Ga Cell Temperature Research Articles

Effects of gallium doping on properties of a-plane ZnO films on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy

The authors report on the structural, optical, and electrical properties of Ga-doped a-plane (112¯0) ZnO films grown by plasma-assisted molecular beam epitaxy. Ga doping level was controlled by changing the Ga cell temperatures from 350 to 470 °C with an interval of 30 °C. With up to Ga cell temperatures of 440 °C, single crystalline Ga-doped a-plane ZnO films were grown; however, the sample with a Ga cell temperature of 470 °C showed polycrystalline features. The typical striated surface morphology normally observed from undoped ZnO films disappeared with Ga doping. ZnO films doped with Ga cell temperatures up to 440 °C did not show a significant change in full width at half maximum (FWHM) values of (112¯0) x-ray rocking curves by doping. The smallest FWHM values were 0.433° (ϕ=90°) and 0.522° (ϕ=0°) for the sample with a Ga cell temperature of 350 °C. The polycrystalline ZnO film with excessive Ga doping at the Ga cell temperature of 470 °C showed significantly increased FWHM values. Hall measurements at room temperature (RT) revealed that electron concentration began to be saturated at the Ga cell temperature of 440 °C and electron mobility was drastically reduced at the Ga cell temperature of 470 °C. The carrier concentration of Ga-doped ZnO films were controlled from 7.2×1018 to 3.6×1020 cm−3. Anisotropic electrical properties (carrier concentration and Hall mobility) were observed in measurements by the van der Pauw method depending on the direction (c- or m-direction) for the undoped sample but not observed for the doped samples. RT photoluminescence (PL) spectra from the Ga-doped single crystalline ZnO films showed dominant near band edge (NBE) emissions with negligibly deep level emission. The NBE intensity in PL spectra increases with Ga doping.

Properties of Cu(In,Ga)Se2 Thin Film Solar Cells on Ga Temperature Variation

In this paper the effect the Ga/(In+Ga) ratio, which was controlled by Ga cell temperature, on the growth behavior of Cu(In,Ga)Se2 (CIGS) thin film and its photovoltaic performance is presented. It was found that both the grain size and the void density of CIGS layer decreased due to higher incorporation of Ga in CIGS by increasing Ga temperature. It was revealed that the CIGS films satisfying the composition ratio of Ga/(In+Ga) = 0.3∼0.4 and Cu/(In+Ga) = 0.84∼1.04, which were obtained at the Ga temperature of 1033∼1035°C, has the comparable diffraction intensity of (112) and (220) peaks. The (112)/(220) peak ratio of either Cu-rich or heavily Cu-poor CIGS films was found to deviate from unity and the solar cells made at these composition range showed lower photovoltaic performances. The highest efficiency of solar cell obtained by adjusting Ga cell temperature was 8.93% on device area of 0.16 cm2 (fill factor, open circuit voltage, and short circuit current were 50.34%, 576 mV and 30.79 mA/cm2, respectively).

Photoluminescence of Ga-doped ZnO film grown on c-Al2O3 (0001) by plasma-assisted molecular beam epitaxy

High quality gallium doped ZnO (Ga:ZnO) thin films were grown on c-Al2O3(1000) by plasma-assisted molecular beam epitaxy, and Ga concentration NGa was controlled in the range of 1×1018–2.5×1020∕cm3 by adjusting∕changing the Ga cell temperature. From the low-temperature photoluminescence at 10K, the donor bound exciton I8 related to Ga impurity was clearly observed and confirmed by comparing the calculated activation energy of 16.8meV of the emission peak intensity with the known localization energy, 16.1meV. Observed asymmetric broadening with a long tail on the lower energy side in the photoluminescence (PL) emission line shape could be fitted by the Stark effect and the compensation ratio was approximately 14–17% at NGa⩾1×1020∕cm3. The measured broadening of photoluminescence PL emission is in good agreement with the total thermal broadening and potential fluctuations caused by random distribution of impurity at NGa lower than the Mott critical density.

