Nonstoichiometric deep levels in Mg-doped GaP epitaxial layers

We have investigated the relation between deep levels in Mg-doped p-type GaP liquid phase epitaxy (LPE) layers and stoichiometry of the surface of the substrates by PHCAP measurement. Concentration of a deep donor level at E_C−1.9–2.1 eV is higher in an n-type undoped GaP substrate annealed with applying phosphorus vapor pressure of 20 kPa than in sample annealed beneath a carbon cover. Next, Mg-doped LPE layers are grown on substrates that have been pre-annealed under phosphorus vapor pressure just before the growth. The densities of deep levels at E_V+0.85 and E_V+1.5 eV in long-time (2 h) pre-annealing sample are greatly decreased, but a deep level at E_C−1.9–2.1 eV shows opposite tendency. The latter is thought to be identical to a deep level detected in the substrate, probably phosphorus interstitial atoms.     

Controlled vapor-pressure heat-treatment effect on deep levels in liquid-encapsulated czochralski-grown GaP crystals

Photocapacitance (PHCAP) measurements have been carried out on GaP crystals grown by the liquid-encapsulated Czochralski (LEC) method with heat treatment under various phosphorus-vapor pressures at different temperatures. Electron traps of E_C−1.1 eV, E_C−1.6 eV, E_C−1.9 eV, and a hole trap of E_V+2.26 eV are mainly detected. The phosphorus-vapor pressure dependence of the E_C−1.9 eV trap density and their diffusion behavior indicate that they are interstitial phosphorus atoms. The densities of both E_C−1.1 eV and E_C−1.6 eV traps are strongly dependent on the shallow impurity concentrations. Moreover, the density of E_C−1.1 eV traps increases with increasing phosphorus-vapor pressure. From these results, we suggest that E_C−1.1 eV traps are the complexes of shallow donors and antisite phosphorus atoms. Deep-level densities in GaP crystals after annealing at 860°C or 960°C for 60 min are decreased almost one order of magnitude lower than those in untreated substrate crystals, which should have occurred via out-diffusion of interstitial phosphorus atoms. However, such an effect is not prominent for 800°C treatment for 60 min.     

Nonstoichiometric deep levels in Mg-doped GaP epitaxial layers: Effects of pre-annealing of substrates

The effects of substrate pre-annealing on deep level density in Mg-doped GaP liquid-phase epitaxy (LPE) layers were investigated by photocapacitance measurement. With annealing under optimum phosphorus-vapor pressure, concentration of deep donor at E_C — 1.9–2.1 eV increased in undoped GaP substrate. Deep level densities in Mg-doped layers were also affected by pre-annealing of the substrates. Densities of dominant deep levels at E_V + 0.85 eV and E_V + 1.5 eV were an order of magnitude reduced and, in contrast, the level at E_C − 2.1 eV in Mg-doped layer increased with long pre-annealing. This level at E_C − 2.1 eV is supposed to be involved with P-rich-type nonstoichiometric defects, such as phosphorus interstitial atoms diffused from the surface of the substrate.     

Quantitative esr analysis of deep defects in LEC-grown GaP

Defects in LEC GaP have been analyzed quantitatively using an ESR method. Transition-metal impurities and native defects were studied in as-grown crystals. The antisite defect, P_Ga, and the isolated gallium vacancy, V_Ga, were emphasized since no other method has given definite, quantitative information on these defects. P_Ga antisites are present in some Cr-doped and some Zn-dopeii samples in concentrations high enough to affect their electrical properties. V_Ga was not detected in as-grown crystals indicating that V_Ga concentrations are smaller than the antisite concentrations at least in p-type material. Typical ESR detection., limits for transition-metal impurities are in the middle 10¹⁴ cm⁻³ range. The method is especially sensitive to Fe traces which were observed in all the samples studied. Work performed while the author was a visiting scientist at TAF Freiburg,     

Transition metal deep centers in GaAs,GaP and Si

A simple model predicts that the energy of a transition element relative to the valence band edge is related to elastic stress and therefore to the radius. This model has been originally applied to a few elements in GaAs, but is shown to give moderately quantitative results for a range of substitutional iron-group transition elements from Sc to Cu in GaAs and to a lesser extent between measurements and theory for GaP and Si. The radius term involved is of the form R_M²δR, where δR is the difference between the metal atom with a full 3d shell and the tetrahedral covalent radius of the atom with +2 coordination. Other transition metal properties, such as distribution coefficients and hole lifetimes also appear to be related to atomic radius, implying that impurity induced stresses may be involved.     

