Defect state assisted tunneling in intermediate temperature molecular beam epitaxy grown GaAs

Current transport in molecular beam epitaxy (MBE) GaAs grown at low and intermediate growth temperatures is strongly affected by defects. A model is developed here that shows that tunneling assisted by defect states can dominate, at some bias ranges, current transport in Schottky contacts to unannealed GaAs material grown at the intermediate temperature range of about 400°C. The deep defect states are modeled by quantum wells which trap electrons emitted from the cathode before re-emission to semiconductor. Comparison of theory with experimental data shows defect states of energies about 0.5 eVbelow conduction band to provide the best fit to data. This suggests that arsenic interstitials are likely to mediate this conduction. Comparison is also made between as-grown material and GaAs grown at the same temperature but annealed at 600°C. It is suggested that reduction of these defects by thermal annealing can explain lower current conduction at high biases in the annealed device as well as higher current conduction at low biases due to higher lifetime. Quenching of current by light in the as-grown material can also be explained based on occupancy of trap states. Identification of this mechanism can lead to its utilization in making ohmic contacts, or its elimination by growing tunneling barrier layers.     

The effect of boron,oxygen, and fluorine in ion implanted GaAs

Effects of boron, fluorine, and oxygen in GaAs have been investigated by electrical characterization using current-voltage, capacitance-voltage and deep level transient spectroscopy techniques. Ion implantation at 100 keV energy was conducted with doses of 10¹¹ and 10¹²/cm². Carrier compensation was observed in each implanted sample. The compensation effect strongly depended on ion implantation condition and ion species. More free carriers were compensated for higher dose and heavier species; however, severe surface damage would also be induced that degrade electrical performance. Rapid thermal annealing treatment showed the heavier ion implanted samples to be more thermally stable. Defect levels for each implanted species were compared and identified. A native shallow defect E4 was easily removed by ion implantation. In higher dose and heavier ion implantation, both electron and hole traps were induced. However, in some cases, heavier ion implantation also removed native defects. Acceptor-type surface states were created by implantation that degrade material electrical characteristics and also reduce the effect of compensation. The damage induced traps were mostly point-defects or point-defect/impurity complexes as evidenced by sensitivity to heat treatment.     

Diode structures from amorphous low-temperature GaAs

The effect of annealing on the electrical properties of a GaAs diode structure, which incorporated a nominally undoped low-temperature (LT) layer on top of conventionally grown p-type GaAs, is examined. Unannealed GaAs grown by molecular beam epitaxy at substrate temperatures below 250°C is amorphous and highly resistive. Annealing at high temperatures converts the undoped LT-GaAs from amorphous to single crystal material. The annealed material is n-type. The current-voltage characteristics of the LT on p-type GaAs structures showed greater asymmetry, with lower reverse leakage currents, as the anneal temperature was increased above 400°C. This reflects the improved crystal quality of the LT layer.     

Defects in molecular beam epitaxial GaAs grown at low temperatures

We have utilized a variable energy positron beam and infrared transmission spectroscopy to study defects in GaAs epilayers grown at low temperatures (LT-GaAs) by molecular beam epitaxy. We have measured the Doppler broadening of the positron-electron annihilation gamma ray spectra as a function of positron implantation energy. From these measurements, we have obtained results for the depth profiles of Ga monovacancies in unannealed LT-GaAs and Ga monovacancies and arsenic cluster related defects in annealed LT-GaAs. We have also studied the effects of the Si impurities in annealed LT-GaAs. The infrared transmission measurements on unannealed LT-GaAs furnish a broad defect band, related to As antisites, centered at 0.370 eV below the conduction band.     

SI GaAS中S~+注入的电学特性

本文研究了经常规热退火和快速热退火后SIGaAs中S~+注入的电学特性.热退火后,GaAs中注入S~+的快扩散和再分布不决定于S~+或砷空位V_(AS)的扩散而决定于离子注入增强扩散.使用快速热退火方法能抑制注入S~+在GaAs中的增强扩散,明显减小S~+的再分布,可以获得适合于制造GaAs MESFET器件的薄有源层.     

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.     

