Role of substrate in electrical properties of GaAs implanted layers

The variation of implantation efficiency in different semi-insulating GaAs materials is shown to be correlated with he purity of the starting substrate, i.e. its Cr concentration and more especially its value of ND?NA (shallow levels). This last value appears to be a key parameter in the electrical properties of implanted layers.     

Effects of channelling on the electrical properties of donor implanted GaAs

We have measured the electrical effects of implanting tin or tellurium ions into GaAs in random and channelling directions. Channelled implants produced higher electrical activities and broader electron concentration profiles than did random direction implants. To avoid channelling, samples should be carefully misaligned from a low-index direction for implant energies of less than about 100 keV, while implantations at higher energies require less stringent control of the implant direction.     

Electrical profiles from zinc implanted GaAs

Hole concentration and mobility profiles have been measured for 1015Zn+/cm2 implanted into GaAs at room temperature. After annealing at 800°C the peak hole concentration for a 60 keV implant was in excess of 1019 holes/cm3 and decreased with increasing energy up to 450 KeV.     

Pulsed laser annealing of zinc implanted GaAs

Samples of n type GaAs implanted with 1015Zn+/cm2 at room temperature were irradiated with a ruby laser of pulse length 0.8 ms. For samples coated with Si3N4 a laser energy of 1.5?2.5 J/cm2 produced electrical activity of 40?50% of the implanted dose. Peak hole concentrations up to about 7 × 1019 cm?3 were measured. Uncoated samples irradiated with similar laser energies were not electrically active.     

Reversible electrical properties of LEC GaAs

Undoped, low-pressure, liquid-encapsulated Czochralski GaAs can be reversibly changed from conducting (ρ ∼ 1Ω-cm) to semi-insulating (ρ ∼ 10⁷Ω-cm) by either slow or fast cooling, respectively, after a 5 hr, 950° C soak in an evacuated quartz ampoule. The semi-insulating wafers are very uniform and lead to tight threshold-voltage control in direct-implant MESFET’s. We have studied crystals in both states by temperature-dependent Hall effect, photoluminescence, IR absorption, mass spectroscopy, and DLTS. It is shown that donor and acceptor concentrations are typically more than an order of magnitude greater than the C and Si concentrations, which are both less than 3 × 10¹⁴ cm⁻³. The EL2 concentration remains relatively constant at about 1.0 × 10¹⁶ cm⁻³. Thus, the normal EL2-Si-C compensation model does not apply. The most likely explanation for the reversibility involves a delicate balance between native-defect donors and acceptors in equilibrium at 950° C, but with the donors dominating after a slow cool, and the acceptors after a fast cool. A consistent model includes a dominant donor at E_c 0.13eV, probably V_As – As_Ga, and a dominant acceptor at E_v + 0.07eV, probably V_Ga Ga_As. In this model, vacancy motion is very important during the slow cool. Such processes must be strongly considered in the growth of bulk, high-purity GaAs.     

Electron beam annealing of zinc implanted GaAs to control profile broadening

Highly doped P+ layers have been obtained by using multiply scanned electron beam annealing. The diffusion of the zinc was controllable at temperatures above 900°C if anneal times were less than 3 s.     

The electrical properties of zinc diffused indium phosphide

The electrical properties of p-type layers of InP, formed by the diffusion of zinc into n-type material, are studied. A wide range of diffusion conditions are used and both homogeneously doped specimens and those containing a zinc atom concentration gradient are produced. The impurity atom distribution of the diffused crystals is characterised by radiotracer analysis. p?n junction measurements on non-homogeneously doped specimens indicate that the number of zinc atoms in a diffused layer is much greater than the number of shallow acceptors. This non-correspondence of atom and carrier concentrations is confirmed by four point resistivity, Hall effect and capacitance-voltage measurements. The first two of these techniques are used to produce carrier concentration profiles which are compared with corresponding radio-tracer profiles. The carrier profiles are achieved by both serial sectioning and multiple specimen techniques. A gold probe point contacting procedure is developed for the Hall Effect measurements from which a plot of carrier mobility versus carrier concentration, in the range 5 × 10¹⁸ – 5 × 10¹⁹cm^?3, is produced for p-type InP.     

Measurement of electrical properties of semi-insulating GaAs single crystals

Semi-insulating GaAs is becoming a more and more important material for the coming multimedia era in the region of communications, such as cellular phones, PHS, wireless LAN, satellite communications, digital ICs for workstations and personal computers.     

Effects of annealing on electrical properties of Se-implanted GaAs

Observes for the first time reversible changes in the sheet carrier concentration of rapid thermally annealed Se-implanted GaAs samples during subsequent heat treatments. The electrical profiles of the Se-implanted samples have also been modified during the annealing processes. These modifications can lead to significant degradation in the performance of devices.<>     

electrical properties of the proton-irradiated semi-insulating GaAs:Cr

The n-p conversion of the conduction type and a decrease in resistivity to 10² Ω cm at 300 K were revealed upon proton irradiation (5 MeV, 300 K, D≈2×10¹⁷ cm^?2) of semi-insulating GaAs:Cr (ρ≈(3–4)×10⁸ Ω cm). Temperature dependences of ρ for heavily irradiated samples indicate a hopping conduction in the temperature range of 400–120 K, with the transition to the conduction with variable-range hopping at T≤120 K. The effects of electronic switching were found in low-resistivity proton-irradiated GaAs:Cr at about 20 K. The isochronous annealing of radiation defects in the temperature range of 20–750°C was investigated.     