Properties of GaP light-emitting diodes grown on spinel substrates

Red and green GaP electroluminescent diodes have been successfully fabricated from GaP grown heteroepitaxially on spinel substrates by a vapor phase/liquid phase two-stage process. Current-voltage and light emission characteristics of the diodes are compared with those grown on bulk substrates. Quantum efficiencies up to 0·1 per cent in the red and 0·01 per cent in the green have been obtained.     

A study of deep centers in microplasma channels in GaP light-emitting diodes with green-emission spectrum

The statistical delay of microplasma breakdown in GaP light-emitting diodes with the green-emission spectrum is studied. The unusual profound effect of deep centers on the statistical delay of avalanche breakdown is observed in the temperature range of 300–380 K; this effect is caused by a variation in the charge state of these centers due to a reduction in the reverse bias applied to the p–n junction. Four deep levels are revealed and their parameters are determined.     

Luminescence decay kinetics in GaP:Bi and GaP:N

The lifetime of excitons bound to isoelectronic traps in GaP is studied as a function of temperature, excitation energy and intensity. The photoluminescence was excited both selectively and above the gap by using a pulsed, tunable dye laser with photon flux varying in the range of 10¹⁴–10¹⁹ photons/cm² per pulse. GaP:Bi shows a strong intensity dependence of τ(T) in the temperature range of 40–60 K due to the thermal activation of the electron. Similarly, GaP:N shows this behavior in the range of 20–100 K. In this case, two activation processes can be identified: release of the bound exciton into the free exciton band and dissociation of the exciton. The observed dependence of τ(T) on both excitation energy and intensity indicate that saturable deep traps (shunt paths) deplete the excess free carriers. These traps can be completely saturated in GaP:N while only partial saturation is achieved in GaP:Bi. The kinetic equations are written for GaP:Bi and solved numerically assuming quasiequilibrium conditions. Fitting this model to the experimental results yields the capture cross sections for carriers by Bi and by the deep traps as well as the concentration of the latter.     

Carrier recombination and deep levels in PbTe

The results of transient photoconductivity lifetime measurements and of impurity photoconductivity as well as Hall effect measurements obtained on PbTe films show that deep electron trapping levels exist in monocrystalline epitaxially grown PbTe. To our knowledge this is the first direct observation of deep levels in the gap of PbTe. The energetic position of the levels and their concentration were determined. The observed levels profoundly influenced the efficiency of PbTe infrared emitter diodes. This was shown experimentally by comparing the luminescence intensity of various diodes with different carrier concentration in the active zone. A strong decrease of luminescence intensity was observed for carrier concentrations greater than 10¹⁷ cm⁻³. This decrease could be explained by recombination via the observed levels in the gap.     

Growth-parameter dependence of deep levels in molecular-beam-epitaxial GaAs

Deep levels in GaAs grown by molecular-beam epitaxy have been examined. Dependence of both electron and hole trap densities on growth parameters such as growth temperature have been obtained.     

Low-frequency emissions from deep levels in GaAs MESFETs

Low-frequency natural oscillations have been observed in ion-implanted GaAs test MESFETs based on undoped LEC grown semi-insulating substrates that originate from deep levels.     

Electron-hole plasma luminescence in GaP

An experimental investigation of the luminescence of strongly excited “pure” GaP has been performed as function of input light intensity and temperature. Three distinct situations are observed. For T < 25K, the spectrum consists of a band, due to the recombination of the electron-hole liquid (EHL), coexisting with an exciton-rich gas phase. At T 40K, the EHL band is still apparent, but the gas has evolved into an electron-hole plasma (EHP). Finally, at T > 60K, only the EHP recombination remains and exhibits a density dependent lineshape as expected. The density at which the metal-insulator Mott transition occurs is compared to the calculations using the Debye-Hückel criterion.     

