Ultrapure GaAs Research Articles

Low-Temperature In-Induced Holes Formation in Native-SiOx/Si(111) Substrates for Self-Catalyzed MBE Growth of GaAs Nanowires.

The reduction of substrate temperature is important in view of the integration of III–V materials with a Si platform. Here, we show the way to significantly decrease substrate temperature by introducing a procedure to create nanoscale holes in the native-SiOx layer on Si(111) substrate via In-induced drilling. Using the fabricated template, we successfully grew self-catalyzed GaAs nanowires by molecular-beam epitaxy. Energy-dispersive X-ray analysis reveals no indium atoms inside the nanowires. This unambiguously manifests that the procedure proposed can be used for the growth of ultra-pure GaAs nanowires.

Interplay of Electron and Nuclear Spin Noise in n-Type GaAs.

We present spin-noise spectroscopy measurements on an ensemble of donor-bound electrons in ultrapure GaAs:Si covering temporal dynamics over 6 orders of magnitude from milliseconds to nanoseconds. The spin-noise spectra detected at the donor-bound exciton transition show the multifaceted dynamical regime of the ubiquitous mutual electron and nuclear spin interaction typical for III-V-based semiconductor systems. The experiment distinctly reveals the finite Overhauser shift of an electron spin precession at zero external magnetic field and a second contribution around zero frequency stemming from the electron spin components parallel to the nuclear spin fluctuations. Moreover, at very low frequencies, features related with time-dependent nuclear spin fluctuations are clearly resolved making it possible to study the intricate nuclear spin dynamics at zero and low magnetic fields. The findings are in agreement with the developed model of electron and nuclear spin noise.

Excitonic effects and Franz–Keldysh oscillations in photoreflectance of ultrapure GaAs epilayers

AbstractA model based on the exciton theory has been presented for lineshape analysis of photoreflectance (PR) spectra. Experimental PR spectra of high‐quality thick GaAs epilayers of different purity were measured to verify the theoretical results. It has been shown that excitonic effects completely prevail over the one‐electron theory in the formation of photoreflectance spectrum even at high electric fields and room temperature, giving evidence of the existence of Franz–Keldysh oscillations enhanced by excitonic effects. A comparison of PR spectra simulated using the exciton theory and the one‐electron approximation has shown that the semiperiods of the oscillations were practically similar. This demonstrates the validity of using the simple “high‐field” model of Aspnes and Studna to determine surface electric fields. In photoreflectance of thick ultrapure GaAs epilayers an oscillating structure was experimentally observed in the region between the excitonic and the fundamental bandgap. This provides a possibility for contactless characterization not only for doped (N ∼ 1014–1018 cm–3) but also for ultrapure (N ∼ 1011–1013 cm–3) GaAs epilayers as well as other semiconductor materials. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Determination of the free carrier concentration in ultra-pure GaAs epilayers by a photoreflectance technique

A new method for the diagnostics of high-quality n-GaAs epilayers grown by vapor phase epitaxy in an open chloride system has been developed and implemented. As the free carrier concentration in the material decreases from 1015 to 1011 cm−3 and the quality of epilayers increases, the photoreflectance spectra of GaAs exhibit a change from the traditional Franz-Keldysh (FK) oscillations (enhanced by the exciton effects) to a weak oscillatory structure observed in the region of exciton transition energies. It is shown that the periods of this structure are close to that of the FK oscillations.

Optical characterization of ultra‐pure GaAs

The problem of ultra-pure thick GaAs layers has been important for a long time. Growth of such material is an important technological problem and the basis of application in electronic devices. Epitaxial (100–1000) μm thick GaAs layers were grown by vapour phase epitaxy in a chloride system (HVPE) on N+ and semi insulating 2″ substrates horizontally; the growth zone extended over ∼19 cm with a temperature gradient of ∼3 °C/cm. The total impurity concentration N was in the range ≤ 1012–1014 cm–3. These epilayers have been investigated using low temperature photoluminescence (LTPL), C–V, and Hall measurements. However, estimation of the electro-physical parameters of ultra-pure (N ≤ 1012 cm–3) GaAs often causes difficulties. Because of the low concentration of impurities (N ≤ 1012 cm–3) electro-physical measurements are difficult in many cases or even impossible. In those cases where electro-physical measurements proved feasible a direct relationship was established between the variation of the electro-physical parameters and the evolution of the LTPL spectra at 2 K. By extrapolating the obtained data to regions of lower impurity concentration, ND and μe values corresponding to certain values of the parameters of the LTPL spectra could be found. Thus, analysis of the LTPL spectra can yield estimates of ND and μe when direct determination is difficult or impossible. We consider that from a detailed analysis of LTPL spectra at 2 K the concentration of deep levels in ultra-pure GaAs can be determined.

