A computer analysis of heterojunction and graded composition solar cells

The development and application of a variable composition solar cell computer analysis program is discussed, The basic device equations as used in the computer analysis are derived and the techniques used to model material parameter variations described. Finally, the results of the computer analysis of several AlxGa1-xAs and GaAs1-xPxsolar cell structures are presented along with a discussion of the effects of interface states and various composition and doping profiles on maximum solar cell efficiency. Interface states are found to be responsible for a severe reduction in the efficiency of GaAs1-xPxcells, but do not significantly affect the behavior of AlxGa1-xAs cells. The depth of the p-n junction below the wide bandgap window layer is a critical factor in determining the loss of minority carriers to interface recombination at the heterojunction.     

Electroluminescent source with luminance greater than 6 × 109cd/m2

GaAs1-xPxinjection lasers were found to emit 6440 Å radiation at an average output level of 2 × 10⁴W/cm²from each of the two Fabry-Perot facets.     

Temporal-line shift of pulsed GaAs1-xPxinjection laser diodes

The time dependence of the coherent emission wavelength shift of GaAs1-xPxlaser diodes during a flat-topped, 240 ns current pulse (20 ns rise time) was measured and found to agree reasonably well with calculations based on a simple thermal model of the diode which gives a square root of time dependence.     

Yellow—Green In1-xGaxP and In1-xGaxP1-zAszLED's and electron-beam-pumped lasers prepared by LPE and VPE

In1-xGAxPx(x= 0.6-0.7) and In1-xGaxAszP1-z(x= 0.7- 0.8 andz= 0-0.2) epitaxial structures for visible LED's and electron-beam-pumped lasers have been fabricated both by vapor-phase epitaxy (VPE) and by liquid-phase epitaxy (LPE). The crystal perfection and luminescent properties of layers grown on     

Single-diode end-pumped Nd:YAG laser

Experimental results on the first miniature Nd:YAG laser end-pumped by a single incoherent infrared-emitting diode are given. Laser operation was verified by measurements of relaxation oscillations, angular divergence, wavelength, and nonlinear output. In order to emit in the 0.81- μm pump band of Nd:YAG, the GaAs1-xPxdiode required cooling to 88 K (-185°C). Cooling the diode increased its efficiency while cooling the rod decreased the pump power necessary to reach threshold. With both laser mirrors highly reflecting at 1.06 μm, the maximum pulse powerP_{D}^{th}emitted by the diode at threshold was 6.2 ± 1.2 mW. The diode power required for laser threshold at room temperature was derived and compared with more recent experiments. Miniature size and long-lived solid-state components make this laser attractive for further development.     

Continuous room-temperature photopumped laser operation of visible-spectrum LPE In1-xGaxP1-zAsz(λ ~ 6700 Å)

A modified version of the standard step cooled LPE growth process is described for the growth of visible-spectrum In1-xGaxP1-zAszdouble-heterostructure (DH) lasers on GaAs1-yPy(y sim 0.30). The quality of the quaternary epitaxial layers is improved by growing a "thick" uniform active region (∼1000 Å) by means of a multiple-layer stack of "thin" (∼100 Å) layers. Each of the "thin" layers grown (cyclically) from an equilibrium melt in the short growth period of <100 ms does not make a transition to the diffusion-limited conditions employed in standard LPE growth processes. This method of liquid-phase epitaxy has been used to grow In1-xGaxP1-zAsz-In1-x'Gax'- P1-z'Asz'(x sim 0.87, z sim 0.44; x' sim 0.66, z' lsim 0.01) DH's that operate as photopumped CW 300 K lasers atlambda sim 6700Å.     

The effect of the material velocity—Field characteristic on the small-signal microwave impedance of punchthrough devices

The theoretical dependence of the small-signal impedance of microwave punchthrough diodes (BARITT) on the shape of the velocity-field characteristic is studied with the aid of a small-signal computer simulation and a simple analytical model, in an attempt to assess what semiconductor material might give the smallest negative Q factor. Four different types of velocity-field characteristics are considered showing, respectively, soft saturation (e.g., holes in Si), negative mobility (e.g., electrons in GaAs), constant mobility (e.g., electrons in GaP), and hard saturation (e.g., electrons in GaAs1-xPx). A clear picture emerges of the effect of the velocity-field characteristics on the microwave negative resistance with Si appearing to be the near optimum material; GaAs provides little if any negative resistance due to poor phasing of the injected ac space charge and the other two materials are intermediate.     

