:50PM, U10
"Magneto-Optical Spectroscopy, Bandstructure, and Lasing Characteristics of Heterostructures with Biaxially Compressed InAsSb Layers:" S.R. KURTZ, R.M. Biefeld, A.A. Allerman, H.J. Hjalmarson, Sandia National Laboratories, Department 1126, MS 0601, PO Box 5800, Albuquerque, NM 87185-0601
With the prospect of reduced Auger rates in compressively strained heterostructures, midwave infrared (3-5 um) lasers with strained InAsSb active regions are attracting renewed interest. In order to understand the electronic properties of these novel heterostructures, we present magneto-optical spectra and the corresponding bandstructure for heterostructures with biaxially compressed layers of InAs1-xSbx (x~0.1). The properties of injection lasers utilizing these heterostructures as active regions are discussed.
InAsSb heterostructures and devices were grown by MOCVD. High resolution magneto-photoconductive spectra were obtained for an InAs1-xSbx / In1-xGaxAs strained-layer superlattice (SLS) and a superlattice constituent, InAs1-xSbx alloy (x~0.1). Transition energies and reduced masses measured previously through magneto-photoluminescence were observed in the magneto-phototconductivity spectra, as well as new transitions revealing information about excited states. The SLS displayed the effect of biaxial strain with the lowest energy hole state being |3/2,+/-3/2> of InAsSb, characterized by a light in-plane hole effective mass. At 30 meV higher energy, an excited state was observed in the SLS with higher in-plane mass, indicating |3/2,+/-1/2> character. Observation of the lh ( |3/2,+/-1/2>) state is in agreement with the type I band offset model for this heterostructure and contradicts large type II band offset models proposed to explain long wavelength anomalies of InAsSb heterostructures.
Pseudomorphic, compressively strained InAsSb multiple quantum well injection lasers, emitting at 3.5-3.6 um were constructed. Optical confinement was achieved with InPSb layers, lattice matched to the InAs substrate. Lasing was observed in gain-guided stripes from 14 K through 135 K with pulsed operation. With immersion in liquid nitrogen, CW operation was observed at 77 K. Typically, the 77 K threshold current was 250 A/cm2 for a 40 um stripe. Under pulsed operation, the characteristic temperature of these lasers is 33 K. Calculations of Auger-1 rates indicate that the characteristic temperature is limited by the low energy lh state observed in magneto-optical studies.
*This work was supported by the US DOE under contract DE-AC04-94AL85000.
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