Low-temperature metalorganic chemical vapour deposition of AlN for surface passivation of GaAs

Aluminium nitride (AlN) thin films have been grown by low-temperature metalorganic chemical vapour deposition (MOCVD) to passivate GaAs. By utilizing hydrazine (N₂H₄), highly resistive amorphous-like AlN films were obtained at growth temperatures around 400°C. At the AlN-GaAs interface, three deep trap levels were found: 0.6 eV (DL1) and 0.9 eV (DL2) below the conduction band minimum and 0.5 eV (DL3) above the valence band maximum. The number of DL1 levels was reduced by preparing As-dimer-stabilised surfaces of GaAs. The capture cross-sections and time constants of DL1–DL3 suggest that these levels originated from point defects, not from precipitates or disorder. Neither precipitation nor reaction was detectable by Auger electron spectroscopy after annealing at 900°C for 20 min, indicating that the AlN-GaAs interfaces are thermally stable. These results demonstrate that these AlN films are applicable as capping films for processing GaAs as well as passivation films.     

Pill-box capless thermal-heat-pulse annealing of ion-implanted GaAs

A capless annealing method for GaAs, employing short (1-10-s) thermal heat pulses from a graphite strip heater, is described. Results with²⁹Si+ implanted into semi-insulating chromium-doped HB and chromium-doped LEC GaAs are presented. The samples were annealed at temperatures as high as 1000°C in a stationary N2atmosphere with the implanted surface in close contact with a flat graphite strip heater surface. The wafers were covered with a graphite lid, effectively confining the volatile arsenic to a very small volume around the sample and providing a uniform temperature environment. Implantation efficiencies were high, and the quality of the implanted surface remained excellent after annealing.     

Si,S,Be,Mg离子注入GaAs白光快速退火特性研究

利用灯光瞬态退火处理Si,S离子注入SI-GaAs样品,在950℃5秒的条件下得到了最佳的电特性,Be,Mg离子注入SI-GaAs样品在800℃ 5秒退火得到了最佳的电特性.Si,S,Be注入GaAs样品在适当的条件下得到了陡峭的载流子剖面分布,而Mg注入的样品有Mg的外扩散和较大的尾部扩散.透射电镜测量表明,Si低剂量和Be大剂量注入退火后单晶恢复良好,而Si和Mg大剂量注入退火后产生了大量的二次缺陷.应用Si和Mg注入GaAs分别制作了性能良好的MESFET和β=1000的GaAIAs/GaA,双极型晶体管(HBT).     

Damage related deep electron levels in ion implanted GaAs

A study has been made of the deep electron levels in semi-insulating GaAs implanted with either ⁷⁸Se⁺ or ²⁹Si⁺ ions and rendered n-type by subsequent annealing without encapsulation in partial pressures of arsenic or arsine. Three implantation related deep states were detected with concentration profiles approximating to the type of Gaussian distributions expected for point defects related to ion implantation damage. Further heat treatment of the samples at 500°C in a gas ambient of N₂/H₂ substantially reduced concentrations of these deep levels. Two of these states were thought to be related to displacements of the substrate atoms. The third, at E_c − 0.67 eV, was found in only ⁷⁸Se⁺ ion implanted GaAs substrates and was thought to be a defect involving both Se and As atoms, rather than intrinsic lattice disorder. It is proposed that the annealing rate of these implantation related deep levels depends crucially on the in-diffusion or creation of arsenic vacancies during heat treatments.     

Rapid Thermal annealing of si implanted GaAs

Rapid thermal annealing (RTA) with incoherent light from tungsten lamps shows high potential relative to the conventional furnace annealing (FA) to activate the implanted dopant. Due to the short time annealing, it could completely eliminate the re-diffusion of dopant and host atom. For the Si implantation with dose of 2 × 10¹⁴ cm², the electrical activity of 78% for RTA was higher than that of the FA. But for this short time, some defects measured by deep level transient spectroscopy (DLTS) were hard to remove. A two-step annealing was suggested by the combination of high temperature RTA (1000° C) and FA (700° C). After the post-FA, the defects would be removed to a great extent, and the electrical activity of dopant also increased. With the dose of 2 x 1013 cm^-2, the activity attained after the two step annealing was 92.5%, which may be the highest value according to our knowledge for rapid thermal annealing on Si ion implanted GaAs.     