Effects of ion implantation on deep levels in GaAs

We have studied the effects of ion implantation in GaAs using the techniques of deep-level transient spectroscopy. Samples included an unimplanted epitaxial buffer layer, a sample implanted directly into that buffer layer and then capped with Si3N4, a sample implanted into that buffer layer through a similar cap, and a sample implanted directly into a semi-insulating substrate and then capped. All implants were with Si29 and both types of implant were annealed at 860°C for fifteen minutes. We find that the total density of deep levels is not changed significantly by direct implantation, capping and annealing but that implantation through a cap greatly in creases the total deep-level concentration. Deep levels found in implanted layers after capping and annealing are primarily characteristic of the substrate or buffer layer into which the implantation is made, unless the implant is through a cap in which case contaminants from the capping process may be evident at high densities.     

Determination of deep levels in semiconductors from C-V measurements

A method is presented that allows deep level density and energy of impurities in semiconductors to be routinely determined from simpleC-Vmeasurements of Schottky barriers. The method is particularly useful for material having levels with very long time constants that are not accessible by noise spectra measurements. The technique is illustrated by measurements of epitaxial n-GaAs which show a donor level at 0.83 eV below the conduction band.     

Cu掺杂氧化锌氧化物的热学性能

基于线性响应的密度泛函微扰理论研究了Cu掺杂纤锌矿结构氧化物ZnO的热学参数和热学性能。结果表明,Cu掺杂导致ZnO氧化物晶胞减小;在计算温度区间,纯的ZnO和Cu掺杂的ZnO的晶格热容均随温度升高不断增大,Cu掺杂的ZnO具有较高的晶格热容;纯的ZnO和Cu掺杂ZnO的晶格热容在最高温度900K时分别达到69.1J·(mol-1·K-1)和152.8J·(mol-1·K-1)。纯的ZnO和Cu掺杂ZnO的德拜温度均随温度升高而不断增大;在175K以下,Cu掺杂ZnO体系的德拜温度高于未掺杂体系,在175K以上,Cu掺杂ZnO体系的德拜温度低于未掺杂体系。Cu掺杂在ZnO中引入了新的振动模式。Cu掺杂ZnO氧化物应具有较高的晶格热导率。     

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.     

Characterization of deep levels in modulation-doped AlGaAs/GaAs FET's

Deep levels in modulation-doped field-effect transistors (MODFET's) fabricated from MBE-grown AlGaAs/GaAs heterostructures, have been characterized by a modified deep-level transient spectroscopy (DLTS) technique. Assuming donor-like traps in the AlGaAs layer, it is shown that the threshold voltage Vtvaries exponentially with time under pulsed-biased conditions. This result is verified experimentally by observing the transient in the drain current IDin long-gate FET's biased in saturation. The resulting Δ √{I_{D}} DLTS spectrum reveals an electron trap with an activation energy of 0.472 eV in Si-doped Al0.3Ga0.7As.     

Influence of growth conditions on deep levels in molecular-beam-epitaxial GaAs

Molecular-bearn-epitaxially grown undoped GaAs layers are studied with regard to the relation between growth conditions and deep-level trap generation. Both carrier concentration and electron trap density have a minimum value in samples grown at close to the minimum As/Ga flux ratio under As-stabilised conditions. Growth under these conditions leads to suppression of defect generation and unintentional impurity incorporation.     

Autosolitons in bistable system of silicon with deep impurity levels

Autosolitons in the bistable system of silicon associated with the thermal and electric-field ionization of In deep acceptor levels at a temperature of 77 K are experimentally found and investigated. It is shown that the positive feedback on the activator is related in the considered model of autosolitons to an increasing dependence of the lattice temperature on growth in the hole concentration, while the damping role of the inhibitor is associated with a decrease in the charge-carrier temperature during phonon-induced scattering. This leads to power-loss reduction and lattice-temperature restriction in the vicinity of autosolitons.     

A study of deep levels in GaAs by capacitance spectroscopy

We show how the DLTS capacitance spectroscopy technique can be used to detect small amounts of deep level impurities in GaAs p-n junctions. The DLTS spectra associated with Cu, Fe, Cr, 0, and two unidentified but commonly occurring deep levels in GaAs are shown. The LPE distribution coefficients are obtained for Cu, Fe, and Cr. The carrier capture cross sections for six levels are measured and give evidence for capture by multiphonon emission.