Feasibility of optically detected cyclotron resonance to study electron mobility in ultrapure GaAs

While the optically detected cyclotron resonance (ODCR) technique is a widely applied technique for the study of semiconductors, the effect of cyclotron resonance (CR) on the carrier capture and recombination processes is not well understood. We report on a comparative study of microwave CR and ODCR in photoexcited ultrapure GaAs at low temperatures. We found that the ODCR spectrum is broader and thus gives lower electron mobility than the CR spectrum under the same experimental conditions. To explain the discrepancy, a rate-equation model is developed for the dependence of the exciton density on the microwave power (free-electron temperature) under the CR conditions. A good agreement between the experimental data and the model calculations was obtained by assuming the exciton formation rate dependence on the hot-electron temperature.

Light scattering by electrons in the excitonic absorption region of GaAs

A study has been made of the effect of the additional generation of photoexcited electrons on the excitonic absorption and luminescence spectra of ultrapure GaAs samples at T=2 K. The observed increase in the absorption coefficient for the ground (n=1) excitonic state is shown to originate from the polariton character of the energy spectrum of this state and to be due to an increase of polariton damping. The increased damping observed under electron generation is caused by polariton scattering from hot electrons as the latter undergo thermalization. As a result, the polaritons are heated. The changes observed in the luminescence spectra are produced by the reverse effect of electron heating and polariton cooling.

Low-temperature electron mobility studied by cyclotron resonance in ultrapure GaAs crystals.

We report on a systematic study of the electron scattering processes in ultrapure GaAs crystals over the temperature range 1.6--100 K by using the classical cyclotron resonance (CR) for a low density of photogenerated electrons (${\mathit{n}}_{\mathit{e}}$\ensuremath{\sim}${10}^{10}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$). The highest electron mobility extracted from the CR data is 2.2\ifmmode\times\else\texttimes\fi{}${10}^{6}$ ${\mathrm{cm}}^{2}$/V sec at T=1.6 K. The electron scattering due to the acoustic-phonon piezoelectric potential is found to be the main intrinsic electron-phonon scattering mechanism in the temperature range 1.6T30 K. Electron scattering by neutral impurities is shown to be significant even in these ultrapure GaAs crystals (unlike Ge and Si). The acoustic-phonon piezoelectric and deformation-potential coefficients as well as the impurity concentrations (\ensuremath{\sim}${10}^{13}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) were obtained from the analysis of the electron-scattering-rate dependence on temperature. A CR line narrowing is observed under increasing microwave power and is interpreted as due to a decrease in the acoustic-phonon piezoelectric scattering efficiency with electron heating. The electron effective mass determined from the CR magnetic field applied along [001] is ${\mathit{m}}^{\mathrm{*}}$=(0.0662\ifmmode\pm\else\textpm\fi{}0.0002)${\mathit{m}}_{0}$.

Characterization of Ca+ ion-implanted GaAs by photoluminescence

Optical properties of Ca+ ion-implanted GaAs were characterized by photoluminescence measurements at 2 K. Four new emissions denoted by (Ca0,X), gCa, SM, and (e,Ca) were found to be produced by Ca incorporation into GaAs. By changing the Ca dopant concentration and excitation intensity, it was revealed that the (Ca0, X) emission is due to excitons bound to a neutral shallow Ca acceptor, and SM probably originates from excitons bound to a Ca impurity in a deep energy level. In addition, in an impure GaAs substrate, Ca could also form a shallow emission gCa, despite the fact that the g emission for well-known shallow acceptors such as C, Be, and Mg was usually found only in ultrapure GaAs. It was demonstrated that Ca is a shallow acceptor impurity in GaAs with a binding energy of 28.4 meV.