Continuous operation of visible-spectrum in1-xGaxP1-zAszlaser diodes (6280 Å, 77 K)

Stripe-geometry In1-xGaxP1-zAsz(x approx 0.84-0.86, z approx 0.38-0.42) double heterojunction laser diodes, grown by liquid phase epitaxy (LPE) on vapor phase epitaxy (VPE) GaAs1-yPysubstrates, are described that operate (CW, 77 K) in the visible atlambda approx 6280-6360Å with differential quantum efficiencyeta_{ext} sim 28percent and power output in the range 1-7 mW.     

A monolithic light-emitting-diode display

A monolithic 5 × 7 array of planar diffused p-n junctions in GaAs1-xPx(x≃0.38) has been built for a light-emitting diode (LED) alphanumeric readout. A character formed by this readout is 0.246 cm high and 0.170 cm wide. The monolithic chip has all p-n junctions, n-contacts, p-contacts, interconnections and terminal metallurgy on the epitaxial layer which represents a departure from the conventional methods of making LED arrays, namely wire bonding discrete chips with contacts on two sides in a hybrid configuration. Each LED in the array is connected to one of the terminals arranged around the periphery of the chip and individually addressed by direct current from a driver on a silicon control chip. For each character position in a display there is one monolithic LED chip and one monolithic silicon control chip solder joined to terminals on a glass plate and interconnected by Cr-Cu-Cr lines evaporated onto the glass substrate. The display is addressed by serial information provided from an ROM which is read into a 35-stage shift register on the control chip which controls the drivers. Thus with two standard parts, any N-character display can be fabricated with considerable reduction in handling since no discrete elements or wire bonds are used.     

Photoluminescence of ZnSxSe1-xepilayers and single crystals

Photoluminescence properties of ZnSxSe1-x(0 ≤ x ≤ 1) epilayers on GaP substrates and of corresponding ZnSxSe1-xsingle crystals are examined. The samples have been grown by vapor phase iodine transport. The blue emission bands of the single crystals are quenched in the epilayers. Comparison of the luminescence properties of ZnSe crystals grown by sublimation without iodine clearly shows that iodine creates deep recombination levels of high concentration. By simple sublimation without transporting agent and subsequent annealing in Ga and Zn deep emission bands are suppressed in favor of blue edge emission. Differences in the spectra obtained by the use of source materials from two manufacturers are assigned to unidentified background impurities.     

Temperature dependence of emission efficiency and lasing threshold in laser diodes

It is found that in GaAs1-xPxinjection lasers both the spontaneous emission efficiency η and the laser thresholdjvary exponentially with the temperatureT: eta = eta_{0} exp(-T/theta_{1})andj = j_{0} exp(T/theta_{2}). θ1and θ2are usually nearly equal and range between 50 and 110°K. The behavior of the external efficiency is correlated with the temperature dependence of the absorption along the propagation path of the radiation: as the temperature increases, the exponential absorption edge shifts to lower energies faster than the emission peak. The difference between the two shifts is nearly linear with temperature. The effect of self-absorption on external efficiency was tested by measuring the light emitted transversely to thep-njunction through a layer of constant thickness. With such a geometry, it is shown that the efficiency should vary asexp [-A exp (T/theta)]. The experiment agrees with this prediction.     

Laser oscillation at 3-4 µm optically pumped InAs1-x-ySbxPy

Quaternary InAs1-x-ySbxPy) and ternary InAs1-xSbxhave been grown on InAs substrates using liquid phase epitaxy. The quaternary layers are lattice matched to InAs fory = 2.2x, and have bandgaps greater than InAs. The ternary layers have lower bandgaps than InAs, but have larger lattice constants. Reasonable quality growth has been obtained up tox = 0.15where the luminescence peak is shifted to ∼4 from 3.1 μm and the lattice mismatch is ∼0.01. Optically pumped laser emission at 3.1 μm has been observed from 77 to 100 °K using an InAs active layer with InAs1-x-ySbxPyclading layers. Laser emission at 3.9 μm has been observed from 77 to 125 °K using a InAs1-xSbxactive layer, withxsim 0.13, and InAs cladding layers. Laser emission from intermediate ternary compositions has also been observed.     

