High Purity GaAs Research Articles

Enhancement of donor-related photoluminescence intensity due to near-band-gap excitation of high purity GaAs

Striking increases in the intensity of donor-related, photoluminescence transitions are observed in undoped (10 14-10 15 cm -3) GaAs for excitation energies (E e) in the vicinity of the band-gap energy (E g). The enhancement has maxima at E e consistent with excitation of the n=2 and n=3 states of the free exciton (X n=2,3) and appears to be correlated to the concentration of ionized donors, suggesting that the effects are related to capture of electron-hole pairs by ionized donors through trapping of X n=2,3. A resonant interaction may be operative as well. The enhancement decreases monotonically as E e increases to values as much as 12 meV above E g.

Mechanisms for the optically detected magnetic resonance background signal in epitaxial GaAs.

The mechanisms for the usually very strong background signal in optically detected magnetic resonance (ODMR) experiments have been studied in high-purity epitaxial GaAs layers. The optically excited carriers are heated by the microwave field, via both cyclotron resonance and other microwave absorption processes involving free-carrier transitions within a Landau subband. The main mechanism for the change of photoluminescence (PL) by the hot carriers in the temperature range 4-10 K studied in this work is concluded to be impact ionization of shallow bound excitons (BE's) and shallow donors by hot free carriers, resulting in a quenching of the shallow PL emissions related to BE's and donor-acceptor pairs. These processes explain the microwave-induced change of PL intensity (the ODMR background signal).

Assessment of n-type GaAs by the Hall effect

Variable temperature (4–300 K) Hall measurements were used to determine the donor and acceptor concentrations in high-purity epitaxial GaAs grown by metalorganic chemical vapor deposition, molecular-beam epitaxy, and vapor-phase epitaxy. It is found that in many cases the onset of impurity conduction at low temperatures limits the successful determination of the impurity concentrations in the material. Furthermore, attention is drawn to the fact that experimentally determined compensation ratios may differ appreciably from those obtained by the much used method of comparing a single 77 K measurement with theoretically calculated tables.

Very high purity GaAs grown by gas source molecular beam epitaxy

We report gas source molecular beam epitaxy (GSMBE) growth of very high purity GaAs in which a peak mobility near 300,000 cm 2/V·s was observed. We investigate this high purity layer by means of deep level transient spectroscopy (DLTS), photoluminescence and transport measurements. We present evidence that an unintentionally doped donor has binding energy less than 2 meV and the temperature dependence of mobility at low temperature significantly deviates from the expected transport behavior.

Evidence for Strong Trapping by Ionized Donors of Free Excitions in Excited States for High Purity GaAs and AlGaAs

AbstractStriking increases in the intensity of donor-related, photoluminescence transitions are observed in undoped (1014-1015 cm-3) GaAs for excitation energies (Ee) in the vicinity of the band-gap energy (Eg). The enhancement has maxima at Ee consistent with excitation of the n=2 and 3 states of the free exciton (Xn=2,3) and appears to be correlated to the concentration of ionized donors, suggesting that the effects are related to capture of electron-hole pairs by ionized donors through trapping of Xn=2,3. The enhancement decreases monotonically as Ee increases to values as much as 12 meV above Eg.

Identification of residual donors in high-purity undoped p-type epitaxial GaAs by magnetophotoluminescence

The residual donor species, Si, S, and Ge, have been identified in high-purity undoped p-type epitaxial GaAs grown by metalorganic chemical vapor deposition and arsenic trichloride vapor phase techniques using the magnetic splittings of ‘‘two-electron’’ replicas of donor bound exciton transitions at low temperature (∼1.8 K) and at a high magnetic field (9.0 T). This technique permits identification of donors in certain high-purity p-type GaAs samples in which donor species cannot be identified by photothermal ionization spectroscopy or any other technique.