Electrical,Rutherford backscattering and transmission electron microscopy studies of furnace annealed zinc implanted GaAs

Electrical, Rutherford backscattering and transmission electron microscopy measurements have been carried out on GaAs samples implanted with 150 keV, 1.10¹⁵ zinc ions/cm² and furnace annealed in the temperature range from room temperature to 900°C. A correlation between three types of measurement technique was established and four distinct annealing stages have been identified. For perfect recrystallization and maximum electrical activation an annealing temperature of 900°C is required. The maximum peak hole concentration was in the range 1–2.10¹⁹ holes/cm³.     

Effect of GaAs surface pretreatment on electrical properties of MBE-ZnSe/GaAs substrate interfaces

Interface properties of MBE-grown ZnSe/GaAs substrate systems formed on variously pretreated GaAs surfaces, which include standard chemically etched (5H₂SO₄:1H₂O₂: 1H₂O), (NH₄)₂S_x-, NH₄I-, and HF-pretreated surfaces, are investigated by capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements. A HF-pretreated and annealed ZnSe/p-GaAs sample showed marked reduction of interface state density, N_ss, with N_ss,_min below 4 x 10¹¹cm^-2 eV^-1 near E_c- E_FS= 1.0 eV. The value is about one order of magnitude smaller than that of the standard chemically etched interface, and comparable to (NH₄)₂S_x- pretreated interface. Nevertheless, C-V characteristics of ZnSe/nGaAs samples, which were measured for the first time, indicate that interface Fermi level, E_FS, is not completely unpinned due to the interface states located above the midgap. A consistent result was obtained by DLTS method in determining E_FS position. The influence of N_ss distribution on vertical current conduction is also analyzed. It is found that U-shaped interface states with N_ss(E) > 1 x 10¹³ cm^-2 eV^-1 above the midgap may cause an excess voltage drop larger than a few volts at the interface.     

Ion implanted GaAs bipolar transistors

GaAs n-p-n bipolar transistors were fabricated by ion implanting Be and Se into bulk n-type substrate material. Devices with mesa collectors exhibit a d.c. current gain hFE ? 8 and a reverse bias leakage current of less than 10 nA.     

Redistribution of implanted oxygen in GaAs

We show that implanted oxygen in GaAs has two different thermal behaviours depending on the implantation dose. In the case of low doses, below 1014 oxygen-cm?2 oxygen diffuses very quickly at 900°C. In the case of high doses, above 1014 oxygen-cm?2, oxygen piles up around its projected range during annealing. This should be due to the formation of precipitates. The presence of substrate impurities in the nucleation process is not suspected.     

Photoluminescence of Be implanted Si-doped GaAs

Degenerately doped n-type GaAs produces band-to-band luminescence with the peak energy dependent on the carrier concentration. In this study the photoluminescence of Si-doped GaAs is examined after implantation with high energy Be ions and annealing. The band-to-band peak energy in the unimplanted (reference) material is shown to be smaller than reported values in Te-doped GaAs of the same carrier concentration. This is attributed to compensation in the Si doped material as a result of its amphoteric nature. For the implanted samples, no luminescence was recorded for the unannealed samples or those annealed at 400°C and 500°C. Comparing the relative peak intensities from material annealed at 600°C for 15 min and 30 min indicates an increase in the number of As vacancies with anneal time. For samples annealed at 700°C and 800°C, the dominant luminescence is associated with Ga_As antisite defects. It is suggested that formation of these defects occurs predominantly only at these higher temperatures. Crystal recovery as measured by the luminescence intensity increased with both anneal temperature and time. For the implanted sample annealed at 800°C for 15 min, the dominant peak height was 25% of that from the reference sample.     

Laser annealing of ainc implanted GaAs

GaAs samples implanted with 1.10¹⁵ zinc ions/cm³ have been annealed using pulses from a ruby laser operating in the freely generated mode. The percentage electrical activities and peak carrier concentrations were found to depend on the implant temperature, the laser energy density and the number of pulses. The maximum hole concentration measured was about 1.10²⁰ cm^?3 following irradiation with four pulses of 3 J/cm². Significant residual damage remained after single pulse irradiation at this energy density value. However, the carrier mobilities were similar to values for good single crystal GaAs.     

Structural, electrical, and optical studies of GaAs implanted with MeV As or Ga Ions

The structural properties of GaAs implanted with high doses of 2 MeV arsenic or gallium ions with subsequent annealing at different temperatures were studied by transmission electron microscopy, Rutherford backscattering spectrometry-channeling, double crystal x-ray diffraction. Optical absorption, electrical conductivity, Hall effect and time-resolved photoluminescence were applied to monitor changes in electrical and optical characteristics of the material. An important conclusion from this investigation is that there was hardly any difference between materials implanted with gallium or arsenic. For implantation of either species, a large number of point defects was introduced and for a high enough dose a buried amorphous layer was formed. Hopping conduction and high absorption below band-to-band transition were observed for both cases. After low temperature annealing of the amorphous material, a high density of stacking faults and microtwins were found. Regrowth rates at the front and back amorphous-crystalline interfaces showed a significant difference. This was attributed to differences in local nonstoichiometry of the material at the upper and lower amorphous-crystalline interfaces. Structural studies showed the presence of some residual damage (a band of polycrystalline material in the center of the regrown area) with some associated strain even after annealing at high temperatures. Recovery to the conduction band transport in annealed samples was observed but mobilities, of the order of 2000 cmWs, were still smaller than in unimplanted GaAs. These results show that, in as-implanted material and even after annealing at lower temperatures, the point defects introduced by the implantation are responsible for the very short photocarrier lifetime. *On leave from Institute of Experimental Physics, Warsaw University, Poland.