Effect of hot electrons on exciton luminescence of ultrapure GaAs in a magnetic field

Exciton photoluminescence in ultrapure GaAs (total shallow impurity concentration ND approximately 1012 cm-3) was studied in magnetic fields up to 20 T. The observed magnetic field enhancement of free-exciton luminescence and exciton cooling were found to depend strongly on the excitation energy, giving direct evidence for the role of hot photoexcited electrons. The observed effect in fact reflects cooling of electrons in a magnetic field due to their increasing acoustic phonon energy relaxation rate and an efficient exciton-electron interaction. In a magnetic field parallel to the sample surface a suppression of the exciton-electron interaction was found because of the spatial separation of excitons and electrons.

Formation of four new shallow emissions in Mn+ ion-implanted GaAs grown by molecular beam epitaxy having extremely low concentration of background impurities

Mn+ ions were implanted into ultrapure GaAs layers grown by molecular beam epitaxy. 2 K photoluminescence revealed that in addition to the well-established Mn-related deep acceptor emission at ∼1.41 eV, four new shallow emissions denoted by (Mn°, X), ‘‘G’’, ‘‘H’’, and (D, A)2 are formed in the near band edge when the Mn concentration [Mn] exceeds 3×1016 cm−3. Both ‘‘G’’ and ‘‘H’’ exhibit no energy shift with growing [Mn] up to 1×1019 cm−3. In contrast, for shallow acceptor- (such as C) doped GaAs with extremely low background concentrations of donor impurities, a series of [g-g]-like energy levels, which present strong energy shifting with increasing acceptor concentration, are universally formed. These results show that pairs between deep Mn acceptors do not produce such [g-g] like energy levels.

Effect of hot electrons on the magnetic field enhancement of exciton luminescence in GaAs

Exciton photoluminescence in ultrapure GaAs (with total shallow impurity concentration N D ∼ 10 12 cm −3) was studied in magnetic fields up to 20 T. The observed magnetic field enhancement of free-exciton luminescence and exciton cooling were found to depend drastically on the excitation energy. The experimental data give strong evidence to the model which is based on an increase in the energy relaxation rate of photoexcited electrons in a magnetic field and an efficient exciton-electron interaction.

Formation of Shallow Energy Levels in Mn+ Implanted GaAs with Extremely Low Background Impurity Concentration

ABSTRACTManganese ions implantation into ultrapure GaAs layers grown by molecular beam epitaxy were investigated by photoluminescence technique systematically in a wide range of manganese concentration up to 1×1020cm-3. Five shallow emission bands denoted by (Mn°, X), “G”, “G' “ “H” and (D, A)2 are formed in the implanted layers in addition to the well known Mn impurity related emission at ~880nm. With increasing manganese concentration to 1×1019cm-3, “G” exhibits no energy shift, suggesting that “G” is different from the behavior of [g-g] emission that is commonly formed in shallow acceptor (such as C) incorporated ultrapure GaAs. (Mn°, X), “G” and “G' “ present no energy shift with increasing excitation intensity, while “H” and (D, A)2 indicate peak energy shift greatly showing typical donor-acceptor pair characteristics. “G” and (Mn°, X) are found to hold similar radiative origin which is different from “G”. Temperature dependence measurement reveals that emission “G” has a thermal activation energy of 5.4meV.

Damage-Free Ion Beam Doping of Carbon During Molecular Beam Epitaxy of GaAs

ABSTRACTRecently, we introduced various acceptor impurities into MBE-grown ultra-pure GaAs by conventional high-energy ion implantation and found many novel shallow emissions associated with acceptor-acceptor pairs. Most of these emissions were easily quenched by extremely small amount of residual donor atoms which were unintentionally introduced during doping processes. For the interpretation of impurity effects, the usage of mass-separated atom as dopant source was strongly suggested. Along this consideration, we developed combined ion beam and molecular beam epitaxy (CIBMBE) technology, in which damage-free doping with high mass purity (M/ΔM=100) is expected to be possible. We here present the results of low-energy (100 eV) carbon ion doping using CIBMBE method. Samples were prepared asa function of growth temperature (Tg=400-700°C) and ion beam current. Net hole concentration, |NA-ND| as high as ~1×1020 cm-3 was obtained in as-grown samples. In 2K photoluminescence spectra, emissions due to acceptor-acceptor pairs exhibit specific energy shift with growing |NA-ND|. Results indicate that carbon doping can be made efficiently even at Tg as low as 500°C without any post heat treatment. These results also tell that by CIBMBE method no serious radiation damages are produced and the undesired impurity contamination can be considerably suppressed.