Growth and characterization of Ga-doped ZnO layers on a-plane sapphire substrates grown by molecular beam epitaxy

Gallium-doped ZnO epitaxial layers were grown on a-plane sapphire substrates by molecular beam epitaxy (MBE) at various Ga cell temperatures from 350°C to 450°C. The ZnO layers grown on a-plane sapphire were c-oriented without any trace of the 30° rotation domains often observed in ZnO on c-plane sapphire. The Ga concentration in Ga-doped ZnO increased from 4×10 16 to 7×10 18 cm −3 with increasing Ga cell temperature. The activation ratio of Ga was about unity when the Ga concentration exceeded 3×10 17 cm −3. The photoluminescence (PL) spectra of Ga-doped ZnO were dominated by an emission at 3.362 eV which can be assigned to emission of exciton bound to Ga-related neutral donors. The intensity of this emission was maximum when the Ga concentration was 2×10 18 cm −3. The high crystalline quality of the Ga-doped ZnO epilayers was confirmed by X-ray diffraction (XRD), Hall effect measurement and Rutherford backscattering spectrometry. Our results show that high-quality Ga-doped n-type ZnO can be grown on a-plane sapphire substrates by using MBE.

Using statistical experimental design to investigate the role of the initial growth conditions on GaN epitaxial films grown by molecular beam epitaxy

The nucleation and buffer growth of GaN on (0001) sapphire by molecular beam epitaxy are investigated using the design of experiments approach. Six factors are simultaneously varied: time and temperature for nitridation, buffer growth temperature, Ga cell temperature, growth time, and nitrogen plasma power during buffer growth. In situ reflection high-energy electron diffraction is utilized to monitor these steps. The quality of the epitaxial layers obtained is examined by means of electron mobility and atomic force microscopy. It is shown that the buffer layer growth rate has the greatest influence on improving the electrical properties of the subsequent GaN epitaxial layer. Depending on the growth conditions, the Hall mobility of the GaN epitaxial layer varies from 24 to 238 cm2/V s. Changes in surface morphology are correlated with improvements in electron mobility. We also discuss interaction effects between the factors. A trend extracted from a least-squares model reveals that 300 K Hall mobility is greatly improved at high growth rate and low nitrogen plasma power during buffer growth.

RBS/Channeling study of Er doped GaN films grown by MBE on Si [formula omitted] substrates

The influence of the Ga cell temperature on the quality of GaN films grown by MBE on p-Si(1 1 1) substrates was studied for cell temperatures ( T Ga) in the range from 865°C to 922°C using the RBS/Channeling technique. The films were in situ doped during growth with Er at a constant cell temperature. The films show a strong dependence of the crystalline quality on the Ga cell temperature with the best films grown at T Ga=915 ∘C. For temperatures T Ga below 880°C the films showed no channeling effect. The thickness increases linearly with the temperature suggesting that changes in the Ga flux influence the growth process. The decrease of the Ga flux allows the incorporation of higher Er concentrations in the films. The data showed that a maximum value of about 0.35 at% was reached under the chosen growth conditions. The Er ions occupy mainly the Ga sublattice in the films with single crystalline quality. A comparison of the angular scans through the 〈0 0 0 1〉 and the 〈1 0 1 ̄ 1〉 axes with Monte Carlo simulations leads to the conclusion that a majority (∼90%) of the Er ions occupies Ga sites.

Mg doping studies of electron cyclotron resonance molecular beam epitaxy of GaN thin films

A modified electron cyclotron resonance plasma source, allowing optically active GaN thin films at growth rates up to 1 μm per h, was utilized in a molecular beam epitaxy environment for Mg doping studies of GaN. The effect of the growth parameters on the Mg incorporation was studied using secondary ion mass spectroscopy and Hall measurements. We found that Mg does not incorporate when GaN is grown under Ga-rich conditions, independently of the Mg flux, in the temperature range of 700–800 °C. Only N-rich growth conditions allow measurable Mg incorporation. This incorporation increases with decreasing Ga cell temperature. Furthermore, under N-rich growth conditions, the Mg content is more sensitive to the lowering of the substrate temperature than to the increase of the Mg cell temperature.