Capless annealing of silicon implanted gallium arsenide

Numerous commercially available semi-insulating GaAs substrates have been implanted with silicon ions and the post implantation annealing carried out using the technique of capless annealing in an arsine atmosphere. Results are presented on the implanted atomic silicon distribution along with carrier concentration and mobility profiles, Hall mobility and percentage activation figures for various implanted substrates. The phenomenon of thermally induced surface conduction layers in semi-insulating GaAs is discussed in the context of a capless annealing technique.     

A deep level transient spectroscopy study of rapid thermal annealed arsenic implanted silicon

Deep level traps are observed in silicon that has been implanted with high doses of arsenic and subsequently annealed by rapid thermal annealing. The doses studied create enough damage to form a surface amorphous layer. Annealing temperatures, implant fluence, and the presence of a surface amorphous layer contribute to the type of trap observed. These results show evidence for a clustering/declustering mechanism of arsenic in silicon during rapid thermal annealing.     

Optical and electrical properties of AlGaN films implanted with Mn,Co, or Cr

The electrical and optical properties of undoped n-AlGaN films with Al mole fraction close to x=0.4 were studied before and after implantation of 3×10¹⁶ cm⁻² 250-keV Mn, Co, and Cr ions. The electrical properties of the virgin samples are shown to be dominated by deep donors with the level near E_c-0.25 eV and concentration of about 2×10¹⁸ cm⁻³. The microcathodoluminescence (MCL) spectra of the virgin samples were dominated by two strong defect bands at 2.5 eV and 3.7 eV. After implantation, the resistivity of the implanted films increased but could not be accurately measured because of the shunting influence of the unimplanted portions of the films. Their resistivity was increased by more than an order of magnitude compared to the virgin samples because of the compensation by defects coming from the implanted layer during the post-implantation annealing. The absorption and luminescence spectra of the implanted samples were dominated by two strong bands near 2 eV and 3.5 eV. The latter are attributed to the electron transitions from the Mn, Co, or Cr acceptors to the conduction band.     

Deep-level dominated current-voltage characteristics of buriedimplanted oxide silicon-on-insulator

Current-voltage characteristics of Au contacts formed on buried implanted oxide silicon-on-insulator (SOI) structures are discussed, which indicate that the dominant transport mechanism is space-charge-limited current (SCLC) conduction in the presence of deep-level states. The deep-level parameters, determined using a simple analysis, appear to be sensitive to anneal conditions used and subsequent processing. Silicon implanted with 1.7×10¹⁸ cm^-2 oxygen ions at 150 keV following a 1200°C anneal for 3 h shows deep level 0.37 eV below the conduction band edge with a concentration of unoccupied traps of ~ 2×10¹⁵ cm^-3 . In contrast, arsenic ion implantation, in the 1200°C annealed material with a dose of 1.5×10¹² cm^-2 at 60 keV and activated by rapid thermal annealing (RTA), introduces a deep level 0.25 eV below the conduction band edge with an unoccupied trap concentration of ~6×10¹⁷ cm^-2     

MOS capacitors obtained by wet oxidation of n-type 4H-SiC pre-implanted with nitrogen

The manufacture process and the electrical characterization of MOS devices fabricated by wet oxidation of N⁺ implanted n-type 4H-SiC are here presented. Different implantation fluence and energy values were used with the aims to study the effect of the N concentration both at the SiO₂/SiC interface and within the SiO₂ film. High doses, able to amorphise a surface SiC layer to take advantage of the faster oxidation rate of amorphous with respect to crystalline SiC, were also evaluated. The electrical quality of the SiO₂/SiC system was characterized by capacitance-voltage measurements of MOS capacitors. The analyses of the collected data show that only the implanted N which is located at the oxide-SiC interfaces is effective to reduce the interface states density. On the contrary, the interface states density remains high (the same of an un-implanted reference sample) when the implanted N is completely embedded in the region consumed by the oxidation. Furthermore, none generation of fixed positive charges in the oxide was found as a consequence of the different N concentrations enclosed in the oxide films. These results were independent of the amorphisation of the implanted layer by the N⁺ ions. Our results demonstrate that by using a suitable N ion implantation and an appropriate wet oxidation treatment, it is possible to obtain a reduced thermal budget process able to decrease the interface state density near the conduction band edge. The proposed approach should be interesting for the development of the MOSFET technology on SiC.     