Effect of arsenic source on the growth of high-purity GaAs by molecular beam epitaxy

We report growth of ultrapure GaAs by molecular beam epitaxy (MBE) and show that the arsenic source has a dramatic effect on the purity. With every thing else the same, by changing the arsenic source from 6N grade double refined chunks to a 7N grade slug which closely fits the 35 cm3 crucible, the deep electron trap and the residual acceptor densities were reduced by nearly two orders of magnitude. When intentionally doped with Si to an electron density of 3×1013 cm−3, the measured mobility at 77 K was 205 600 cm2 V−1 s−1, which increased to 294 700 cm2 V−1 s−1 at 42 K. These mobilities are the highest ever observed in intentionally n-doped MBE GaAs. Low-temperature photoluminescence studies of both undoped and Si-doped GaAs layers grown with the As slug show a marked decrease in acceptor- and defect-related luminescence over that observed when the As chunks are used. Our study conclusively proves that the major source of residual acceptor impurities and typical MBE GaAs traps observed by deep level transient spectroscopy is the chemical impurities present in the arsenic source, although the exact nature of these impurities is yet to be identified. These impurities or their complexes with the native crystal defects, and not the native defects alone, are responsible for the deep traps.

Selective optical compensation effect of two new near-band-edge emissions in simultaneously acceptor (Zn+) and donor (Se+) ion-implanted GaAs

Newly discovered Zn-associated emissions in GaAs, ‘‘g’’ and [g-g], were found to be completely suppressed by the simultaneous presence of Zn and Se atoms. This selective optical compensation (SOC) effect observed when acceptor (Zn) and donor (Se) atoms exist in combination was established by simultaneously implanting Zn+ and Se+ ions into almost carbon-free ultrapure GaAs made by molecular-beam epitaxy. It is suggested that the failure to observe ‘‘g’’ and [g-g] emissions in acceptor-incorporated GaAs prepared by conventional epitaxial methods may be ascribed to this SOC effect, in which donor atoms were unintentionally introduced into the epitaxial layer.

Photoconductivity effects in extremely high mobility modulation-doped (Al,Ga)As/GaAs heterostructures

Single period modulation doped Al0.35Ga0.65 As/GaAs heterostructures were grown by molecular beam epitaxy. The mobilities and sheet carrier concentrations were measured as a function of lattice temperature in the dark and in room light. Mobilities as high as 8490, 105 000, and 221 000 cm2/Vs at 300, 78, and 10 K, respectively, were obtained for samples measured in the dark. When measured in light, these mobilities increased to 9090, 136 000, and 286 000 cm2/Vs at the same respective temperatures. In all cases the sheet carrier concentrations were between 4.5×1011 and 9.5×1011 cm−2. These values represent one of the best dark values reported to date and are significant with respect to field effect transistor applications. The 300 K mobility of 9090 is equivalent to the best mobilities obtained in ultrapure GaAs (n⩽1013 cm−3). The change in the mobility and sheet carrier concentration is the result of a persistent photoconductivity effect which is attributed to the ionization of electrons from traps in the (Al,Ga)As layer. Electron hole pair generation, though smaller, also affects the electron mobility because of increased screening.

High purity GaAs by liquid phase epitaxy

The growth of ultra-pure GaAs by the liquid phase epitaxy technique is reported. This material has exhibited net carrier concentrations as low as 1012 cm-3 and peak Hall mobilities up to 250 000 cm2/V sec at 51°K. High drift mobilities of 350 000 cm2/V sec have been observed at 4°K under high electric field conditions. Über das Wachstum von ultra-reinem GaAs, das durch Flüssig-Phase-Epitaxie gewonnen wurde, wird berichtet. Ladungsträger-Konzentrationen vou nur 1012 cm-3 und maximale Hall beweglich keiten bis zu 250 000 cm2/V sec wurden beieiner Temberatur von 51°K erhalten. Beihohen Feldstärken und einer Temperatur vou 4°K wurden hohe Driftgeschwindigheiten von 350 000 cm2V sec beobachtet.