Avalanche gain in GexSi1-x/Si infrared waveguide detectors

Avalanche gain in GexSi1-x/Si heterostructures photodiodes has been measured for the first time. Absorption of infrared radiation occurs in a GexSi1-x/Si strained-layer superlattice (SLS) which serves as a waveguide core, and the avalanche multiplication takes place in one of the Si-cladding layers. Multiplication factors as high as 50 have been obtained for a 1.1-µm wavelength response (x = 0.2). The external absolute sensitivity operating at a multiplication of 10 is 1.1 A/W at 1.3 µm for an uncoated device.     

Applications of ternary III-V compounds to high-speed microwave modulation

High-speed microwave modulation is obtained by exploiting the intervalley electron transfer time in certain ternary III-V compounds. Saturation of the net carrier drift velocity due to intervalley electron transfer at threshold fields below 3 kV/cm is reported in GaA1-xPxand AlxGa1-xAs for x = 0.315 ± 0.01 and 0.38 ± 0.02, respectively. The equivalent circuit of a bulk device fabricated from such material is derived and verified through small-signal RF measurements which, in addition, directly yield the high-field differential mobility and electron diffusion coefficient of the material. Operated as a microwave switch, isolation levels in excess of 20 dB with 4-dB insertion loss are reported at X-band with GaAS1-xPxdevices. It is shown that contact resistance can present serious limitations, although refinements in contacting technology should result in improved performance, making devices useable through millimeter-wave frequencies. Switching speed, the measurement of which is limited by laboratory pulse generation and detection capabilities, is estimated to be well under 200 ps and a theoretical limit of 20 ps has been predicted. The importance of this work lies in the fact that these switching speeds can be obtained with no sacrifice of incident RF power-handling capability, since there is no minority-carrier charge storage in these majority-carrier devices. In addition, the devices are stable at their transit-time frequency due to the absence of negative differential mobility in the material.     

Diffusion and recombination 1/f noise in long n+-p Hg1—xCdxTe diodes

1/f noise in long n⁺-p Hg1-xCdxTe diodes with x = 0.30 is studied at 193 K. The 1/f noise is considered to be generated by diffusion and recombination fluctuations. A distinction is made between cases a (all minority carriers contribute to the 1/f noise) and b (only the excess minority carriers contribute to the 1/f noise). Measurements on long nonplanar diodes show that case a is valid, indicating that all minority carriers contribute equally to the 1/f noise; this should be valid for any long-junction device in which the current flow is by diffusion and recombination of minority carriers. The lifetime τnof the electrons in the p-region is measured by the input impedance method, and the Hooge parameter αHof the device is evaluated. τnis of the order of 10^-6to 10^-7s and depends somewhat on bias. Near zero bias αHis of the order of 5 × 10^-3in close agreement with Handel's coherent state 1/f noise theory, which yields αH= 4.6 × 10^-3. Due to the nonplanar geometry of the studied diodes, the measurement of τnis not always equally reliable. Larger values of τnare accompanied by larger values of αH, because the noise measurements give αH/τn, and its value practically independent of bias. We also evaluated τnby putting αH= 4.6 × 10^-3; the τnvalues are then much closer and agree rather well with Honeywell lifetime tables. Preliminary measurements at 113 K also indicate coherent state 1/f noise, whereas data at 273 K give αH= 5 × 10⁵, in agreement with the Umklapp 1/f noise theory.     

High value implanted resistors for microcircuits

Boron ion implantation has been used to fabricate high sheet resistance p-type junction resistors in silicon substrates. Thermally grown SiO2and conventional photolithography were employed to define the resistor geometries. Ion doses in the range 0.5 × 10¹³to 10 × 10¹³ions/cm²with energies ranging from 30 to 55 keV followed by anneal at 950°C were used. The temperature coefficient of resistance (TCR), voltage coefficient of resistance (VCR), junction Characteristics, and noise level of these resistors have been studied for sheet resistances ρx, from 0.8 to 11 kΩ/square. Over this range of sheet resistances the TCR increases smoothly from approximately 800 to 4000 PPM/°C with the lower TCR corresponding to the lower sheet resistance. For 3 kΩ square implanted resistors, the variation of resistance with temperature closely matches that found for a standard boron base and resistor (B&R) diffusion having a sheet resistance of 140 Ω/square. The junction leakage and the noise level of the implanted resistors can be made comparable to that obtained for diffused resistors. The implanted resistor exhibits a positive VCR, which increases with increasing sheet resistance as a result of depletion-layer pinch-off action from the substrate. Details of the implant conditions and process control are discussed. Experimental results demonstrating the compatibility of the resistor implantation process with microcircuits using low current, high β diffused bipolar transistors are presented.     