Stoichiometric structures of defects in high-purity GaAs grown by the liquid encapsulated Czochralski method

We analyze some existing data obtained on a GaAs sample grown by the liquid encapsulated Czochralski (LEC) method from a near-stoichiometric melt after the sample was cycled through various thermal processes. By using the constraint of constant deviation from stoichiometry we are led to suggest that the defects observed or inferred to exist in the sample have the following properties: (1) the acceptor associated with the 1.45-eV photoluminescence signal has the stoichiometric structure of GaAs if doubly charged or of VGaGaAs if singly charged; (2) the (presumed) donor at Ec−0.134 eV has the stoichiometric structure of VAs; (3) another (inferred) acceptor has the stoichiometric structure of VGa; and (4) the very shallow donor at Ec−0.003 eV is the precursor of EL2 and becomes EL2 upon reaction with VGa or its stoichiometric equivalent.

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.

Growth of device quality GaAs by chemical beam epitaxy

The growth of high purity GaAS with excellent uniformity and very low defect density by chemical beam epitaxy using triethylgallium and arsine is described. The residual background impurity is mostly carbon. A mobility of 518 cm2/Vs with a hole density of 3.6 x 1014 cm−3 has been obtained for a growth temperature of 500° C. The electrical quality is further evaluated by fabricating a Si doped epilayer into MESFET device using 1 μm gate length. A transconductance of 177 mS/mm has been measured. The results indicate that chemical beam epitaxy is a very attractive growth technique for GaAs integrated circuits.

Effects of polyimide passivation on the photoluminescence of high-purity epitaxial GaAs

Polyimide has been used as interlayer dielectric and passivation material. We studied the optical passivation property of polyimide by examining the spectral changes in the photoluminescence (PL) spectra of high-purity epitaxial GaAs coated with polyimide. Before full cure, the polyimide is nearly transparent to visible and near-infrared radiation. After full cure, the typical PL of GaAs was unobservable at liquid nitrogen or higher temperatures, but clear and distinct at liquid-helium temperature. The resolution of the GaAs exciton spectrum was improved in the coated sample. The polyimide-GaAs system also resulted in very weak luminescence extending over a wide spectral range from the red to near infrared.

Photoluminescence identification of residual donors in undoped GaAs grown by metalorganic chemical vapor deposition

A photoluminescence technique was used to identify residual donors in the 1014 cm−3 range in high-purity GaAs grown by metalorganic chemical vapor deposition. The measurements were taken at zero magnetic field. A narrow-linewidth tunable laser was used to resonantly enhance a specific narrow-linewidth ‘‘two-electron’’ satellite line of the donor-bound exciton, enabling determination of the 1s-to-2s donor energy separations. Ge was identified as the dominant residual donor. Lower levels of the donors X1 and X2 were observed in some samples. These are attributed to Si (or possibly Sn) and S donors.

Control of residual impurities in very high purity GaAs grown by organometallic vapor phase epitaxy

Very high purity GaAs layers with 77 K electron mobility values as high as 210 000 cm2/V s and a compensation ratio as low as ≊0.05 (NA+ND≊1014 cm−3) have been grown by organometallic vapor phase epitaxy. 4.2 K photoluminescence and magnetophotoluminescence spectra of these layers confirm their high purity. The degree of material purity and the compensation are found to be controllably dependent on the growth conditions, the optimum growth temperature being about 650 °C at a V:III ratio of 17.5.

Limits of electro-absorption in high purity GaAs, and the optimisation of waveguide devices

Limits of electro-absorption in high purity GaAs, and the optimisation of waveguide devices

Hydrogenation of GaAs and Application to Device processing

ABSTRACTExposure of GaAs and AlGaAs to a hydrogen plasma has been shown to result in a significant change in the electrical and optical properties. The changes are related to the electrical deactivation of the deep and shallow impurities by hydrogenation. Spectroscopic and electrical measurements have shown that Si donors and C acceptors in high purity GaAs can be passivated by hydrogenation. Hydrogenation of p-type GaAs and AlGaAs has resulted in highly resistive material. SiO2 was found to be a suitable mask for the hydrogenation process. Single and multiple stripe geometry lasers have been fabricated by properly masking the laser structure. The lasers produced using the hydrogenation process have low threshold currents and are capable of cw room temperature operation.