MBE growth of vertical-cavity surface-emitting laser structure without real-time monitoring

Evaluation of producing a vertical-cavity surface-emitting laser (VCSEL) epitaxial structure by molecular beam epitaxy (MBE) without resorting to any real-time monitoring technique is reported. Continuous grading of Al x Ga 1− x As between x=0.12 to x=0.92 was simply achieved by changing the Al and Ga cell temperatures in no more than three steps per DBR period. Highly uniform DBR and VCSEL structures were demonstrated with a multi-wafer MBE system. Run-to-run standard deviation of reflectance spectrum center wavelength was 0.5% and 1.4% for VCSEL etalon wavelength.

Cubic GaN Heteroepitaxy on Thin-SiC-Covered Si(001)

We have investigated the growth conditions of cubic GaN (β-GaN) layers on very thin SiC-covered Si(001) by using gas-source molecular beam epitaxy as functions of SiC layer thickness, Ga-cell temperature and substrate temperature. Under the present SiC formation conditions on Si substrates by carbonization using C2H2 gas, the SiC layers with the thickness between 2.5 and 4 nm result in the epitaxial growth of β-GaN on thus SiC-formed Si substrates. At the highest GaN growth rate of 110 nm/h ( a Ga-cell temperature of 950 °C), β-GaN layers grown at a substrate temperature of 700 °C show a nearly flat surface morphology and the fraction of included hexagonal GaN becomes negligible when compared to the results of β-GaN layers grown under other conditions of Ga-cell and substrate temperatures. Thus obtained β-GaN films have good performance in photoluminescence intensity although the FWHM of band-edge recombination peak is still wider (137 meV) than the reported values for the β-GaN on 3C-SiC and GaAs.

Self-organization of GaN/Al0.18Ga0.82N multi-layer nano-columns on (0 0 0 1) Al2O3 by RF molecular beam epitaxy for fabricating GaN quantum disks

GaN nanostructures (GaN nano-column) were grown on (0001) Al2O3 by RF-radical source molecular beam epitaxy (RF-MBE) through a self-organization process. An averaged diameter of the GaN nano-column was minimized to 40–45nm by optimizing a Ga cell temperature under a fixed nitrogen supply. The self-organization process of the GaN nano-column was applied for the fabrication of the GaN/Al0.18Ga0.82N multi-quantum disk (MQD). The GaN nano-column with an averaged diameter of 46nm was fabricated, in which the 10 paired GaN(6nm)/Al0.18Ga0.82N(9nm) multi-layer structure was built in to form quantum disks. The blue shift in a photoluminescence (PL) peak wavelength at room temperature was observed for the MQD sample, probably due to the quantum-size effect.

Cubic GaN Heteroepitaxy on Thin-SiC-Covered Si(001)

We have investigated the growth conditions of cubic GaN (β-GaN) layers on very thin SiC-covered Si(001) by using gas-source molecular beam epitaxy as functions of SiC layer thickness, Ga-cell temperature and substrate temperature. Under the present SiC formation conditions on Si substrates by carbonization using C2H2, gas, the SiC layers with the thickness between 2.5 and 4 nm result in the epitaxial growth of β-GaN on thus SiC-formed Si substrates. At the highest GaN growth rate of 110 nm/h (a Ga-cell temperature of 950 °C), β-GaN layers grown at a substrate temperature of 700 °C show a nearly flat surface morphology and the fraction of included hexagonal GaN becomes negligible when compared to the results of β-GaN layers grown under other conditions of Ga-cell and substrate temperatures. Thus obtained β-GaN films have good performance in photoluminescence intensity although the FWHM of band-edge recombination peak is still wider (137 meV) than the reported values for the β-GaN on 3C-SiC and GaAs.

