Photoconductivity and photoluminescence studies in copper diffused InP

Cu diffusion was carried out in p-InP at 300°C for one hour followed by 600°C for one minute. High photoconductivity (I_ph/I_d = 2.6 × 10⁵ at 200K) was observed in this sample. Information about Cu related deep levels was obtained from dark conductivity, photoconductivity and its spectral response. A Cu related photoluminescence (PL) band was observed at 1.216 eV and its line-shape and line-width analysis carried out. The configuration coordinate diagram of the band was calculated and showed small lattice relaxation. In n-InP Cu diffusion at 650°C for two hours resulted in two PL bands at 1.20 and 1.01 eV. The former was similar to the 1.216 eV band in p-InP. The PL of the 1.01 eV band was also studied in detail and the corresponding configuration coordinate diagram derived. The parameters of the Cu related bands obtained from the line-shape and line-width analysis are compared with those reported due to Mn and Fe in InP.     

Current controlled liquid phase epitaxial (CCLPE) growth of InGaAs on (100) InP

High quality epitaxial layers of In_xGa_1−xAs (x = 0.53) were grown on semi-insulating (Fe-doped) (100) InP sub-strates. The layers were grown at a constant furnace temperature of 640°C by passing a direct electric current (0–10A/cm²) from the substrate to the melt. In order to minimize the out-diffusion of Fe atoms from the bulk of the substrate during the melt saturation, the substrate was kept at a cold temperature region (340°C) within the growth chamber and remotely loaded in the graphite boat just prior to the initiation of the growth cycle. In addition to pre-venting the out-diffusion of Fe atoms, this procedure sub-stantially reduced the thermal degradation of the InP sub-strate surface. The above technique produced high quality layers having uniform thickness and good surface morphology. A study of the dependence of growth rate on the applied current density yielded an average growth rate of 0.06μm/ A-min. Room temperature Hall measurements on layers grown by CCLPE resulted in Hall mobilities μ₃₀₀ = 8900cm²/V-sec at a carrier concentration of 6.2 × l0¹⁶cm⁻³. The improve-ment in the mobility achieved by the CCLPE technique is attributed to a reduced out-diffusion of scattering centers from the substrate into the growth layer, as well as to the higher quality of epitaxial layers normally achieved by CCLPE.     

Electronic properties of low-temperature InP

We have investigated InP layers grown by low-temperature (LT) gas source molecular beam epitaxy. Using high-pressure hall effect measurements, we have found that the electronic transport in the LT epilayers is determined by the presence of the dominant deep donor level which is resonant with the conduction band (CB) located 120 meV above the CB minimum (E_CB). We find that its pressure derivative is 105 meV/GPa. This large pressure derivative reveals the highly localized character of the donor which via auto-ionization gives rise to the high free electron concentration n. From the deep level transient spectroscopy and Hall effect measurements, we find two other deep levels in the band gap at E_CB−0.23 eV and E_CB−0.53 eV. We assign the two levels at E_CB 0.12 eV and E_CB−0.23 eV to the first and second ionization stages of the phosphorus antisite defect.     

InP growth and properties

Bulk polycrystalline InP is synthesized from the elements via a gradient freeze process. Hall data for a typical boule are N_d-N_a= 4.7 × 10¹⁵/cm³ and Μ77 = 28,000 cm²/V-sec. Photoluminescence data indicate that zinc is present as an acceptor impurity in the polycrystalline InP and in nominally undoped LEC single crystals grown using the synthesized InP as charge material. A series of doping experiments have determined the effective segregation coefficient to be 1.6 × 10⁻³ for Fe in InP. Semi-insulating InP crystals with resistivity > 10⁷ ohm—cm have been grown consistently from melts doped with 150 ppm Fe.     

Chromium contamination in VPE InP from photoconductivity spectra

A photoconductivity study has been made of nominally undoped VPE n-InP materials grown on semi-insulating InP:Fe substrates. The photoresponse spectra obtained indicate features characteristic of Cr deep acceptors. By comparing the magnitudes of the photoresponse with that of a back-doped InP:Cr reference sample, the concentration has been estimated at between 2 and 8×1014 cm?3.     