Symmetric-gain, zero-offset, self-aligned, and refractory-contact double HBT's

We report double-heterostructure bipolar transistors using refractory emitter and base contacts (WSix/InAs for n-type, and W with rapid diffusion of Zn for p-type) and self-aligned implantation. These devices have near-symmetric gain (∼ 100) across a fairly large current range (∼ four decades), negligible offset voltage, and a weak current gain dependence on device size. This results from suppression of surface recombination obtained using a P-GaAlAs electron barrier at the extrinsic-base surface. We also demonstrate use of a technique for deconvolving various recombination mechanisms. The major gain-limiting mechanisms in these devices are shown to be (a) residual damage from ion implantation and its effects on base transport factor, and (b) Shockley-Read-Hall recombination in the depletion regions and its effect on injection efficiency. Devices similar to the above showed gains approaching theoretical values of ∼ 1000 at high currents, and greater than 10 at picoampere currents when the processing was designed to obtain high lifetime in the implanted region.     

Improved MODFET performance through ion implantation in the gate region

AlxGa1-xAs/GaAs modulation-doped FET's (MODFET's) are reported which utilize for the first time a shallow low-dose p-type implantation under the gate region to improve the source-drain breakdown voltage, and the gate-channel forward turn-on and reverse breakdown voltages. Extremely low output conductances of less than 0.2 mS/mm are also obtained and the open-circuit dc voltage gains gm/gdexceed 250. Such improvements are important for achieving high RF power in microwave device applications, and have similarly significant implications for digital circuitry.     

Transmission electron microscope study of oxidation-induced

Boron-implanted silicon devices have been shown to be affected by the ambient conditions during implantation, anneal and drive-in diffusion. The presence of oxygen appears to be of major significance. This study was undertaken to observe directly by transmission electron microscopy defects existing in silicon implanted and annealed under varying conditions which might account for changes in device characteristics. Boron ions at doses of 10¹³ to 10¹⁶ B⁺ /cm² were implanted at 180KeV into silicon through 2000Å. thick oxide layers formed by steam at 950°C or dry oxygen at 1150°C. After implantation specimens were annealed to 950° or 1000°C for 1/2 hour periods in dry nitrogen or argon. At 950°C precipitate colonies were found in the steam-oxidized specimens and were also present in those annealed to 1000 C, whereas, to date, no such colonies were found on the specimens which had been oxidized in dry oxygen at 1150°C. Energy dispersive x-ray analysis in an electron microscope shows that the particles contain copper and are possibly a copper suicide. Precipitation occurs only along dislocations formed on annealing the ion-damaged specimens Colonies were found as deep as 0.5μm below the oxide film. Tentatively, it is proposed that precipitation of the copper is observed only in specimens implanted through the steam formed oxide because of an increase in the number of silicon oxide nuclei due to the faster rate of oxidation. These nuclei form at dislocations and become nucleation sites for copper precipitation. The source of the copper contamination has not yet been determined.     

Implantation of sodium ions into germanium

The donor properties of Na atoms introduced by ion implantation into p-Ge with the resistivity 20–40 Ω cm are established for the first time. Na profiles implanted into Ge (the energies 70 and 77 keV and the doses (0.8, 3, 30) × 10¹⁴ cm⁻²) are studied. The doses and annealing temperatures at which the thermoprobe detects n-type conductivity on the sample surface are established. After implantation, the profiles exhibit an extended tail. The depth of the concentration maximum is in good agreement with the calculated mean projected range of Na ions R _p . Annealing for 30 min at temperatures of 250–700°C brings about a redistribution of Na atoms with the formation of segregation peaks at a depth, which is dependent on the ion dose, and is accompanied by the diffusion of Na atoms to the surface with subsequent evaporation. After annealing at 700°C less than 7% of the implanted ions remain in the matrix. The shape of the profile tail portions measured after annealing at temperatures 300–400°C is indicative of the diffusion of a small fraction of Na atoms into the depth of the sample.