Growth and characterization of high quality LPEE GaAs bulk crystals

High purity GaAs bulk crystals were grown by liquid phase electroepitaxy (LPEE). One inch diameter more than 4 mm thick ingots were free of polycrystalline edges and Ga inclusions. Typical free carrier concentration was in the 10 14 cm -3 range. Low temperature photoluminescence (PL) measurements identified the Si acceptor as the dominant residual impurity which was distributed very uniformly throughout the ingots. In addition, a correlation was established between residual impurity concentration and key growth parameters, such as temperature, baking time of the Ga-As solution, and the solution volume.

Chemical beam epitaxial growth of high-purity GaAs using triethylgallium and arsine

The growth of high-purity GaAs by chemical beam epitaxy using triethylgallium and arsine is reported. Purity of the epilayer is affected by the cracking efficiency of arsine, V/III ratio, and the substrate temperature. Samples generally show p-type conductivity with carbon as the residual impurity. The growth conditions to achieve net carrier concentration below 1014 cm−3 are identified. The low-temperature photoluminescence spectrum shows well-resolved excitonic structures, an indication of excellent optical quality.

Identification of residual donors in high-purity epitaxial GaAs by magnetophotoluminescence

The magnetophotoluminescence peaks in the ‘‘two-electron’’ satellites of the donor bound exciton transitions corresponding to the shallow donors S, Si, Ge, Sn, and Te have been identified by correlation of photoluminescence measurements with photothermal ionization measurements on the same high-purity epitaxial GaAs samples. The magnetophotoluminescence measurements were made at 1.7 K and a magnetic field of 9.0 T. These results permit the use of magnetophotoluminescence measurements for the identification of residual donor impurity species in high-purity GaAs.

Abnormal outdiffusion behavior of the deep level EL2 at the surface layer of undoped semi-insulating GaAs

p-type thermal conversion of high-purity undoped semi-insulating GaAs after annealing at 850 °C in H2 gas has been investigated. It has been found that the p-type carrier concentration at the surface layer quantitatively matches the residual carbon concentration in the bulk. The concentrations of the other acceptor impurities (Mn, Zn, Cu, Cr) in the sample are confirmed to be negligibly small. The depth profile of the carrier concentration indicates that the concentration of the EL2 level increases as the sixth power of the depth near the surface region and that it exhibits a linear dependence on depth in the deeper region. A possible mechanism for this abnormal behavior of the EL2 level is discussed.

Hydrogen passivation of C acceptors in high-purity GaAs

The effects of hydrogenation in high-purity p-type GaAs grown by molecular beam epitaxy and metalorganic chemical vapor deposition have been investigated by low-temperature photoluminescence and Hall-effect measurements. Before hydrogenation, photoluminescence measurements showed the dominant acceptor in the original samples was C, while after hydrogenation, the concentration of electrically active C acceptors was significantly reduced and the samples were highly resistive. These electrical and spectroscopic results show that C acceptors in GaAs can be passivated by hydrogenation.

Cyclotron-resonance study of polarons in GaAs-AlxGa1-xAs heterostructures.

Cyclotron-resonance transmission and emission experiments were performed in the two-dimensional electron system of an ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As-GaAs heterojunction using a nonresonant three-level excitation technique. It is found that in comparison to bulk GaAs, the polaron effect is reduced in the investigated energy range. The reduction of the polaron effect in the two-dimensional electron gas can be explained by occupation number effects. In addition, we determined the polaron effect in a hot-electron situation. At low electric fields the mass increase with energy is caused mainly by nonparabolicity. At electron temperatures above 100 K the polaron contribution to the effective mass reaches values observed in high-purity bulk GaAs.