Band offsets at the (1 0 0)GaSb/Al2O3 interface from internal electron photoemission study

From electron internal photoemission and photoconductivity measurements at the (1 0 0)GaSb/Al₂O₃ interface, the top of the GaSb valence band is found to be 3.05 ± 0.10 eV below the bottom of the Al₂O₃ conduction band. This interface band alignment corresponds to conduction and valence band offsets of 2.3 ± 0.10 eV and 3.05 ± 0.15 eV, respectively, indicating that the valence band in GaSb lies energetically well above the valence band of In_xGa_1−xAs (0 ? x ? 0.53) or InP.     

Effect of annealing conditions on the uniformity of undoped Semi-Insulating InP

Recently, it was found that undoped semi-insulating InP can be obtained by highpressure annealing of high purity materials. The reproducibility and the uniformity was, however, not satisfactory. In the present work, we found that not only Fe concentrations but also Cr and Ni concentrations in annealed wafers were slightly increased during annealing. Since it seems that the origin of the contamination was due to the vapor source of red phosphorus, conductive InP with a trace amount of Fe was annealed under low phosphorus vapor pressure in order to reduce the contamination. By preventing the contamination of Cr and Ni, preparation of semi-insulating InP became highly reproducible. The minimum Fe concentration for realizing semi-insulating InP was found to be 1 x 10¹⁵cm⁻³. It was also found that the better resistivity uniformity can be obtained at higher annealing temperatures.     

The behavior of unintentional impurities in Ga0.47 In0.53As grown by MBE

A number of factors contribute to the high n-type background carrier concentration (high 10¹⁵ to low 10¹⁶ cm⁻³) measured in MBE Ga_0.47In_0.53As lattice-matched to InP. The results of this study indicate that the outdiffusion of impurities from InP substrates into GalnAs epitaxial layers can account for as much as two-thirds of the background carrier concentration and can reduce mobilities by as much as 40%. These impurities and/or defects can be gettered at the surfaces of the InP by heat treatment and then removed by polishing. The GalnAs epitaxial layers grown on the heat-treated substrates have significantly improved electrical properties. Hall and SIMS measurements indicate that both donors and acceptors outdiffuse into the epitaxial layers during growth resulting in heavily compensated layers with reduced mobilities. The dominant donor species was identified by SIMS as Si, and the dominant acceptors as Fe, Cr and Mn.     

Electrical and optical characterization of Mg,Mg/P,and Mg/Ar implants into InP:Fe

Mg, Mg/P, and Mg/Ar implantations were performed into InP:Fe with an energy of 80 keV for obtaining shallow p⁺ layers suitable for device applications. After rapid thermal annealing at 850 or 875°C for 5 or 10 s, activations between 10 and 50% and mobilities as high as 110 cm²/Vs were obtained for the different doses employed. For the implantations with 10¹⁴ cm₋₂, differential Hall measurements showed hole profiles with peak concentrations in the mid-10¹⁸ cm^-3 range and Hall mobilities of 90 cmWs. However, secondary ion mass spectrometry profiles showed a clear pileup of Mg at the surface and in-diffusion tails deeper than 2 μ. Phosphorus or Ar co-implantation reduced the Mg in-diffusion and increased the activation, but not as clearly as in the case of Be implants. Photoluminescence (PL) measurements demonstrated the good crystalline quality of the material after all the annealing cycles employed. In the photoluminescence spectra, together with narrow emissions close to the gap wavelength, two broad bands, centered at about 1.3 and 0.87 eV were found, this last being the dominant emission of the PL spectra from the layers with higher implanted doses. The origin of this band is tentatively assigned to complexes involving Mg and a defect.     

Characteristics of deep levels in Al-doped ZnSe grown by molecular beam epitaxy

We investigated the characteristics of deep levels in heavily Al-doped ZnSe layers grown by molecular beam epitaxy, whose electron concentration is saturated. Low-temperature photoluminescence showed deep level emission around 2.25 eV, and its intensity increases with Al concentration. This deep-level is located at 0.55 eV above valence band maximum, implying a point defect such as a self-activated center, Al_ZnV_Zn. Deep-level transient spectroscopy was used to investigate non-radiative trap centers in Al-doped ZnSe layers, and showed the presence of two electron trap centers at depths of 0.16 and 0.80 eV below conduction band minimum, with the electron capture cross-sections of 810⁻¹² and 1×10⁻⁷ cm², respectively. It is suggested the carrier compensation in heavily Al-doped ZnSe layers be ascribed to the deep levels.     