Precise thickness measurement within a few monolayers by X-ray diffraction from InGaAs/GaAs strained-layer superlattices

We propose a technique to measure the thickness of a GaAs layer with a precision of a few monolayers (MLs) by high-resolution X-ray diffraction (HRXRD) from InGaAs/GaAs strained-layer superlattices (SLSs) on GaAs substrates. Using this technique to monitor the GaAs growth rate, we have successfully controlled the Ga beam flux within ± 1% in molecular beam epitaxy (MBE) growth for continuous 40 runs during four days by increasing the Ga cell temperature to compensate the decrease of the Ga beam flux caused by the consumption of the Ga source. Precise thickness measurements are also demonstrated in the growth on InP substrates by using InAlGaAs/InGaAs SLSs and InAlGaAs/InAlAs SLSs.

Precise thickness measurement within a few monolayers by X-ray diffraction from InGaAs/GaAs strained-layer superlattices

We propose a technique to measure the thickness of a GaAs layer with a precision of a few monolayers (MLs) by high-resolution X-ray diffraction (HRXRD) from InGaAs/GaAs strained-layer superlattices (SLSs) on GaAs substrates. Using this technique to monitor the GaAs growth rate, we have successfully controlled the Ga beam flux within ± 1% in molecular beam epitaxy (MBE) growth for continuous 40 runs during four days by increasing the Ga cell temperature to compensate the decrease of the Ga beam flux caused by the consumption of the Ga source. Precise thickness measurements are also demonstrated in the growth on InP substrates by using InAlGaAs/InGaAs SLSs and InAlGaAs/InAlAs SLSs.

Photoassisted growth of gallium nitride by gas source molecular beam epitaxy

Thin films of GaN have been achieved via gas source molecular beam epitaxy with and without substrate illumination from a 500 W Hg arc lamp. X‐ray photoelectron spectroscopy showed a nominally stoichiometric GaN film without contaminants regardless of illumination. Illumination and Ga cell temperature changed the texture of the polycrystalline GaN from (0001)∥(100) to (0001)∥(111) and back again. Near single crystal films were deposited with an orientation relationship of w‐GaN (0001)∥β‐SiC (100) and (1010)∥(011). Resulting films were characterized by scanning electron microscopy as well as x‐ray and electron diffraction.

High indium content graded channel GainAs/AlinAs pseudomorphic MODFETs

We report on the electrical and microstructural properties of InP/GaxIn1-xAs/Al0.48In0.52As modulation doped layers having compositionally graded active channels with different channel thicknesses. The layers were grown by solid source molecular beam epitaxy on Fe-doped InP substrates. The undoped GaInAs two dimensional electron gas channel layers were grown having indium compositions graded fromx = 0.53 at the substrate buffer tox= 0.65 at the heterointerface by varying the Ga cell temperature during growth. Active channel thicknesses of 20 nm and 30 nm were compared with lattice matched layers. Transmission electron microscope image analysis indicates no misfit dislocations in these structures. Hall-effect measurements at 300 K show an increase in the mobility from 8,380 cm2/Vs for the lattice matched layer to 12,500 cm2/Vs for the 30 nm pseudomorphic layer. Small gate-length, 0.25 μn, MODFETs were fabricated to determine effective velocity values from transconductance (gm) and current gain (h21) measurements. The peak dc extrinsicgm increased from 367 mS/mm for the lattice matched layer to 668 mS/mm for the 30 nm pseudomorphic layer. The effective electron carrier velocity increased from 1.57 × 107 cm/s for the lattice matched layer to 1.88 × 107 cm/s for the 30 nm pseudomorphic layer. Our results show that compositional grading is a useful technique to obtain thick pseudomorphic layers with good transport properties.