Photoluminescence in silicon implanted with silicon ions at amorphizing doses

Luminescent and structural properties of n-FZ-Si and n-Cz-Si implanted with Si ions at amorphizing doses and annealed at 1100°C in a chlorine-containing atmosphere have been studied. An analysis of proton Rutherford backscattering spectra of implanted samples demonstrated that an amorphous layer is formed, and its position and thickness depend on the implantation dose. An X-ray diffraction analysis revealed that defects of the interstitial type are formed in the samples upon annealing. Photoluminescence spectra measured at 78 K and low excitation levels are dominated by the dislocation-related line D1, which is also observed at 300 K. The peak position of this line, its full width at half-maximum, and intensity depend on the conduction type of Si and implantation dose. As the luminescence excitation power is raised, a continuous band appears in the spectrum. A model is suggested that explains the fundamental aspects of the behavior of the photoluminescence spectra in relation to the experimental conditions.     

Deep levels in Si-implanted and rapid thermal annealed semi-insulating gaAs

Deep levels have been investigated in Si-implanted and rapid-thermal-annealed semiinsulating GaAs:Cr, which was grown by a horizontal Bridgman method. Samples were implanted with a Si-dose of (1 - 5) x 10¹² ions cm^-2 with 100 keV energy, and treated by a two-step rapid thermal annealing process at 900 and 800° C. After these processes, three electron deep levels at 0.81, 0.53 and 0.62 eV below the conduction band and three hole deep levels at 0.89, 0.64 and 0.42 eV above the valence band were observed. The new deep levels E_c - 0.53 eV, E_c - 0.62 eV, andEv + 0.64 eV in fact, dominate the implantation and/or the thermally damaged region, but are not found in the bulk. These results indicate that high-density deep levels may be induced near or within the implanted region by rapid heating and cooling, and that these defects may effect carrier activation.     

Analysis of rapid thermal annealings of boron and arsenic in polysilicon emitter structures

The codiffusion of implanted boron and arsenic during rapid thermal annealing in a polysilicon/monocrystalline silicon system used for high speed bipolar integrated circuit technology has been investigated. Such a process allows a uniformly high concentration in the emitter regions. It induces a very shallow emitter-base junction depth for an n-p-n transistor. On the contrary, in p-n-p configuration, when the arsenic dose is lower than the boron dose, boron deeply penetrates the single-crystal silicon, causing problems for fabricating shallow junctions. The rapid thermal annealing-induced redistribution of arsenic and boron implanted at various implant doses in a 380 nm low-pressure chemical vapor deposition (LPCVD) polysilicon layer has been studied by secondary ion mass spectroscopy measurements. A strong lowering of the minority dopant diffusion in the polysilicon film is observed due to the high concentration of the other dopant. This effect is mainly related to grain boundary trap saturation while the electric field issued from dopant activation in the fastly recrystallized amorphous layer due to arsenic implantation in the upper region of the LPCVD film has a lesser effect. Transmission electron microscopy measurements show that recrystallization extends deeper than the amorphous layer, which is responsible for the increase of grain size. Sheet resistance measurements have been performed as an approach to the electrical behavior for these structures.     

Ion implantation in gallium arsenide MESFET technology

The authors emphasize controlled shallow doping of GaAs by ion implantation and its limitations to state-of-the-art GaAs IC technology. The authors discuss the electrical activation behavior of implanted silicon in GaAs upon subsequent capless or silicon nitride capped rapid thermal annealing (RTA). It is demonstrated that atomic H diffuses into the implanted region of GaAs from a plasma-enhanced chemical vapor deposition Si₃N₄ cap during the deposition as well as during subsequent annealing, and the H retards the electrical activation kinetics of the implanted Si. Thru-Si cap dopant implants into GaAs have been studied to enhance dopant concentration in the surface region of the GaAs by recoil-implanted Si from the cap. Application of ion implantation to achieve buried-p layers in GaAs is also briefly discussed