Electrical properties of Si:Er/Si layers grown by sublimation molecular-beam epitaxy

Temperature dependences of the concentration and electron Hall mobility in Si:Er/Sr epitaxial layers grown at T = 600°C and annealed at 700 or 900°C have been investigated. The layers were grown by sublimation molecular-beam epitaxy in vacuum (～10^?5 Pa). The energy levels of Er-related donor centers are located 0.21–0.27 eV below the bottom of the conduction band of Si. In the range 80–300 K, the electron Hall mobility in unannealed Si:Er epitaxial layers was lower than that in Czochralski-grown single crystals by a factor of 3–10. After annealing the layers, the fraction of electron scattering from Er donor centers significantly decreases.     

Vapor growth of InP for MESFET’S

Epitaxial layers of InP have been grown by the conventional In/PCl₃/H{ion2} technique. With the aim of fabricating FET’s structures, we have studied the growth of low doped buffer layers and the doping by H₂S. It has been shown, that the purity of the layers increases from experiment to experiment and that low doped layers, in the 10¹³ – 10¹⁴ cm^-3 range, are obtained after growth of about 10 layers. Evidence for the purity of these layers have been obtained from Hall, photoluminescence and SIMS measurements. Cr and Fe outdiffusion from the substrate has been studied by SIMS. Fe is found to diffuse from the substrates, even in the case of substrates which are not intentionally doped with Fe. Some FET’s have been fabricated on epitaxial structures with and without buffer layers: the static characteristics of the transistors are encouraging (I_Dss = 24 mA, g_m = 19 mS for a gate of 2 μm and 200 μm in length and width, respectively); the pinch-off is better in devices fabricated from structures with buffer layers.     

Semi-insulating InP grown by low pressure MOCVD

Fe-doped semi-insulating InP layers have been successfully grown in a vertical flow, low pressure metalorganic chemical vapor deposition (LPMOCVD) system, and used as current blocking layers in buried crescent (BC) laser structures emitting at 1.51 μm. Triethylindium ((C₂H₂)₃In), phosphine (PH₃) and iron pentacarbonyl (Fe(CO)₅) were used as the reactant gases. Process variables have been identified which produce high resistivity (10⁷ to 10₈ Ω-cm) InP having featureless surface morphology, and layer thickness and doping uniformity. There is optical and x-ray diffraction evidence for the presence of an unidentified In-Fe-P second phase associated with markedly degraded surface morphology under nonoptimized growth conditions. Early BC lasers incorporating LPMOCVD grown, Fe-doped InP blocking layers have operated CW with threshold currents as low as 12 m A and optical output > 18 mW.     

Flexible Cu(In,Ga)Se2 solar cell on stainless steel substrate deposited by multi‐layer precursor method: its photovoltaic performance and deep‐level defects

Cu(In,Ga)Se₂ (CIGS) films on soda‐lime glass and stainless steel (SUS) substrates with several [Ga]/([Ga] + [In]), GGI, and Fe concentrations are fabricated by so‐called “multi‐layer precursor method”. From optical deep‐level transient spectroscopy, deep‐level defect located at 0.8 eV from valence band maximum (E_V) is observed. This defect becomes recombination center when GGI is over 0.4, thereby decreasing cell performances. Fe‐related deep‐level defect is moreover detected in CIGS film on SUS substrate situated at 0.45 eV from E_V. Its density is consistent with Fe concentration in CIGS films. According to SCAPS simulation and experimental results, Fe concentration of above threshold (1.0 × 10¹⁶ atom/cm³) decreases carrier lifetime and carrier density and has more harmful influence on cell performances with GGI of above 0.4. On the other hand, Fe concentration of below threshold (1.0 × 10¹⁶ atom/cm³) has no detrimental impact on cell performances. Namely, conversion efficiency (η) is slightly changed by below 2%. CIGS solar cell on SUS substrate with η of 17.5% is fabricated by decreasing Fe concentration to approximately 5.2 × 10¹⁶ atom/cm³ although higher than the threshold value. Copyright © 2016 John Wiley & Sons, Ltd.     