Thermodynamic study on the origin of oval defects in GaAs grown by molecular-beam epitaxy

The relationship between oval defect density and GaAs growth conditions for molecular-beam epitaxy is investigated. Oval defects are proved to form continuously during growth, with density dependent on Ga cell and substrate temperatures. The correlation between oval defect formation and chemical reactions occurring in the Ga cell and on substrate surfaces, e.g., Ga-Ga2O3, GaAs-Ga2O3, and C-Ga2O3, including the vaporization of Ga and Ga2O3, is investigated thermodynamically. Ga2O formed by the reactions between Ga and Ga2O3 and between C and Ga2O3 in the Ga cell is determined to be related to oval defect formation. Next, substrate surface reactions are studied to investigate the behavior of Ga2O emitted from the Ga cell. The activation energy related to the reaction suggests that only Ga2O adsorption and desorption occur on the substrate. Thus, Ga2O adsorbed on the substrate determines oval defect formation. Prevention of Ga oxidation by chamber baking suggested by the above investigation results in a decrease in oval defect density by a factor of 10. Here, Ga2O is also proved to be the defect origin. It is formed by the reactions between Ga and H2O and between C, Ga, and H2O remaining even in ultrahigh vacuum conditions. These results suggest that both the oxygen and the H2O remaining in the MBE chamber should be reduced to eliminate oval defects.

Molecular-beam epitaxial growth of graded band-gap quaternary GaxAlyIn1−x−yAs multilayer heterostructures on InP: Application to a novel avalanche photodiode with an ultrahigh ionization ratio

We first describe the growth of device quality compositionally graded quaternary GaxAlyIn1−x−yAs on InP by molecular-beam epitaxy (MBE). Device quality material without misfit dislocations were grown to satisfy the lattice matching condition x+y≂0.47 as closely as possible during the growth of the entire graded region. This was accomplished by keeping the In flux constant (constant cell temperature) while changing the Ga and Al cell temperatures simultaneously to keep the total Ga plus Al flux constant while grading x (and consequently y). This was done using a desktop computer and a data acquisition/control system interfaced to cell temperature controllers and shutter actuators. Next we describe growth and operation of a novel p-intrinsic-n type semiconductor (p-i-n) avalanche photodiode (APD) which incorporates such compositionally graded layers in the i region. The i layer contains a multitude of asymmetric wells with one side abrupt and the other graded, each well surrounded by (Al,In)As. This device yielded a very large hole to electron ionization ratio β/α=50 at −12-V bias and 200 Hz. This value for β/α is the highest reported in any III–V compound. Avalanche multiplication occurs when carriers heated by the electric field in the barrier layers impact ionize carriers which are dynamically stored in the wells. Due to the grading of the exit well/barrier interface and under reverse bias condition there is a substantial reduction in the density of dynamically stored electrons and there is negligible multiplication of electrons resulting in a high β/α ratio.

Transport properties of Sn-doped AlxGa1−xAs grown by molecular beam epitaxy

Transport properties of Alx Ga1−x As layers having AlAs mole fractions between 0.17 and 0.375 and grown by molecular beam epitaxy were investigated. The Al cell temperature was kept at 1075 °C while the Ga cell temperature was changed between 910 and 950 °C to obtain the aforementioned AlAs mole fractions. The donor levels of Sn in Alx Ga1−x As obtained from the temperature dependence of the net donor concentration are below 3, 30, and 40 meV for x=0.17, x=0.29, and x=0.375, respectively. Al0.22 Ga0.78 As layers having a net electron concentration of about 1.7×1018 cm−3 and grown at 630 °C exhibited a mobility of 868 and 1095 cm2/V sec at 300 and 78 °K, respectively. The mobility was found to be strongly dependent on the substrate temperature during the growth. The lower substrate temperatures resulted in lower electron mobility.