Hydrogenated amorphous silicon p–i–n solar cells deposited under well controlled ion bombardment using pulse‐shaped substrate biasing

We applied pulse‐shaped biasing (PSB) to the expanding thermal plasma deposition of intrinsic hydrogenated amorphous silicon layers at substrate temperatures of 200 °C and growth rates of about 1 nm/s. Fourier transform infrared spectroscopy of intrinsic films showed a densification with increasing deposited energy and a reduction in void content, whereas dual‐beam photoconductivity measurements showed an increase in Urbach energy above 4.8 eV/Si atom. From dark conductivity and photoconductivity measurements, we determined a maximum photoresponse of 2 × 10⁶ at 3 eV/Si atom, which decreased at higher deposited energies because of a higher dark conductivity as a result of a lower band gap. p–i–n solar cells with PSB applied during the intrinsic layer deposition showed initial energy conversion efficiencies of 7.4% at around 1 eV/Si atom. Decreasing open‐circuit voltage at >1 eV/Si atom can be related to a lower band gap, whereas the short‐circuit current drops at >4.8 eV/Si atom, predominantly because of hole collection losses as determined from quantum efficiency measurements. The reduced fill factor for >1 eV/Si atom was presumably related to a decrease in mobility‐lifetime product because of an increase in defect density. Copyright © 2011 John Wiley & Sons, Ltd.     

Optical and crystallographic properties of high perfection InP grown on Si(111)

The growth of InP by low-pressure metalorganic chemical vapor deposition on vicinal Si(111), misoriented 3° toward [1-10], is reported. Antiphase domain-free InP is obtained without any preannealing of the Si substrate. Crystallographic, optical, and electrical properties of the layers are significantly improved as compared to the best reported InP grown on Si(001). The high structural perfection is demonstrated by a full width at half maximum (FWHM) of 121 arcs for the (111) Bragg reflex of InP (thickness = 3.4 μm) as obtained by double crystal x-ray diffraction. The low-temperature photoluminescence (PL) efficiency is 70% of that of homoepitaxially grown InP layers. The FWHM of the near-gap PL peak is only 2.7 meV as compared to 4.5 meV of the best material grown on Si(001). For the first time, InP:Fe layers with semi-insulating characteristics (ρ > 3 × 10⁷ Ω-cm) have been grown by compensating the low residual background doping using ferrocene. Semi-insulating layers are prerequisite for any device application at ultrahigh frequencies.     

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.     

An optimal annealing technique for ohmic contacts to ion-implanted n-layers in semi-insulating indium phosphide

An optimal annealing process was developed for sintering AuGe ohmic contacts to ion-implanted semi-insulating InP substrates. Contacts were annealed using a standard furnace, graphite strip heater and a lamp annealer. Alloying at 375°C for 3 min was found to be most suitable for achieving good contact morphology and lowest contact resistivity. Of the three techniques, the lamp annealing technique was found to give the best results when contacts were annealed under a SiO₂ cap. Contact resistivity as low as 8 × 10⁻⁶ cm² was obtained for ion-implanted n⁺ layers in semi-insulating InP.     

Microwave field-effect transistors from sulphur-implanted GaAs

Sulphur implanation into semi-insulating Cr doped GaAs has been used to fabricate MESFETs with 1.5 μm gatelength showing microwave gain equivalent to epitaxial FETs (MAG = 9 dB at 10 GHz) but higher noise. Room temperature implantation of S at an energy of 30 keV and a dose of 5 × 10¹² cm^?2, sputtered SiO₂ and Si₃N₄ as encapsulants and heat treatments from 820 to 900°C have been used. Electrical activation was found to depend critically on the substrate material. Si₃N₄-encapsulation gave slightly higher electrical activation than SiO₂.     

Uniformity of deep levels in semi-insulating InP obtained by multiple-step wafer annealing

The uniformity of deep levels in semi-insulating InP wafers, which have been obtained by multiple-step wafer annealing under phosphorus vapor pressure, was studied using the thermally stimulated current (TSC) and photoluminescence (PL) methods. Only three traps related to Fe, T₀ (ionization energy E_i=0.19 eV), T₁ (0.25 eV), and T₂ (0.33 eV), probably forming complex defects, were observed in the wafer and they exhibited a relatively uniform distribution. PL spectra relating to phosphorus vacancies observed in some regions of the wafer are correlated with a small TSC signal having an ionization energy of 0.43 eV.